Proposed Change in Snow Load Requirements for Houses

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Technical Note (National Research Council of Canada. Division of Building

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Proposed Change in Snow Load Requirements for Houses

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....

NATIONAL RESEARCH COUNCIL OF CANADA

No.

DIVISION OF BUILDING RESEARCH

414

'fEClHlN ][CAlL

NOTJE

RESTRICTED CIRCULATION

ｾ

PREPARED BY DBR ｈｯｵｳｩｮｾ and CHECKED BY

Building Structures Sections

APPROVED BY _R.F.L.

ｾ February 1964

PREPARED FOR NRC Associate Com.m.ittee on the National

Building Code

SUBJECT PROPOSED CHANGE IN SNOW LOAD REQUIREMENTS FOR HOUSES

At the 35th m.eeting of the ACNBC in Novem.ber 1962 a proposal for the reduction of design snow loads for houses from. 80 per cent of the ground load to 60 per cent was referred to the Advisory Hoosing Group for study and

recom.m.endations. The m.atter was considered by the Advisory Housing Group

at their m.eeting in April 1963, at which tim.e this Group proposed" ... that the design snow load be reduced to 60 per cent of the ground load but to not less than 30 psf for rafters only . . . . "

The ACNBC accepted this recom.m.endation in principle at their

36th m.eeting in May 1963, and suggested that a special study group be appointed to exam.ine the effect of the recom.m.endation on Ho using Standards, with a view to preparing a statem.ent for the Com.m.ittee that would sum.m.arize the specific am.endm.ents to be m.ade to the Housing Standards in order to effect the

recom.m.endations of the Advisory Housing Group. Mem.bers of the Housing

and Building Structure s Sections of the Division of Building Re search have

prepared this statem.ent in consultation with representatives of CMHC and FPRB.

GENERAL INTENT

In considering this m.atter the following background inform.ation is relevant:

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"

,

- - - -- - - .

-2-Design Snow Loads

The reduction in the design roof snow load for houses from 80 per cent of the ground load to 60 per cent was originally recommended by the Revision Committee on Housing Standards and is based on information

collected in the DBR survey of snow loads on roofs. These data show that

for the majority of roofs the average roof load is seldom more than 60 per cent

of the ground load. Since then the survey has indicated, however, that in

a minority of roofs that are well sheltered (such as a roof of a house in a small clearing in a fore st) the roof load may reach 80 to 90 per cent of the ground load.

It might also be noted here that the Revision Committee on Loads at

their meeting on November 21 st has decided, in principle, to recommend to the ACNBC a design snow load for exposed roofs of 60 per cent of the ground load.

Imbalance of Strength in Conventional Roofs

The nailed connections are the most critical part of a conventional roof bui It in accordance with the pre sent Housing Standards as they are

considerably weaker than the wood members. This was shown by many loading

te sts of the Division of Building Re search and is indicated in Technical Note No. 395 on the Strength of House Roofs prepared by the Division of Building

Re search. A copy of the note is attached to this report. It was con sidered

by the Advisory Housing Group at their meeting in April 1963 when their motion on snow loads was passed.

It should be noted that the motion by the Advisory Housing Group is aimed at two things:

(a) reducing the design snow load for house roofs to 60 per cent

of the ground load;

(b) improving the existing imbalance between the strength of the

nailed connections and the strength of the wood members in conventional roofs by maintaining the pre sent nailing

require-ments. (This would have the effect of maintaining the strengths

of conventional roofs at their present level because nailing has been shown to be the weakest link.)

Other Roofs

As any reduction of the design snow load would apply to all house roofs, consideration should be given to the effect of such a reduction on trusses and flat roofs.

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.;

.

-3-For trusses, it is recommended that the criteria be revised to

maintain truss strength at the present level relative to good conventional

construction. The se criteria should be kept under review to permit reductions

as supporting information becomes available.

Application of this reduced snow load to flat roofs would still leave such roofs considerably stronger than conventional roofs in the same area. Minimum Design Roof Load

At present there is no minimum design roof load in the Housing Standards or in the National Building Code other than the values established

by snow loads of houses and snow or wind loads of other buildings. This can

result in very low design loads in some areas, e. g. 12 psf (Claresholm) and

15 psf (Calgary) in Alberta. The basis for the 30 psf minimum load' proposed

by the Advisory Housing Group has been examined and it is suggested that there

is some precedence for setting this at 20 psf (see later discus sion).

These aspects, together with the changes to be made to the span table s, the nailing table s, and the truss criteria in the Housing Standards to effect the recommendations outlined here, are briefly reviewed in the following section.

EFFECT OF PROPOSED SNOW LOAD REDUCTION ON HOUSING STANDARDS

Span Tables

If the de sign roof load is reduced to

o.

6 time s the ground snow load

(at present this is 0.8 times the ground load), the roof framing tables listed

for 30, 40 and 50 psf roof snow loads, which are now applicable to

37i,

50 and

62i

psf ground load areas, would then become valid for 50, 66-2/3 and

83-1/3 psf ground load areas, respectively. Revisions to Housing Standards

The only changes required in the Housing Standards would be the addition of the following statement under Clause 9. 3.4. 3( 2) on Page VII:

"De sign snow loads on roofs shall be

o.

6 times the ground

snow loads listed in Supplement No. 1 to the National Building Code. "

NOTE: This makes no distinction between sheltered

and unsheltered roofs nor does it allow for drift

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I •

-4-Nailing Table

As it is not intended that nailing requireInents be reduced, the roof nailing for any given area Inust reInain as specified at present, even though the roof load is to be reduced.

The nailing table on Page 84 (Table XIX) of the Housing Standards

for 30, 40 and 50 psf roof load now applie s to 37t, 50 and 62t psf ground load

areas. With the roof load reduced to 0.6 of the ground load the 30, 40 and

50 psf headings in this table would then read 22t, 30, 37t, in other words,

in order for the nailing to be Inaintained at its present level the nUInber of

nails shown in this table Inust be increased by a factor of 0.8 _ 1 33 33

0 . 6 - ' or per

Revisions to Housing Standards

Table I of this report is the proposed substitution for Table XIX,

Page 84, of the Housing Standards.

Trus s Criteria

cent.

The current requireInents in the Housing Standards state that roof trusses Inust withstand at least twice the design roof load plus the ceiling

load for 24 hours and Inust not deflect Inore than 1/360 of the span when loaded

with ceiling load and design roof load for one hour. If it is the intention to retain

trusses at their present strength in a given area, even though the design snow

load is reduced, the trus s criteria Inust be adjusted to take into account the

lowered design loads. This would, in effect, Inean that the current requireInents

Inust be revised to specify that roof trusses be capable of supporting

ＨｾＮＺ

x 2) or 2-2/3 tiInes the roof load plus the ceiling load for 24 hours, and

Inust not deflect Inore than 1/360 of the span when loaded with the ceiling load

(0.8)

plus 0.6 or 1.33 tiInes the roof load for one hour.

Revisions to Housing Standards

Clause 6.26 (d) on Page 119 should be deleted and the following

substituted:

"LuInber Roof Trusses - Roof trusses in houses shall be capable of withstanding a load equal to the ceiling load plus

2-2/3 tiInes the design roof snow load (but not less than

60 psf) for 24 hours. Roof trusses shall not deflect Inore than

1/360 of the span after being loaded with the ceiling load plus 1-1/3 the design roof snow load (but not less than

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-5-MINIMUM ROOF LOAD

In considering the question of minimUIn design roof load it

should be noted that no reasons were given by the Advisory Housing Group for their choice of 30 psf for this purpose; this, however, has been a minimum figure established by CMHC for their qualification of trusses. Concern has also been expressed that a minimum load is desirable to enable the .roof to carry loads imposed during construction.

If 30 psf is accepted as the minimum design load, there are certain complications that must be faced. The first of these is that the new nailing table for roof framing would also have a 30 psf lower limit, which would specify 33 per cent more nails than is required in the current table for this snow load. This will mean an upgrading in nailing in those municipalities currently using a snow load of 30 psf or less. These include Victoria, Vancouver, Calgary, Eimonton, Moose Jaw, Regina, Saskatoon, Chatham, St. Thomas, St. Catherines, Sarnia,

Windsor, and possibly Toronto.

The second complication is that the roof truss requirements will be affected. Ifthe requirements are revised, as propose d, to

increase the required failure load to 2-2/3 the design load and the 1/360 deflection related to 1-1/3 the design load, the effect will be that trusses for all areas where the design roof load is at present below 40 psf will be upgraded. It was not the intent of the Advisory Housing Group to increase current requirements.

It would seem reasonable, therefore, to re -examine the 30 psf sugge sted minimum. It is, quite naturally, very difficult to establish a theoretical basis for a minimum design load, but one approach is to investigate what other authorities have done. A survey of some of the major U. S. standards and code writing bodies was made and the following is a summary of current U. S. practice:

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•

-6-C ode or Standard

1. Minimum De sign Loads in

Buildings and other Structure s, National Bureau of Standards. A. S. A. Spec. A58.l-l955.

2. Minimum Property Standards

U. S. Federal Housing Administration

3. Uniform Building Code (1958)

International Conference of Building Officials

4. B. O. C. A. Basic Building

Code (1961) Building Officials Conference of America

5. Modern Standard Building

Code (1959) Midwest Conference of Building Officials

6.

Southern Standard Building

Code (1960) Southern Building Code Congress

Min. Design Live Load

20 psf 20 psf (slopes 3/12 or less) 15 psf (slope s over 3/12) 20 psf (slope s Ie s s than 4/12) 15 psf (slopes 4/12 or more) 20 psf 30 psf 20 psf

As can be seen from the above values, a 20 psf minimum design

live roof load is more common than the suggested 30

psi

minimum recommended

by the Advisory Housing Group. The adoption of the 20 psf minimum design

load would mean that there would be no upgrading of nailing or truss design for the municipalitie s in the lower snow load areas, nor would there be an upgrading in requirements for roof trusses.

There is one slight disadvant age in that current span table s are not calculated for 20 psf snow loads; areas with 20 psf or lower roof loads, however, could use the 30 psf spans until such time as spans for 210 psf loads are calculated, or they could vary the rafter spacing to adjust to the lighter load (1. e. rafter spacing for 20 psf would be 50 per cent greater than for 30 psf loads for similar spans).

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r-- ....ｾＮ •

TABLE I>:<

NUMBER OF 3t -IN. NAILS REQUIRED TO CONNECT RAFTERS TO JOISTS

Rafter Tied to Every Joist Rafter Tied to Joist Every 4 ft

House Width House Width House Width House Width

Roof Rafter up to 26 ft up to 32 ft up to 26 ft up to 32 ft Slope Spacing (C to C) "0 (1) (1) (1) (1) Cl! rn H rn H rn H rn H 0 rn 0 rn 0 rn 0 rn 0

....:1-

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E

(1)

E

(1)

E

(1)

E

...

... 4-< ... 4-< ... 4-< ... 4-< ｾＴＭ＼ H rn H H rn H H rn H H rn H o 0' 0 p.. 0 0 p.. 0 0 p.. 0 0 p.. 0 ｾ rn 4-< 0 4-< 4-< 0 4-< 4-< 0 4-< 4-< 0 4-< (J) ... rn M rn rn ("f') rn rn ("f') rn rn ("f') rn ,.Q _p.. _p.. _p.. _p.. _p.. _p.. _p.. _p.. 4-< ... 0 - ₀ ₀ 0 0 0 0 0 0 0 _N _<:t< _N _<:t< _N _<:t< _N _<:t< ｾ 4/12 16 in. 3 4 5 4 5 7 9

-

-24 in. 5 7 8 7 9 11 9

-

_I', t 5/12 1-6 in. 3 3 4 4 5 6 6 8 10 8

-

-24 in. 4 5 7 6 7 9 6 8 10

-

-6/12 16 in. 3 3 3 3 3 4 5 7 8 7 9 11 24 in. 3 4 5 4 5 7 5 7 8 7 9 11 7/12 16 in. 3 3 3 3 3 3 4 5 7 6 7 9 24 in. 3 3 4 4 5 6 4 5 7 6 7 9 9/12 16 in. 3 3 3 3 3 3 3 4 5 4 5 6 24 in. 3 3 3 3 3 4 3 4 5 4 5 6 12/12 16 in. 3 3 3 3 3 3 3 3 3 3 3 4 24 in. 3 3 3 3 3 3 3 3 3 3 3 4

ｾＬ This table has been prepared on the assumption that the minimum roof load will be 20 psf (see later discussion), and that ＲＴｾ is rounded to 20, 37t to 40 psf.

NOTE

(1) Nailing necessary to fasten ceiling joists together at the splice over the bearing partition is the same as shown in the above table except that one more nail is necessary in all cases.