Snow loads on sloped roofs: report on a pilot survey carried out in the Ottawa area during the winter of 1965-66

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Snow loads on sloped roofs: report on a pilot survey carried out in the

Ottawa area during the winter of 1965-66

NATIONAL RESEARCH COUNCIL CANADA

DIVISION OF BUILDING RESEARCH

SNOW LOADS ON SLOPED ROOFS Report on a Pilot Survey Carried Out in the Ottawa Ar ea During the Winter of 1965 -66

by

Halvor ｈｾｩ｢ｾ

ANALYZED

Internal Report No. 335 of the

Division of Building Research

OTTAWA November 1966

The Division of Building Research has carried out a survey of actual snow loads on roofs across Canada for a number of years with the broad objective to provide the basis for continued improvement of the design snow loads in the National Building Code.

Although a considerable refinement in design loads has already been achieved in the 1960 and 1965 Codes, one

of the areas that needs further study is that of snow loads on sloped roofs and particularly the relationship between roof slope and snow load.

This report describes a pilot survey carried out in and around Ottawa during the winter of 1965-66, dealing with sloped roofs only. The report contains a brief de-scription of slope reduction factor s used in other codes in addition to the results of the survey and a brief

dis-cussion on the release mechanism for snow sliding off roofs. The report was prepared by Halvor HpibSZ( Civil Engineer, Institute of Farm Buildings, Norwegian Agri-cultural College, Vollebekk., Norway, who was a guest worker in the Building Structures Section from 1 January 1966 to 31 May 1966.

Ottawa

November 1966

R. F. Legget Director

TABLE OF CONTENTS

GENERAL PURPOSE OF THE SURVEY PLAN OF THE SURVEY

REGULATIONS IN OTHER CODES SNOW SLIDE RELEASE MECHANISM RESULTS AND DISCUSSION OF RESULTS

Method of Measuring Snow Depth and Density

Calculation Methods Weather

Loads Plotted as Function of Slope Roofing Material

Exposure to Wind

Heat Loss and Solar Radiation Relationship Between Drift Loads

and Area Covered by the Drifts CONCLUSION AND SUGGESTIONS

FOR FURTHER STUDIES ACKNOWLEDGEMENTS REFERENCES Page 2 2 3 4 7 7 7 8

9

9

10 10 11 11 13 14

by

Snow loads on roofs are an important factor that must be taken into account in building design in many countries,

especially in northern countries with frequent snow storms and heavy snow accumulation during the winter. The building codes of most of these countries contain standards for snow loads. Many of these standards are, however. rather in-accurate and not in keeping with the accuracy of other factors in structural design.

Because of the variability in the meteorological factors and the many pos sibilities in building shape, in landscape and sheltering effects, there is a great need for snow-load

measurements to be taken on roofs under different conditions before code standards can be reasonably improved.

In Canada, a wide-ranging research project on snow loads on roofs has been carried out for several years, and the design rules for snow loads on roofs in the National

Building Code (l) are relatively advanced. It is still desirable, however, to continue the snow load research in order to get a better approach to proper design values for snow loads on differ ent roofs. One of the areas that needs further study is that of snow loads on sloped roofs and particularly the relationship between roof slope and snow load.

This report describes a pilot survey carried out

in and around Ottawa during the winter of 1965-66 dealing with sloped roofs only. This survey must be considered strictly as a pilot survey because it covers one year only. since the author, a guest worker in the Division of Building Research, had only one winter available for his study. The report contains a brief description of slope reduction factors used in other codes (Section IV) in addition to the results of the survey and a brief discussion on the release mechanism for snow sliding off roofs (Section V).

- 2

I I GENERAL PURPOSE OF THE SURVEY

A main aim of this survey was to improve, if possible, the pr esent National Building Code treatment of snow-Load reductions on sloped roofs by measuring actual snow loads on a range of roofs covering different slopes, roofing materials and exposur es to wind.

A further intention was to determine the best possible way of carrying out the snow-load measurements on roofs and to find an economical and safe way for providing additional data on snow loads on sloped roofs for building code purposes.

II I PLAN OF THE SURVEY

The survey was based on earlier experiences with snow-load measurements made by the Division of Building Research. The survey was to be carried out mainly in the Ottawa area, with supplementary studies to be made in areas of different climates, particularly the Saskatoon area and the Maritimes. Because of the snow conditions this winter, however, snow-load measurements were carried out only in the Ottawa area.

Approximately 50 buildings were chosen in the Ottawa area for the survey. The snow depths and density were to be measured both on the roofs and on the ground after every heavy snowfall, the measurements to be carried out within a period of a maximum of 3 days. The buildings chosen were mainly un-insulated farm buildings. Seven of the farm buildings had

mansard (gambrel) roofs. The rest had gable roofs or flat roofs. Sorn e of the farm buildings wer e, to a certain extent, heated by animals. Eight of the buildings chosen were insulated, heated dwelling houses, and these were included in the survey late in January, after the first snowfall where measurements were taken.

The roofs had different slopes and roofing material and were differently exposed to the wind and sun. In Table I the buildings are listed with a short description of the roofs with regard to their type, size, slope, roofing material, orientation and sheltering conditions.

Each full gable roof and each full mansard roof was normally given a number. Exceptions are made for roofs where there are different materials on the two roof sides (Nos. 4 a- b, sa-b, 22 a- b and 41 a- b); for roofs where there are different slopes on the two roof sides (Nos. 8, 9, 10, 11, 21 a- b, 26 a- b, 28 a- c, 62 a- b, 64a- b and 66 a-b); and for lean-to roofs (Nos. 12, 40 c, 44b and 4S b).

A frequency diagram for the roofs is shown in Figure 1 with the slope as variable, the slopes being taken in S-deg steps and the number of roofs being grouped individually for metal, asphalt and wood shingle roofings.

The average size of the roofs was 1760 sq ft. Average

height to the eave of the buildings included was approximately 12i ft.

IV REGULATIONS IN OTHER CODES

The regulations for snow loads on sloped roofs contained in a number of codes of countries where snow load is an important factor in building design ar e reviewed in the following notes.

In the codes reviewed, the procedure in defining the design load on sloped roofs is not uniform. It is for this reason that a "slope reduction factor" (K), which in some codes is given directly and in the rest of the codes has to be derived, was used. The design snow load on sloped roofs (Sa) could then be found as

Sa

=

K. S where S is the design load for a flat roof, and K is a function of the slope. Sa is given per unit of area of the horizontal

roof projection.

Many of the regulations have a slope r eduction factor of one up to a certain slope, usually 20 to 30 deg. The factor then decreases linearly to zero at a fairly steep slope, usually SO to 70 deg. This is shown in Figur e 2 wher e the slope -r eduction factor is given from the codes of Austria, Canada, France, Germany, Great Britain, Norway, Rumania, Sweden, United States of America and the U. S. S. R.

Figure 3 shows stepwise reduction factors {from the codes in Israel and Japan}, and Figure 4the slope-reduction factor from the codes in Czechoslovakia and Italy. Figure S indicates the slope reduction factor from the codes in Portugal and

Switzerland, where the reduction for slope starts already at 0° slope angle.

4

No extensive research work on snow-load reduction on sloped roofs s e erns to have been done up to this time. The slope-reduction factors in different codes must therefore be r egar ded principally as estimates, based on experience.

Although the conditions for snow accumulation and snow sliding off roofs may vary from one country to another, the degr ee of variation in the slope-reduction factors seems to be questionable.

V SNOW SLIDE RELEASE MECHANISM

The basic reason for the obvious fact that, in many climates, sloped roofs accumulate smaller snow loads than flat (i. e. horizontal) roofs is that on sloped roofs the snow cover is often released in the form of a slide or avalanche.

Snow slides usually from a roof when the bond between the snow cover and the roof surface is broken. This problem can be tr eated as a special case of the general avalanche release problem. The discussion in this section is mainly based on a study of the literature on snow avalanches, which relates the release mechanism to basic concepts of mechanics and certain transformations of the snow (2, 3, 4).

Deposited snow is a viscous, plastic material, consisting of ice grains and pores filled with air. The mechanics for

materials with both cohesion and internal friction are therefore applicable to deposited snow, although possibly other mechanical models might be suitably applied in certain calculations. Although both the cohesion and the internal friction, from which the str ength properties of the snow cover could be defined are not fully known,

s orn e basic figures can still be outlined.

The release of an avalanche depends on many factors. Figure 6 shows the forces acting upon an arbitrarily chosen part of a snow cover resting on a sloped surface. These forces are in balance before the release. The main problem concerns the determination of the shear str ength of the weakest layer (stratum) in the snow cover. The tensile force at the upper end surface, a compression force at the lower end surface and shear forces at the lateral surfaces of the snow mass considered are, however, also important to the stability.

Shear Strength

The shear strength, shown in Figure 6, consists of a cohesion part and a friction part. The equation for the shear strength could be written as follows (measured on unit areas):

where

=

C

+

a

tg cP o 0 0 (Coulomb's law) C o

a

o

= cohesion at r est, dependent on molecular forces between the snow particles. The

cohesion is a function of temperatur e and of the density of the snow cover.

=

effective pressure on the weak layer considered.

= angle of internal friction depending on the shape and size of the snow crystals and the compaction of the snow cover. The CPo might be presumed to consist of a true

internal friction angle (cpo I) and a part (6cpo I) dependent on the compaction of the snow, the latter part representing the internal work due to dilatation (when moving is started). The dilatation might be positive or negative (contraction) (5).

The general equation TR

=

C

+

a

tg cp could probably be applied to most types of snow. There are, however,

differences between avalanche release behaviour in coherent and in cohesionles s snow.

(a) Coherent Snow

Figures 7 and 8 give examples of release mechanism In two typical snow covers. In the two cases, only the shear strength of a presumed weak layer is taken into consideration, with no regard to possible tension, compression or lateral forces.

Figure 7 shows snow with low coherence. The sloped snow cover is stable until the shear resistance TR

=

C

+

a

tg cp

is exceeded by the releasing force T

=

O/A of Hie snow mass cone er n e d, This would occur either by an additional load which increases the force 0, or by a decrease of cohesion C, due to metamorphosis or melting. Figure 8 shows snow with high coherence.

- 6

(b) Cohesionles s Snow

New snow has a high natural slope angle due to interlacing of the branches of the snow crystals. As metamorphosis takes place, the fine branches of the crystals disappear, the crystals become rounded and the natural slope angle decreases. The natural slope angle may also decrease when the snow grains become rounded due to melting.

The surface of a snow cover consists often of a layer of cohesionless snow. Although new snow may have a natural slope angle of 60° or even higher, surface slides will occur when the natural slope angle through metamorphosis becomes lower than the slope of the snow cover (see Figure 9). It can also be seen that new snow can cling to very steep roofs (roof No. 6 b, a

=

57°).

The thickness of the snow cover in relation to the area considered is relatively high on roofs compared to ground slopes, especially when the snow loads ar e heavy. The tension, COm-pr es sion or lateral forces, ther efor e, will be more significant for snow slides from roofs than they are in regular avalanches.

The pressure along the lower edge of the roof snow can often be neglected. Only when the sliding is pr evented by ice built up in the gutter, etc., need this force be taken into account. The lateral forces can also be neglected if the sliding is not prevented by obstacles such as chimneys, ventilator s , etc. The tensile for ce at the ridge, however, often plays an important part in pr eventing snow from sliding off the roofs. This is well known from experience in areas wher e removing of the roof snow load is fr equently done in winters with exceptionally high snow loads. The photograph in Figur e 10 demonstrates that snow covers can develop a fairly high tensile strength.

Insofar as snow slides from roofs ar e concerned, the most COmmon break occurs as a shear break between the snow cover and the roof surface. The photograph in Figure 11 shows a typical break in bond between the snow cover and the roof surface. The roofing material was aluminum sheeting (roof No. 40, photograph taken on the same day as that in Figure 9). On a rougher roof surface, however, it is possible that a shear br eak would have taken place inside the snow cover, either over part of or the entire roof area.

VI RESU LTS - DISCUSSION OF THE RESULTS (a) Method of Measuring Snow Depth and Density

The snow depth was measured with a yardstick both on the roofs and on the ground after the four heavy snowfalls of the season. A sufficient number of depth measurements on the roofs were taken so that it was possible to draw lines of equal snow depths as shown, for example, for roof No. 15 in Figure 12. The snow depths were measured vertically downwards. On same of the steepest roofs it was more convenient to measure the depths at right angles to the roof surface, but these measurements were afterwards transformed into vertical depths. Where the snow layer on the roofs was even or very thin the snow depths were only estimated; on the ground they were measured at several locations around the building, usually ten places at every farm. On farms where the drifting had been heavy the ground snow depths were not measured.

Where the snow loads were significant the snow density was m ea sur ed both on the roofs and on the ground. The density measurements were taken with the Norwegian type of sampler described in Figure 13. After the heaviest snowfall (30 January) the measurements took four days instead of three days as planned. (b) Calculation Methods

For each roof the observations were plotted on a sketch of the horizontal roof projection, made up to a scale of 1 : 120 for the small and medium sized roofs and to a scale of 1 : 240 for the lar gest roofs. Lines of equal depths were drawn on the sketches.

The average depth of the snow cover on the roofs was calculated from the total snow volume d i vi d e d by the projection of the roof area. In calculating average depths bare rims at the roof edges were omitted. For each roof, only the results of the day with the maximum load condition were used. Figure 14 shows a sketch of snow load distribution for one individual roof, with a table giving the load data for the roof.

- 8

The average ground depth was obtained for each farm. In SOme cases where ground-depth measurements were not taken becaus e of too much drifting, the average ground depth available from the other farm s for that particular snowfall was us e d, The ground loads were based on the average depths and the density measurements taken at that farm; where density measurements were not available, the average density taken on that indiv idual day was used. Relatively lar ge variations in the rn e a su.r ed ground loads were found - this is in line with previous experience and should be kept in mind when considering certain anomalies in the roof-to-ground-load ratios.

The observed roof and ground loads are given in Table I I (metal roofing), Table II I (bituminous roofing) and Table IV (wood shingles). Only the maximum average recorded for the winter is shown, together with the maximum drift load and the average ground load on the same day. In the tables, every mansard roof is included as two roofs, one for each slope. The design snow loads according to the National Building Code of Canada, 1965 are calculated for each slope, based on a basic snow load coefficient

=

0.8. The coefficient O. 6 for exposed roofs was not used, in order to provide a uniform basis for comparison (a 60 psf ground load was used throughout and only the slope r eduction factor varied). In Tables I I, I I I and IV the relation between average roof load and ground load is calculated for every roof. In section (h) the relationship between drift loads and areas covered by various drift-load levels is discussed. The calculations were made only for exposed roofs. They were carried out on the basis of drift depths in relation to average depths on the roofs.

(c) Weather

The winter of 1965 -66 was rather unusual. Although the total fr esh snowfall was almost normal (slightly above normal at Uplands Airport) several midwinter thaws resulted in rather low ground loads and also in rather low roof loads. The maximum snow load on the roofs occurred for most roofs after the snow storm of 30 January. This was a very heavy snowfall resulting in 19.1 in. of fresh snow in a fairly short period. This snowfall was reported to be one of the heaviest 24-hr snowfall on record. The recorded accumulated snowfall after this storm was 71. 8 in. for Uplands Airport and 54. 1 in. at the Experimental Farm. For the whole winter the accumulated fresh snowfall was 93.9 in. at the Airport and 71. 5 in. at the

Experimental Farm. The normal for the Airport is 86 in. and for the Experimental Farm, 77 in. Insofar as wind is concerned indications were that the winter winds during and after snow storms were on the light side and that drifting was below normal. (d) Loads Plotted as Function of Slope

In Figures 15 to 19 the roof-to-ground-load ratio (in per cent of the ground load) has been plotted against the slope angle for exposed and sheltered metal, asphalt and wood roofs. It will be seen that the effect of increasing slope is, as would be expected, to reduce the load, but that the results fall into a very wide band in spite of the fact that the roofs have been categorized as mentioned above. Also shown for comparison are two lines giving the application of the K factor to 100 per cent ground load as well as the NBC design roof load. In the next sections various influences will be discussed.

(e) Roofing Material

As mentioned before, the slope reduction factors, K, for various countries including Canada have been shown in Figure 2. The slope reduction formula used in the National

Building Code of Canada can be written more simply as C :: 0.8 K where K

=

70 - a (valid only for 30· :5: a :5: 70°). In Figures 15 to 19 two lines4Rave been included, showing the K factor applied to the measured ground load (l00 per cent) and the NBC design roof load for comparison.

Differences in snow deposition and accumulation were found with the different roofing materials present. It is perhaps surprising that sheltered metal roofs appear to have higher loads than sheltered asphalt roofs (but it should be remembered that there are differences in the ground load used in calculating the roof-to-ground-load ratio). Since, however, the weather and snow conditions were such that deposition of snow on the roofs was mainly from a single snowfall, without accumulation over a longer period, the differ ences between the differ ent materials may not be as significant as would be the case in a winter

without thaws. It should also be remembered that in each group ther ewer e Iar ge variations from one roof surface to another; SOme roofs in the metal group, for instance, were rusted while others were not; some roofs were very slippery, particularly the aluminum roofs, others were not.

10

It will be seen from Figures 15 to 19 that, because of the limited snowfall during the winter and the other special conditions, it is difficult to point out clear differ ences between different roofing materials although s orn e differences have been observed in the sliding off of the snow.

Because of the time involved in observing all roofs in the survey it often happened that at the time the next survey was taken all the snow from the first (earlier) snow storm had slid off or melted away. Any clear conclusions from this w int e r ts survey are, therefore, difficult to draw with

r egar d to the effect of the roofing material.

It can be seen in one photograph that the layer of snow on a metal roof had slipped down 6 to 8 ft and that the snow was held by the effect of lateral forces, produced by a dormer (Figure 20). Observations on the following day showed that all the snow had slid off.

(f) Exposur e to Wind

Some difficulties were found in defining roofs as

IIs h e lt e r e dll or lIexposedll. Some roofs that were called IIs h e l t e r e dJ ' _{in Table I were not sheltered on all sides and} happened to be exposed during the main snow storm of the

1965 -66 winter season, becaus e of an unusual wind direction. Not many heavy drifts wer e found. Some of the drifts obs erved wer e str ongly influenced by the irr egular shape of the roofs. It should also be noted that no distinction was made between large and small roofs. The photographs in Figures 21 and 22 show the load on a well-sheltered roof and on an exposed roof, from which the difference will be noticeable.

(g) Heat Loss and Solar Radiation

It is appar ent from the results that heat los s and solar radiation caused melting on many of the roofs. This was more noticeable for the dwelling houses, but after only one winter's observations it is difficult to make any definite statements about the effect of heat loss. This factor should be reviewed after several winters' observations are available. At that time it will be necessary to review how the temperature influences the cohesion and the friction coefficient between the roof surfac es and the snow cover.

Because the 1965 -66 snow accumulations were so low the effects of heat loss could not be fully observed. A discussion of this aspect of the problem is contained in the conclusion.

(h) Relationship Between Drift Loads and Area Covered by the Drifts In earlier snow load reports the average and the maximum load have been discussed. In the structural analysis of a building it is important to know where the maximum loads occur and how large an area the heavy loads are covering. For each load level, between the maximum drift and the average load, there will be a corresponding area that will be covered by this particular load. This is important to know from an engineering point of view.

In order to approach this problem properly, the attempt was made to calculate the average load over the worst 80, 60, 40, 20 and 5 per cent of the area in relation to the average roof load. This is shown in Figure 23 where, for any percentage of roof area considered, one may read the maximum local load to average load ratio.

The relationship was determined from snow depths and not from loads. In the pr esent case only the lar ger roofs wer e studied, that is, roofs exceeding approximately 1500 sq ft. The author believes that the relationship of snow load as a function of area shown in Figure 23, need only be examined for windy areas and for exposed roofs as sheltered roofs would usually experience approximately uniform loads.

VI I CONCLUSION AND SUGGESTIONS FOR FURTHER STUDIES It has already been mentioned that the information collected and presented in this report is based on one winter only. Further-more, the winter was somewhat unusual, and the conclusions drawn from this report must therefore be rather limited. As stated at the beginning of the report this survey must be considered

essentially as a pilot survey.

The main conclusion of the present survey is a confirmation of the earlier indications that the snow loads on sloped roofs are generally quite low (6). This is mainly the result of the fact that, even on fairly moderate slopes, snow slides are released quite easily. The mechanism of snow slides is such that the heavier the load, the greater the chance of the 32° isotherm moving up from the roof insulation into the surface of contact between roof and snow and thus releasing the snow in the form of a slide.

12

It should be remember ed that on completely unheated buildings, deep snow layers can be retained even on very steep slopes and on very slippery roofing (as shown, for example, in the photograph of Figure 24). Consequently, the familiar question arises whether heat loss should be considered as a valid reason for r educing the snow loads. In the past, this question has been answered in the negative. It is evident, however, that on sloped roofs the effect of heat loss is far more significant than on flat roofs. Furthermore, it s e ern s that in many cases the probability of a building remaining unheated for a whole winter is so low that in areas where the maximum load is produced by an accumulation of snow load over a long period, the heat los s might well be taken into account. In fact, the present slope reduction already r epr es ents in part a tacit and rather vague recognition of the beneficial effect of heat los s ,

With regard to the application of the results of this pilot survey for building code purposes, it is too early to state at the rn orn erit if and in what form an improvement of the 11K"

factor used as a slope reduction can be made. There is, however, one addition to the National Building Code that could be made now. In view of the fact that on SOme roofs, special devices are sometimes used to prevent the snow from sliding, a statement should be included in the Code to the effect that if snow sliding is prevented by special devices on a sloped roof this should be accounted for in its design. Further observations on sloped roofs are desirable to establish the loads during winter s with heavy snow loads and in differ ent climate areas of Canada.

The regular snow load survey of the Division of Building Research has shown that there are considerable differences between the average loads and the maximum loads on roofs, depending on the shelter of the buildings and a number of other factors. It is, however, difficult to find sufficient buildings within a reasonable distance from the National Research Council in Ottawa to carry out the snow studies on sloped roofs only in the Ottawa area. Furthermore, the climate is not reliable enough during the winter as far as snow accumulation is concerned. The snow load measure-ments on sloped roofs, therefore, should be extended to other areas where one could rely more upon snow accumulations on the roofs over a longer winter period.

The proposed further studies should be carried out

with the same measurements to be taken as during this winter's survey. The snow-load measurements on the roofs should, however, be taken only when the loads ar e expected to be at their highest during the winter. The observations should be

carried out with depth measurements to such detail that lines of equal snow depths as shown in Figures 12 and 14 can be drawn. The observations should be plotted on the site, using a prepared sketch of the roofs in a scale approximately 1:100, possibly 1 :200 for the largest roofs. In order to make it faster to plot the

obs ervations and interpolate for equal depth lines to be drawn on the site, the sketches should be made up with grid lines. The calculations of average and maximum loads should be carried out soon after the observations are taken.

It is suggested that the additional observations on sloped roofs should be made a part of the Building Structures Section's regular survey of snow loads on roofs.

ACKNOWLEDGEMENTS

The author wishes to expr ess his gratitude, especially to Dr. R. F. Legget, Director of the Division of Building Research for the permission to spend a period of study as a guest worker at the National Research Council. The

author's appreciation must also be recorded for the facilities provided by the Division for his work on snow load on roofs.

Mr. W. R. Schriever, Head, Building Structur es Section gave invaluable assistance in the preparation of this report. Discussions with Mr. D. A. Lutes were especially useful, To these and other members of the Section and the Division too numerous to name, warm thanks are due for help,

- 14

-REFERENCES

1. National Building Code of C:mada, 1965. Issued by the Associate Committee on the National Building Code, National Research Council, Ottawa, Canada.

(NRC 8305)

2. Roch, Andre. Mechanism of Avalanche Release.

Snow, Ice and Permafrost Research Establishment, Translation ＵＲｾ Wilmette, Il.Iinoi s , April 1956.

3. Ram s Ii , Gunnar. Sn)6 og Sn¢'skred (Snow and Snow Avalanches). Landbruksdepartemente1 s s m a s kr ift e r No. 96,

osi», 1951.

4. Seligman, G. Snow Structure and Ski Fields, Belgium, 1962.

(This book includes many other references.)

5. Bygg. Vol.

1.

AB Bygmas'.arens Forlag, Stockholm, 1959.

6. Snow Loads on Roofs. (Reports of DBR snow load survey is sued annually as Internal Reports of the Division. )

Report 106 - Sept. 1956; 134 - 1956-57; 163 - 1957-58;

184 - 1958-59; 204 - 1959-60; 228 - 1960-61;

260 - 1961 -62; 279 - 1962 -63; 324 - 1963 -64.

Roof

Type of Size

Slope

Heigh t

Roofing

No.

Roof

( sq

(degree) to the

_Material

General Description

ft

)

Eave

(ft

)

1

Gable

1310

33

10

Metal

Machine store, exposed to wind, orient.

a ppr-ox , N-S,

step

Ｒｾ

ft

beloH adjacent

roof No.2 •

.

2

G8ble

830

33

ｬＲｾ

Metal

Machine store with exposure and orient.

equal to roof No. 1.

3

Gable

1490

40

16

Wood

Hay barn, exposed to wind,

orient.

Sh Lng Le

approx. E-\'J, adjacent to rna n s ar-d roof

at righ tangle.

4

a

Gable

960

43

14

Wood

Attic space above animal quarters, only

Shin9:1e

western part of a gable roof, exposed to

wind, orient. approx. N-S.

4

b

Gable

960

43

14-

Metal

Attic space above animal quarters,

eastern part of roof No.4, step 1 ft

below adjacent roof No.5.

Sa

_Gable

₁₀₁₀

₄₁

I

10

Wood

Conditions as for adjacent roof No. 4 a•

':), i :-:".le

-b

_Gable

₈₄₀

₄₁

₁₂

_Metal

_{Conditions as for adjacent roof No. 4 b•}

ｾ

6

Hansard

2310

30/S7

23

Metal

Hay barn,

sheltered by trees,

orient.

approx. N-S.

7

Mansard

14060

23/47

10

Metal

Machine store and uninsulated working

room for gardening, exposed to wind,

orient. appro.x.

E-\\; •

TABLE I

(Continued) Roof Type of Size Slope Height Roofing

No. Roof ( sq (degree) _{to the Material} General Description

ft) Eave

(ft )

8 Gable 8750 25

12

Metal Attic above animal quarters, sheltered by

trees. orient. approx. N-S.

5

dormers.

9 Gable 660 20 8 Metal Western part of ｾ｡｢ｬ･ roof, conditions

as for roof No. on the same ｢ｵｩｬ､ｩｮｾＮ

10 Gable 1710 15 10 Metal Western part of ｾ｡｢ｬ･ roof, conditions

as for roof No. on the same building.

11 Gable 1710 17 11 Metal Eastern part of gable roof, conditions

as for roof No. on the same building.

12 Lean-to 920 12 ₄ Metal Roof above manure bin, extension of

roof No. 8. eastern part.

13 Mansard 2620 35/58 16 Metal Hay barn, exposed to vrind , orient.approx.

E-｜ｾｊＮ

14 Fla t 5160 0 11 Asphalt Animal quarters,adjacent to No. 13,

wi th exposed to wind, orient. approx. N-S. gravel

15 Flat 3530 0 10 Asph.wi th Animal quarters, adjacent to No. 13,

.'!:ravel exposed to wind. orient. approx. ｾＭｓ •

16 Gable 2150 13 12 Metal Machine store, exposed to wind, orient.

approx. N-S.

17 Gable 3170 19 12 Metal Cattle shed with deep litter, exposed to

Wind. orient. aoprox. E-｜ｾＡ •

18 Gable 4490 19 20 Metal Roof above fodder storage adjacent to

No. 17, exposed to wind with some shelter-ing trees is north of the roofs, orient.

•

approx • N-S.

Roof

Type of

Size

Slope

Heigh t

Roofing

No.

Roof

(sq

(degree)

to the

Material

General Description

ft

)

Eave

(ft

)

19

Gable

1720

34

ＱＲｾ

Asphalt

Store,

exposed area,

some shel tering

Shingle

effects from other buildings, orient.

a

ppr-ox , E-W.

20

Gable

2800

42

15

Asphalt

Store, near No.

19,

exposed to wind,

Shingle

orient. approx. N-S.

21

8

Gable

920

30

Ｘｾ

Betal

Northern slope of a

two-sloped

storage

building, exposed to wind, orient.

approx.

E-W.

21

b

Gable

460

36

12

Metal

Southern slope of the saMe building

No.

21

8 •

same conditions.

22

a

Gable

1600

23

10

Asphalt

Northern slope of a

two-sloped

storage

Shingle

building,

exposed to wind,

orient.

approx.

E-W.

22

b

Gable

580

32

13

Metal

Southern slope of the same building,

No.

22

a • same conditions.

23

Gable

2100

28

11

Asphal t

Attic above heated

storage area exposed to

Shingle

wind. orient. approx. B-W.

24-

Gable

1570

30

10

Asphalt

Attic above heated

stor2ge area eXfosed to

Shingle

wind.

orient. approx. E-W.

25

Gable

4130

22

9

Asphalt

Attic above animal qua r- ters, exposed to

Shinf;le

wiDd.

orient. approx.

E-\"r.

26

a

Gable

2160

22

11

Asphalt

Roof above anima

1

quar-ter-s, ad jacen

t

to

Shingle

NO!O.

25

and

27,

exposed to wind,

orient.

....

ｾａｒｔｾ I (Continued)

I

Roof

I

Type of

I

Size Slope IHeight Roofing No. Roof (sq (degree) Ito th e _Material

ft ) !Eave (ft )

ＲＶｾ

Lean-to I

I

I

I

2030

_I

9

I

Ｘｾ I Asphal t

!

I

I I

I

Shingle

I

I

I

Asph a I t 27

I

Gable

1

24 0 0 I 40

I

I 15 I Shingle

.j

I 2S a

I

I Gable

I

990

I

35 , 14 Met9.1 General Description

Roof adjacenteto ｎｾＮ 25 and 27, conditions.

as for No. 26 • ,

Hay store, adjacent to ｎｯｾ 26 a and

26

b

exposed to wind. orient. approx.

E-W •

2S b

i

Gable

I

I

1660 30

I

₁₂

I

I

Metal

I

(old) I 29

I

Gable 1

549 0

i

IS

T

;

Gable

I

440

I

30

, I 30 2Sc

l

<1 : Gal

I - :

27 , j 1e

I

1980

I

-+

i

i

2100

I

30/60 !

!

I

I 10 j Ｘｾ

,

I

lS

,

r

I Me tal (new) ｽｾ･ ta1

!

Metal

,

T

I

Metal I

I

Netsl - - " Metal

I

Conditions as for roof No. 2Sb•

Attic over animal quarters, partly sheltered, orient.approx.

E-W.

Roof over store room, partly sheltered, orient. approx.

N-S.

Roof over manure store room, exposed to wind, orient. approx.

E-W.

\

I

I

I

!

Hay barn under the same conditicns as

_ _ _ _ .J__ＭＮｅＮＹｾＮ

-

1 30/60

I

16

I

Metal 24

I

i i

Gt.l

j 34 I

I

2180 I I rJIens[:rd I ｾＭ_i 35

I

Gcble

I

₄₀₄₀

I

Roof

Type

Size

Slope

Height

Roofing

No.

of

(sq

(degree)

to the

General Description

Roof

ft

)

EBve

Material

(ft

)

36

Gable

4040

24

ＱＰｾ

Metal

Attic above animal quarters, exposed

to wind. orient. appr-ox , N-S.

37

Gable

3100

33

12

Metal

Attic above animal quarters, heavy heat

loss through the roof, exposed to wind,

orient. approx. N-S.

38

Gable

1650

35

11

Asphalt

Attic above machine shop, partly

Shingle

sheltered, orient. approx. E-W.

39

Mansard

1890

29/52

18

Mets.l

Hey barn, sheltered by trees,

orientation approx. N-S.

40

Mansard

3500

37/55

ＱＷｾ

Netal

Hay barn above animal quartern, partly

sheltered by trees, orient. approx. N-S.

40

c

Lean- to

670

₃₄

14

Metal

Cold attic adjacent to No.

40,

partly

sheltered to wind, orient. approx. N-S.

41

a

Gable

700

31

Ｖｾ

Metal

Northern part of roof

No.

41,

shel tered

by h

i

zh trees. orient. appr-ox , E-W.

41

b

Gable

700

31

7

Wood

Southern

ｰ｡ｾｴ

of roof

41,

conditions

Shingle

as for roof

41

a •

42

Gable

1460

45

18

Wood

Hey barn, adjacent to roof

43,

partly

Shingle

sheltered. orient. approx. E-W.

43

Geble

2090

₄₅

16

Wood

Hay barn, adjacent to roof

42,

orient.

Shingle

approx. N-S.

44

a

Gable

2030

40

ＲＰｾ

Metal

Hay barn, partly sheltered by trees,

orient. approx. N-S.

44

b

Lean- to

550

26

9

fvle ta

1

Attic above animal room placed at the

TABLE I (Continued) I

Roo Type of Size Slope Height Roofing

No.

Roof (sq (degree) to the _Material General Description

ft ) Ed.ve

(ft )

45 a Gable 1140 38 Ｘｾ Metal Machine store, she1 tered, orient.

approx. ｅＭｾＢｬＮ

45b Lean-to 320 15 6 Metal Lean-to at the east gable of the

machine store.

b.6 Gable 3630 18 11 Metal Cold stor2ge aree for boat ｾｯ､･ｬｳ at ｾ＿ｇＬ

partly sheltered by trees, orient.

s ppr-ox , N-S.

47 Gable 1360 18 11 Metal Adjacent building between

No.

46 and

No. ｴｾ

7.

Same conditions as for No. 46.

48 Gable 3630 18 _ｬｏｾ 11eta 1 Adjacent building to No. 47, same

conditions.

49 Gable 3630 18 _ＱＰｾ Metal Cold storage are8 at

:'nc.

prrtly sheltered

by trees-. orient. approx. N-S.

50 Gable 1360 18 ＱＰｾ Metal Adjacent building to No. 49 and No. 51.

Same conditions as for No. 49.

51 Gable 3630 18 _ＱＰｾ Metal Adjacent building to No. 50, same

conditions.

52 Hip Roof 1880 23 10 Asphal t Attic above dwelling house, sheltered

Shingle by trees.

53 Flat 620 0 10 Asphalt Roof above garage, adjacent to No. 52,

Sheeting sheltered by trees.

54 F'l a t 700 0 20 Asphalt Attic above dwelling house, sh e I tered,

Sheeting by trees.

55 Gable 1210 26 9 Asphalt Attic above dwelling house adjacent to

re ,

Roof (sq (degree) to the _Material General Description

ft ) EAve

(ft )

S6 Flat _4°0 0 9 Asphalt Roof above ｧＸｲ｡ｧｾ adjacent to No. 55.

Sheeting Sheltered by trees.

57 Get::Ie 1800 ₁₄ 16 Asphalt Attic above dwelling house, sheltered Sheeting from North. East and West.

58

,

Flet 620 0 11 Asphalt Garage roof adjacent to No. 57 and with Sheeting the same conditions.

59 Gable 2790 22 9k. Asphalt Attic above dwelling house, shel tered Sheetinp: by hiM trees.

60 Gable 390

I

22 91:; Asphalt Lower part of garage roof, adjacent Sheetinl': to No. 59 sheltered by trees.

61 Flat 620 0 10 As pha I t Garage roof ｡､ｾ｡｣･ｮｴ to No. 62, sheltered Sheetinp: by trees.

62a Gable 1200 ₃₆ ｬｬｾ Asphal t Roof above dwelling house, sheltered by Shee tinp; trees.

62 b I Gable 200 '-4 19 As ph a1t Raised part of the roof above the dwelling

Shee tinr; house No , 62 sheltered by trees.

63 Fls t 4hO 0 _Ｘｾ As ph a It. Carport roof adjacent ｴｾ building ｾｯＮ 64,

.,i

th gravel sheltered by trees.

64a Gable 6,0 11 17 Graveled 'The one side of R gable, roof above Asphalt dwelling house sheltered by trees.

64b Gahle J

ooo

13 8 Graveled The other side of gable roof No. 64,

ａｳｰｨｾｬｴ conditions as for No. ＶＴｾＮ

65a Gable 530 11 27 Graveled The one side of gable, roof above dwelling As oh a I t house No. 65. sheltered by trees.

65b Gable 10SO 9 ｂｾ Graveled The ｯｴｾ･ｲ side of gable, roof sbove

_I

Asphal t dwelling house No. 65, sheltered by

trees.

66A Gable 740 Ie 17 Graveled The one side Ｐｾ gahle, ｲｯｯｾ above Asphal t dwelling house No. 66, sheltered by

trees.

66b Gs b Le 1050 7 11k GrAveled The other side of gable. Roof abcve;

Asphalt dwelling House No. 66, sheltered b_J

TABLE

II

Maximum Average and Maximum Drift Loads

(Metal Roofing)

Roof

Slope

Code

Observed loads in psf

Roof

Number

(Degrees)

Load

Load to

NBC

Ground

Ave.

Max.

Ground

Remar-k s

1965

Roof

Drift

Load

Ratio

1

33

44

5.0

3

9

0.60

Exposed

2

₃₃

₄₄

5.0

3

9

0.60

Exp.

4

b

43

32

14.0

2

3

0.14

Exp.

5

b

41

35

14.0

2

₄

0.14

Exp.

6

8

30

48

14.0

9

15

0.65

Shel tered

6b

57

16

14.0

10

12

0.71

Shel.

7

8

23

48

8.1

3

10

0,)7

Exp.

7

b

47

28

3.1

0

0

0

Exp.

8

25

48

9.7

9

35

0.93

sne

i ,

9

20

48

9.7

10

12

1.03

SheL

10

15

48

9.7

9

11

0.93

Shel.

11

17

48

9.7

9

14

0.93

Shel.

12

12

48

9.7

10

12

L03

Shel.

13

9

₃₅

₄₂

_13.0

₄

₇

_0.)1

_Exp.

13

b

58

14

13.0

0

0

0

Exp ,

16

13

48

13.0

7

12

0.54

Shel.

17

19

48

13.0

10

15

0.77

Exp.

18

19

48

13.0

7

13

0.54

Exp , .?I8

30

48

5.1

5

10

1.98

Exp ,

2l b

36

41

10.0

7

14

0.70

Exp ,

22b

32

46

10.0

8

17

0.80

Exp ,

1965 Roof Drift Load RatLo 28s 35 42 5.0 1 3 0.20 Shel. 28b 30 48 5.0 6 8 1.20 SheL 28e 30 48 5.0 0 0 0 SheL 29 18 48 10.0 6 25 0.60 Shel. 30 19 48 5.0 4 12 0.80 She1. 31 27 48 10.0 3 17 0.30 Exp. 32 30 48 10.0 1 10 0.10 Exp, a 48 5.0 1 3 0.20 Exp. 33 30 33b 60 12 5.0 2 5 0.40 Exp. 348 30 40 5.0 1 3 0.20 Exp. 34b 60 12 5.0 2 5 0.40 Exp. 35 24 48 10.0 6 32 0.60 Exp. 36 24 48 10.0 5 26 0.50 Exp. 37 33 44 5.0 1 3 0.20 Exp , 398 29 48 7.9 3 5 0.)8 She1. 39b 52 22 7.9 4 7 0.52 SheL 40 s 37 40 10.0 4 8 0.40 SheL 40b 55 18 10.0 0 5 0 She1. 40c 34 43 10.0 2 5 0.20 ShEIl. 418 31 47 26.0'.1- 13 14 0.50 Shel. 448 40 36 10.0 1 5 0.10 Exp. 45s 38 38 16.0 ₄ 11 0.25 SheL 45b 15 u8 16.0 5 5 0.31 She1. 46 18 48 5.5 4 18 0.73 Exp , 47 18 48 5.5 4 9 0.73 Exp , 48 18 48 10.0 5 18 0.50 Exp. 49 18 48 5.5 4 14 0.73 Exp. 50 18 48 5.5 4 12 0.73 Exp. 51 10 48 5.5 4 16 0.73 Exp. Loca tion Sheltered Exoosed

AverAge Ground Load 11.6 paf 8.3 psr

ｾｾｔｨｩＸ high ground load was found only in this high and extremely

well-sheltered location near Navan snd is probsbly the result of reduced melting.

TAB L E III

MpxIr>'uII' Aver'age and Maximum Drift Lond s (Vl1riOIlS Bituminous Roofinga)

Roof Slope Code Observed loads in psf Root

Number (Degrees) Load Load to

NBC Groul"ld Ave. MBX. Ground ｩＱｾＢｬｧｲｫｳ

1965 Roof Drift Load

Rst I o 14 0 48 13.0 5 7 0.38 Shel tared 15 0 48 13.0 6 11 0.46 Shel. 19 34 43 5.1 3 3 0.59 Exposed 20 42 34 5.1 3 7 0.59 F.xp. 22B 23 48 5.1 3 4 0.59 F..xp. 23 28 413 4.1 1 4 0.24 F..xp. 24 30 48 4.1 1 3 0.::'4 Exp , Ｒｾ 22 48 4.1 3 23 0.73 Exp. 268 ₂₂ 48 4.1 5 9 1.22 F..xp. 26b 9 48 4.1 4 9 0.98 Exp. 27 40 36 10.0 1 17 0.10 F.xp. J8 35 42 10.0 2 4 0.20 She 1. 52 23 48 14.0 7 20 0.50 snei , 5'3 0 4Fl 14.0 5 8 0.J6 Shel. 54 0 48 17 .0 7 8 0.41 She 1. 55 26 48 17 .0 6 8 0.J5 Shel. 56 0 48 17 .0 9 17 0.53 Shel. 57 14 4 11 12.0 6 8 0.50 Shel. 58 0 48 12.() 6 6 0.50 Shel. 59 22 48 lS.0 7 13 0.44 Shel. 60 22 48 16.0 10 24 0.63 Shel. 61 0 48 16.0 12 14 0.75 Shel. 628 36 41 16.0 6 13 0.38 Shel. 62b 14 48 16.0 5 9 0·31 snei , 63 0 48 18.0 13 14 0.7<' Shel. 648 11 4El 18.0 10 11 0.56 Shel. 64b 13 48 18.0 12 ₁₄ 0.h7 Shel. 658 11 4El 14.0 6 9 0.43 Shel. 65b 9 40 14.0 3 12 0.57 She!. 668 10 48 14.l 9 11 0.64 snei . 66b 70

4B

14.0 15 18 1.07 Shel. Locs tion

Average Ground Load

Sheltered 14.25 psI'

Exposed 6.40 pst

Maximum Average and Maximum Drift Loads

(Wood Shingles)

Roof

Slope

Code

Observed loads in psf

Roof

Number (Degrees)

Loed

Load to

RemArks

NBC

Ground

Ave.

Max.

Ground

1965

Roof

Drift

Load

Ratio

3

40

36

14·0

2

12

0.14

Expos'ed

4

8

43

32

4.7

5

5

1.06

Exp.

5

a

41

35

4.7

5

7

1.06

Exp.

41

b

31

47

26.0

*

14

15

0.54

Sheltered

42

45

30

8.7

3

22

0.34

Exp.

43

45

30

8.7

6

22

0.69

Exp ,

Loce tion

Average Ground load

Sheltered

26.1

paf

Exposed

1)035

psf

*

This high ground load was found only in this high and

extremely well-sheltered location near Navan and is

probably the result

of reduced me

Lti

ng ,

ｾ

ｾ

ｾ

0-w

.- IS

Vl

:r:

HJ

u

«

w

e::::

o

u.. Vl u..

S

o

o

e::::

u..

o

o

0

z

o

10

20

30

40

Slope, degrees

SO

60

LEGEN D

D

Metal roofing (SI roofs)

ｾ

Asphalt roofing (31 roofs)

ｾ

Wood roofing (6 roofs)

FIGURE

1

DISTRIBUTION OF THE ROOFS ACCORDING TO THEIR SLOPES

Austria, Canada"'u

Great Britain OO u

Germany, Rumania,

U.S.A.'" and U.S.S.R,u

Norway, Sweden

Connecticut (USA)

10

20

30

40

50

60

70

80

Slope, degrees

0

-

u

0.8

ro u. c

0.6

0

-

u

_0.4

:::J "C Q) a:::

0.2

Q) Cl. 0 Vl

0

0:<

The American Standards Association's "Minimum

Design Loads" A-58, uses a minimum snow load of

20 psf horizontal roof projection (other

local

codes in force in the U. S. A. might have special

regulations. )

**

U. S. S. R. has special shape factors in addition

to the regulations for slope.

"'**

Canada has detailed, special shape factors In

addition to the slope reduction factor.

0 0 0 ) 0

Great Britain use an over-all minimum ground

load of 30 ps

t.

FI GURE

2

SLOPE REDUCTION FACTORS (K) IN CODES WHERE THEY

HAVE TO BE USED TOGETHER WITH A CLOSER DEFINED

(I

Local authorities are responsible for

stipulating the ground load in the area

co n cern ed.

Possibilities of heavy unbalanced

accumulations should be especially

cared for in the design.

(1(1

Snow loads on roofs are taken into

account only in the northern part

of the country, the loads depending

on elevation above sea level.

I n locations where heavy loads

usually occur this should be

accounted for in design.

/1//3780-1

FIGURE

3

STEPWISE SLOPE REDUCTION FACTORS

FROM THE CODES IN JAPAN AND ISRAEL

ｾＭＭＭＭＭＭＭｉ

ta

Iyo;. '"

1 0 4 t - I 1 - - -

Czec ho slova ki a'"

10

20

30

40

50

60

70

80

Slope, degrees

0

-::;0.8

ro u..

c

O.

6

0

-::;0.4

:::J "'C

ｾ

O.

2

Q,) a. 0 V')

0

*

Ground load is dependent on the location and the

elevation above sea level.

For slopes exceeding 40° the load may be taken as

50 kg/sq m (10 ps

t)

for any height above sea level.

.)*

Ground loads are dependent on elevation above sea

level.

For slopes less than 10° the design snow

load has to be increased 25% if the possibilities of

draining water off the roof are bad.

FIGURE 4

SLOPE REDUCTION FACTORS (K) IN THE CZECHOSLOVAKIAN

...---Switzerland ....

1 - 4 - - - , - - - -

Po r tug aI

* ....

ｾｩＭＭＭＭ

Cos a.

10

20

30

40

50

60

70

80

Slope, degrees

ｾ

1.0

-...

e

...

0.8

u ro l.1-C

0.6

e

...

u

_0.4

::J "0 CL>

0.2

a:::::

CL> c..

e

(/)

0

* The ground load is dependent on the

elevation above sea level.

The slope

reduction should only be taken into

account when the snow sliding off

the roof is not restricted.

** The ground load is dependent on

I

0

cat ion and eI evat ion abo v e sea I e

ve

I.

The slope reduction should only be

taken into account when the snow

sliding off the roof is not restricted.

The design snow load should not be

taken less than 30 kg/sq m (6psf).

/'IR 1780-5

FIGURE

5

SLOPE REDUCTION FACTORS (K) IN THE CODES

of the snow cover considered

ｾＭｗ･｡ｫ

layer in the snow cover

Releasing force

Shear force,

Q =

G sin

v

Stabilizing forces

1.

Cohesion, Co

}

Shear strength

2

_.

Fr . ctt

_I

_Ion,

F = N

_.f'

= G

_{cos}" . g}

"It

t

A.

=

C0

h esion + f ric t ion

If'

3.

_{Tensile force at the upper end surface, Pt}

4.

Compression force at the lower end surface, Pc

5.

Shear (or diagonal) forces at the lateral surfaces.

FIGURE

6

FORCES ACTING ON AN ELEMENT OF A SLOPED SNOW COVER

T-

Shear stress -

Q/A

0--

compression

stress

a.

S now mass

considered

ｯｾﾢ

(Contact

are a - A)

C

ＧＭＭ｟ＮＮＭｌ｟ｾＫＭＭＭＧＭＭＧＭＧ _ _+--Z.""

b.

Cohesion, C

Static friction

When the shear stress

(T)

exceeds the shear resistance

(Til·

C

+

(J'

tg

¢)

a rupture could take place.

FIGURE

7

a.

SLOPED SNOW COVER WITH ACTING FORCES

a.

C

b.

_{Kinetic friction}

For stresses, as shown in the diagram, the equilibrium is

stable unless external forces, heavy enough to cause break,

are added.

II"

1740 - 8

FI GURE

8

a.

SLOPED SNOW COVER WITH ACTING FORCES

\_,

Figure 9: Surface Slides on Steep New Snow Cover (Roof 66; Slope 57°)

Figure 10: Old Snow Cover Showing High Tensile Strength (Roof 41aj Slope 31°)

\

Figure 11: Shear Break Between Snow Cover and Roof Surface

mansard

roof

_.6 6

.5

5

Slope 0°

FIGURE

12

EXAMPLE OF LINES OF EQUAL SNOW DEPTH BASED

ON DEPTH MEASUREMENTS (Roof 15; Slope 0°)

Pipe made of thin metal

sheet welded or soldered

to a solid pipe with

cutting edge.

I nside area approximately

100 sq cm.

Total length

approximately 30 in.

FIGURE

13

NORWEGIAN TYPE SNOW LOAD SAMPLER

i

Area •

50'

x

80' • 4000

s

q

It.

"

'"

'"

" '" ;>

"

'"

'"

./ ./

Av.

7"

Max 14"

6"

FIGURE

14

Av.

Ii"

Max

3

11

Location:

Near Orleans,

Ontario

Roof no .:

30

Slope:

19°

Observ. date:

25.1.66

Wind direction: East

Ground depth:

7

11

Ground load:

5

pst

Roof

Av. depth:

41

11

Av. load:

4

psf

Max depth:

14"

Max load:

12 p

st

Roof snow density

Depth

12

11

and

91

11

:r ..

0.160 gmICC}0.871

0.175

gm/cc

_ps

fl'

_In.

Ground snow density

'7"

O.

14

2 9

m/ cc }

O. 7

f

4/,.

ps

In.

EXAMPLE OF ACTUAL RECORD SHEET WITH CALCULATED AVERAGE AND

80

-I

30

40

50

Slope, degrees

I I I I I I I I I I I

-

ｾ

Line representing

K

\

Line approx-

-"

factor applied to

imating desig'l

-, ground load

'\IOad for

","observed at each

various

-'" f7rm

. , slopes.

"

\wttawa

-ｾｾ

'7

=

\

ｾＬ

\

-,

\

I I

ｾ

I

60

70

I I I

•

I

20

I I

90

I

-80

I

-70

I-60

ｾ

50

ｾ

40

I

-30

ｾ

20

ｾ

10

ｾ

o

I I

10

I I

100

ｾ

,_.

><

.

o o

--

e:: I1l u ｾ

"'

o ｾＮＮＮＮｊ

"'

ｏｾ ....Je:: _ ::J o 0 o L.. ｾ C)

FIGURE

15

ROOF TO GROUND LOAD RATIO VS SLOPE FOR SHELTERED METAL ROOFS

-80

-I

70

60

50

40

I I

I

30

I

20

I

I

10

I

I

10

I--o

ｾＭＺＭＭＭＭＮＭＭＧ

Line approXimatin g - ;

90

I--

Line representing

ｾ

d

'

I

d f

K

f

_ac

t

_or

....

_{" .}

es

i

qn

oa

_I

or

_\

I' d t

"

various

S

opes.

80

I--

app led

,0

d · " (Ottawa area)

\

groun

oa

"

70

I--

observed at

•

"

,

Ｚｾ

:

each farm.

" ,

\

"

".

\

\

:\.

,

r. \

I

Ｌｾ

100

><

...

<Ll CL

-

e: <Ll u ｾ o o

-"1;J n:J "0 ｾ

40

I--n:J 0 " 0

30

I--...Ie: _:::l o 0

20

I--o ... a::: <.:)

Slope, degrees

FIGURE

16

ROOF TO GROUND LOAD RATIO VS SLOPE FOR EXPOSED METAL ROOFS

110

I • I I I I I I I I I I I T I

T

100

ｾＭＭＭＭＭＭｾ

Line approximating

\

-... c

90

Line representing

,

design load for

Q)

_-

_{K factor applied to}

_{,various slopes.}

_\

-U

L-

80

I -

ground load

(Ottawa area)---\

-Q)

observed at each

"-

0-70

I

-fa r m.

ｾ

\

-.

\

0

-,

0

60

-

--

_><

-,

\

50

-

-,

\

-"0

-,

m

40

0

-

-,

-"0 _-J m \ 0 "0

₃₀

-

-,

,

\

--J C

--

::J

20

0 0 I -

-0

L-'\

e::: <-=>

ｉＧｾ

10

I-

-0

I

I I I I I I I I I I I

10

20

30

40

50

60

70

80

Slope, degrees

FI GURE

17

ROOF TO GROUND LOAD RATIO VS SLOPE FOR SHELTERED BITUMINOUS

-

.... - - - - . - -

ｾ

Line approx-

\

Line represen-

ｾ

Imating design

ting K factor

, l o a d for various,

applied to

slopes. (Ottawa

\

' a r e a )

...

ground load

observed at

"

,

each farm

" \

,

,

-,

_,

\

'

,

\

"

100

....

ｾ

90

u ｾ

80

QJ

0-70

.

o o

-

60

50

"0

:;

40

"0-..1 ro 0 " 0

30

-..Ie _::J o 0

20

o ｾ e::: ｾ

10

OL...---l.._...L...._.l...----l._...L.._..L.----l._---L..._...I....---L_---L.._....L..-___JL...--1._...l----I

10

20

30

40

50

60

70

80

Slope, degrees

FI GURE

18

ROOF TO GROUND LOAD RATIO VS SLOPE FOR EXPOSED BITUMINOUS

•

•

-

-

-

-

Ｍｾｌｩｮ･

approximating

\

Line representing K

ｾ

design load for

factor applied to ground

" ,"various slopes.

\

loa d

0

bser v ed ate ac h far m. '"

ｾｏ

tt

a

w

a are a) - - - \

ｾ

\

"

"

\

\

"

_"

\

\

"

_"\

\

<,

110

100

....

c

₉₀

QJ U

...

80

QJ C.

70

-0 0

₆₀

...

) (

50

"0 ro

40

0 "0 ...J ro

30

0 "0 ...J C

-

::3

20

0 0 0

...

e::: c..:>

10

0

10

20

30

40

50

60

70

80

Slope, degrees

FIGURE 19

ROOF TO GROUND LOAD RATIO VS SLOPE FOR EXPOSED WOOD ROOFS

Figure 20: Snow Cover Held by Lateral Forces Produced

by Dormer. (Note the Compression Folds. )

\

Figure 21: Snow Load on Well-Sheltered Roof

(Roof 41; Slope 310

Figure 22: Snow Load on Leeward Side of Exposed Roof (Roof 25; Slope 22°)

<">

I

1

I

I

I

I

I

I

1

-

-•

-III

-IIIIII11111111

t

•

_II

_ｾ

ｾ III II/TIl TIllTTl1"T' }

-

_{Average roof load}

-I

I

I

I

1

I

I

I

I

"0 0 . > " 0 4 Vl ro V l O 0.> ....J I.0 . .

-><

0 o ｌ｡ＮＮｉｾＳ ro 0.> 0.> ｉＮＭｾ « r o

l.-e

0.> 0.>

>

>«2

t.::>0.>

.c

ro ... 1 . -0.> 0

ｾｾＱ

0.. "C . -ro::::: o ::J ＮＮＮＮｊｾ

ｾ

ro

0

u

o

Vl

0

....J ro

20

40

60

80

100

Percentage of Roof Area

FIGURE

23

LOCAL LOAD OVER A GIVEN AREA EXPRESSED

AS A MULTIPLE OF THE AVERAGE ROOF LOAD

VS PERCENTAGE OF ROOF AREA.

(Shaded

area

covers 19 of the 21 loads u sed.J

Figure 24: Deep Snow Layers Retained on Steep Aluminum Roofs (Rogers Pass, B. C., 30 January 1965)