Field instructions for observers at "A" stations of snow loads on roofs

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Technical Note (National Research Council of Canada. Division of Building Research), 1957-10

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NOTE

'Jr

EClHIN II CAlL

NOT FOR PUBLICATION

PREPARED BY D. E. Allen CHECKED BY W.R.S. APPROVED BY R.F.L.

PREPARED FOR October, 1957.

SUBJECT Field Instructions for Observers at

"A"

Stations of Snow Loads on Roofs

These instructions are intended for

"A"

Station observers taking part in a survey of snow loads on roofs in

selected locations across Canada. The instructions are in

three parts (A) List of equipment and instructions for choosing roofs and applying gauges, (B) Instructions for the field

measurements and (C) Instructions on filling out Form A-2 and when to take measurements.

(A) tIS:COF EQUIPMEllT AND INSTRUCTIONS· FOR CHOOSING ROOFS: Observers will be supplied with the following items:

(1)

Pads containing forms on which to record snow measurements and other pertinent information: One pad shall be· used for

each roof under observation.

Form

A-I,

ttDetails of Roof and Other Relevant

Information. " Two copies are included in each pad: both are to be completed and one returned to the Division of Building Research. The other should be kept by the observer.

Plan of Roof and Surrounding Area , Two copies are

included on the pad: one to be returned to the Division and one to be kept by the observer.

Form A-2, ttObservation of Snow Depth and Density." There are 30 copies of this fOrni on the pad. One of these should be completed each week and sent to the Division.

(2) Snow Density Kit. This kit includes the following: Snow sample cutter

Back plate (for snow sample cutter) Container (graduated in ｾＮ｣ＮＩ

Buckets Yardstick Knife

2

-(3)

Snow Depth Gauges for Roofs

Depth gauges for shingled pitched roofs Depth gauges for flat roofs

(The observer will be sent whichever depth gauges he needs.)

Choosing the Roof

The study will be facilitated by the comparison of similar roofs. The survey will therefore, be based on two common types of roofs (flat ald gable) in selected locations. The observer should choose, if possible, roofs of the following types:

(i) Gable roof (with shingles): heated bungalow with a 5 in 12 pitch, with unheated and ventilated attic spaoe and with insulation in the attic floor. If a roof with a

5

in 12 pitch is not available a roof with a 6 in 12 or 4 in ｬｾ can be used (Figure I). A section from a long building might be used.

(ii) Flat roof (with parapet): heated. bUilding With well insulated roof areas up to 10,000 sq. ft. (Figure 2). The wing of a larger roof could be used.

(ill) Flat roof (without parapet): As in (ii).

The observer should try to choose roofs on new bUildings which can be used over a period of years. The specific number and choice depend on the location.

Exposure

In most locations it is desirable to have both a

shielded and an exposed roof. A shielded roof is one surrounded by higher objects which protect the roof from high winds. .Ail

exposed roof is not obstructed from the wind by high objects. (It may be surrounded by buildings of the same height.)

Gauge Arrangement

Snow gauges are supplied to the observers for measuring snow depth on the roof. There is one type of gauge for shingled roofs and one type for flat roofs. The gauges are

3

feet long and marked in feet and inches.

Locations for snow gauges should be chosen according to the standard arrangements shown in Figures 1 and 2. The observer should show the gauge locations which he proposes on the plan of the roof in Form A-I and number them.

One copy of Form A-I should be mailed to the Division of Building Research. The appropriate snow gauges will then be sent to the observer.

Attaching Snow Gauge

The gauge for shingled, roofs is clipped to ｴｾ･

shingles: the bottom triangular plate 1s placed under a layer of shingles with the stud between two shingle tabs (Figure 3). The upper plate is put over "the, shingles and the wing nut is screwed on to clamp the shingles between the plates. The gauge arm is fitted over the pin.

The gauge for the flat roof will rest directly on the roof.

(B) INSTRUCTIONS FOR FIELD MEASUREMENTS

Measurements on the Roof

The depth of snow on a pitched roof will. normally be apParent from the ground since it is necessary orily to get a depth to the nearest inch. On flat roofs it may be diffi-cult to take readings from the ground because of obstruction of view, and the observer may have to get on the roof or use a ladder to raise his eye level above the roof.

Measurements on the Ground

A yardstick is supplied for measurements of snow depth on the ground.

. The observer should take a number of readings of snow depth on the ground area around the house at the same time as the snow gauge readings. Measurements could be taken, for instance, near each corner of the house in undisturbed snow, (i.e. where no drifting has occurred).

Density Measurements

The density (or unit weight) of, the snow is required to allow the calculation of the actual load of the snow cover per unit area. The average density of the snow cover is

measured by taking a number of samples of snow of known volume from vertical cut in the snow and melting this snow in order to obtain, by volume measurement, the water equivalent.

Location of Density Measurements

A measurement should be tal en on each large plane surface of the roof. On a simple gable roof, for instance, the density should be taken on each side and observed in the

4

-deeper snow of the roof surface, not too close to the edge of the roof. These locations should be shovm on Form A-2. Measurements should be taken at least 5 feet away from the snow gauges to prevent disturbance of the snow around the gauges.

At the same time, on the ground near the bUilding, one or more density measurements should be made in a snow section with an average depth of snow.

Each observer is supplied with a set of instruments to measure density:

Snow Sample Cutter.- The cutter shown in Fig. 4 has a thin saw-tooth cutting edge for use in both hard and soft snow. The removable back plate B-2 has hooks which engage pins D on the cylinder. The internal volume of each outter is 250 0.0.

between-the planes X-X and Y-Y.

The cutter is always used horizontally and therefore a vertioal out should first be made through the snow with a flat-end shovel. The outter is pushed into the snow while the cylinder is rotated around its axis to avoid oompressing the snow sample. A knife is, used to trim the sample level with the forward and rear edges of the cylinder. The baok cover may be used to prevent very slippery samples from sliding out of the cutter after the sample has been isolated. Bucket and Graduated Container.- Each bucket

(containing the snow samples from one location

only) shall be covered, labelled, and taken indoors to melt the snow. The volume of the water shall be measured in the graduated container as soon as

possible after the melting is complete to prevent excess evaporation. As one cubic centimeter of water weighs one gram the number of c.c. read on the

graduated cylinder with the number of samples obtained will yield the average density according to Table I.

It is important therefore, to record how many samples are in the bucket.

Density measurements should be made at approximately 4-inch vertical spacing. If the depth is small more than one reading in each layer should be made to give four or more samples to the bucket. There must be the same number of readings in each layer of the snow cover to give a true average. If the snow is extremely hard in some layers

JUdgment nmst be used in taking the measurements, keeping in mind that over-all density is estimated by finding the average density of the cross-sectional layers.

, v1tlen cutting samples from a vertioal section of snow, samples should be out first from the top layer, then the

next layer and so on, to the roof or the ground. This

oould be done on a pitohed roof near, but not too olose to, the edge of the roof, while the observer stands on a ladder, and on an aocessible flat roof or the .ground by outting a small trenoh.

Samples of soft snow should be out oarefUlly to avoid oompressing the snow. Snow samples that do not oompletely fill the internal volume of the outter should be disoarded. Snow-storm Data

The observer should obtain snow-storm data requested on Form A-2 from the looal meteorologioal office whenever possible.

When desoribing new snow these desoriptive terms should be used where they apply: wet, dry. light, heavy, drifting, non-drifting, rain, hail, ioe-pellets (sleet), snow, granular snow, (e.g., heavy wet snow later mixed with ioe pellets and rain).

If meteorologioal data oannot be obtained, the

observer, if possible, should describe wind speed and direo-tion as indioated in Table

3.

This Table oontains the

Beaufort soale to help assess wind speed. Desoriptive terms suoh as "gusty" oan be used to advantage.

(C) FILLING OUT FORM A-2 AND VIHEN _{TO TAKE }\'1EASURErJLENTS}

The snow depth data sheets should be filled out soon after eaoh snowfall over 2 inohes and at least onoe aweek

(preferably on a regular (ay every week) as indicated in Form A-2. If there is a complete thaw of snow on the roof dUring the winter, it should be recorded on one of the weekly observation sheets. If there is snow on the ground only, its depth should be recorded. Observation should begin with the first heavy snowfall.

(1) Depth measurements on the ground and roof should be made onoe a week and after eaoh snowfall over 2 inches.

(2) Density measurements should be made along with depth measurements on the roof and ground when there is an average depth of

5

inches or more. Density measurements are needed espeoially during heavy accumulations of snow when the

l

6

-(3)

Snow-storm data should be included for each storm yielding a snowfall of over 2 inches.

Vfuere much drifting occurs a general description of the snow on the roof by means of sectional diagrams should accompany depth measurements. Photographs accompanying observations would be most useful.

A general word description of snow on other roofs in t he locality would also be useful especially when the snow depths are greater than usual.

The purpose of this investigation is to improve and refine present snow load requirements for buildings in Oanada. Ultimately, it is hoped that the records obtained will produce definite information that will enable

recom-mendations to be "made to achieve a further- economy in building. This project must, of necessity, be carried on for many years before any reoommendations can be made. Actual measurement of snmv on roofs is the only way in which to aohieve a basis for determining snow load requirements. It must be stressed, then, that the value of the project is directly related to the care taken by each observer in taking the measurements.

A Final Personal Note

In order to keep the foregoing instructions as brief and concise as possible, they have had to be drafted in a necessarily impersonal manner. Will all those who use this note and follow these instructions please be assured that the members of the Division of Building Research who are concerned with the snow load project realize very well

(from personal experience!) the work and inconvenience which the taking of the records herein described calls for. All observers may rest assured that their efforts will be greatly appreciated and that the combined results should lead to a significant advance in knowledge of snow loads and therefore to the improvement of bUilding in Canada.

TABLE I

Weight Read on Scale in Grams Number of Samples Density 4 5 6 7 8 9 10 11

12

gm/cc 80 100 120 140 160 180 200 220 ₂₄₀ 0.08 90 113 135 158 180 203 225 248 270 0.09 100 125 150 175 200

_f

₂₅ 250 275 300 0.10 110 138 165 193 220 248 275 303 330 0.11 120 150 180 210 240 270 300 330 360 0.12 130 162 195 228 260 293 325 358 390 0.13 140 175 210 245 280 315 350 385 420 0.14 150 188 225 263 300 348 375 413 450 0.15 160 200 240 280 320 360 400 440 480 0.16 170 213 255 298 340 383 425 468 510 0.17 180 225 270 315 360 405 450 495 540 0.18 190 238 285 333 380 428 475 523 570 0.19 200 250 300 350 400 450 500 550 600 0.20 210 262 315 368 420 473 525 578 630 0.21 220 275 330 385 440 495 550 605 660 0.22 230 288 345 403 460 518 575 633 690 0.23

e

240250 300313 360375 438420 480500 540563 625600 688660 750720 0.240.25 260 325 390 455 520 585 650 715 780 0.26 270 338 405 473 540 608 . 675 743 810 0.27 280 350 420 490 560 630 700 770 840 0.28 290 363 435 508 580 653 725 798 870 0.29 300 375 450 525 600 675 750 825 900 0.30 310 388 465 543 620 698 ｾ 775 853 930 0.31 320 400 480 560 640 720 800 880 960 0.32 330 413 495 578 660 743 825 908 990 0.33 340 425 510 595 680 765 850 935 1020 0.34 350 438 525 613 700 808 875 963 1050 0.35 360 450 540 630 720 810 900 990 1080 0.36 370 463 555 648 740 833 925 1018 1110 0.37 380 475 570 665 760 855 950 1045 1140 0.38 390 488 585 683 780 878 975 1073 1170 0.39 400 500 600 700 800 900 ·1000 1100 1200 0.40 410 513 615 718 820 923 1025 1128 0.41 420 525 630 735 840 945 1050 1155 0.42 430 538 645 753 860 968 1075 1183 0.43 440 550 660 770 880 990 1100 0.44 450 588 675 788 900 1013 1125 0.45

ASPHALT SHINGLE PITCH

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.1-FIGURE

GAUGE

ARRANGEMENT

FOR

GABLE

ROOF

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GAUGE TO ·PARAPET WALL

2

2 Fl: - 6 IN. 3 FT.

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GAUGE ARM FT. IN. UPPER BAR r---SHINGLE '---- LOWER PLATE \ \ UPPER BAR I I I

LOWE R TR IA N GULAR PLATE

ｾＭｇａｕｇｅ ARM

FIGURE

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ROOF/