An Expert Reviews a New Roof System

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à [email protected].

Questions? Contact the NRC Publications Archive team at

[email protected]. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

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

Research); Volume 4

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC :

https://nrc-publications.canada.ca/eng/view/object/?id=7f63a5cb-05a9-4d7c-9d4a-27aec6e82f37

https://publications-cnrc.canada.ca/fra/voir/objet/?id=7f63a5cb-05a9-4d7c-9d4a-27aec6e82f37

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

An Expert Reviews a New Roof System

Ser

NA7110

N21h8

no.

4

2

N A T I O N A L R E S E A R C H COUNCIL

C A N A D A

D I V I S I O N OF BUILDING R E S E A R C H

H O U S I N G N O T E N O . 4

AN

EXPERT REVIEWS

A

NEW

ROOF

SYSTEM

by

R. E.

Platts

Reprinted from .National Builder, Vol.

XI,

PETAIL B/

A - A

An expert reviews

a

new

roof system

Among the outstanding new fea-

tures of the Mark

Experimen-

tal Project, built by the

NHBA

Research Committee last year on

the

RCAF

Station Rockcliffe

near Ottawa, was a roof-ceiling

component system using prefab-

ricated, stressed skin plywood

panels. Here, Mr. R. E. Plans

of the staff of the Division of

Building Research, NRC dis-

cusses its advantages and possi-

bilities

in

a special article for

National Builder readers.

Over the years builders in Canada have shown increasing interest in shop- built house components. Faster summer or winter closing-in, less on-site labour with better job control. closer costing and faster turnovcr with lower overhead for the builder are often cited as obvious reasons for their use.

Although factory fabricated windows

and other building components have gained total acceptance, however, little progress has been made until recently in developing complete components for the roof.

The roof offers a large unbroken rec- tangular area that may be easily sub- divided into simple repeated components, but the common arrangement of a slo- ping roof and a horizontal ceiling tends to defeat its panelizing. The number of panels must usually be doubled to form the separate surfaces. In exception to this, the Research Committee of the National House Builders' Association recently used a unique method of in- cluding sloped roof and flat ceiling in one panel of stressed skin plywood con- struction in the Mark 111 experimental project built near Ottawa.

While this design offers one solution to the problem of combining a flat ceil- ing and sloping roof surface in the same wedge shaped panel, a considerably more simple panel construction is possible where a cathedral type ceiling is used o r where the panels are used for flat roofs.

In these applications stressed skin de- sign can provide a panel of simple struc- ture in which insulation, cladding and

finish are included in one light panel at inherently low cost.

Stressed skin design uses light, strong sheet materials as structural "skins", which are bonded to stabilizing webs to form an enclosed panel. The panel acts as a group of I-beams; in bending, the skins act as flanges, taking the axial stresses, and the webs act in shear and also stabilize the skins against wrinkling.

Stressed skin panels and structural sandwich panels are very efficient in the use of materials; they can easily span from side walls to a ridge beam or centre partition to form a roof and ceiling complete except for roofing.

A sample panel is sketched in Fig.

1. It may be noted that only 4-in. thick- ness with %-in. plywood skins and 1- by 4-in. wood webs is sufficient to span

12 ft under snow load of 50 P.S.F. with a deflection of 1/240 span.

Other simple arrangements are used equally effectively.

Builders may, of course, ask why such panels are not more widely used. There are several answers: stressed skin panel fabrication is not a backyard operation; a well organized and roomy shop that is heated and has good supervision is

Q O O F

AssFMBLY

F I G .

I

-

A

G T R E S S E W

needed. Low volume production in small shops has not achieved cost reductions over framed construction. Large prefab- ricators and a few large builders in the United States have achieved significant- ly lower costs with large volume stressed skin production. In many cases, how- ever, large volume production is diffi- cult to achieve, because various muni- cipal codes restrict the unfamiliar de-

signs. This difficulty wiU continue to

some extent even though complete stressed skin houses were developed by the U.S. Forest Products Laboratories in 1937 and have performed well since then in many areas.

Engineering practice for stressed skin design is well defined, proven and ac- cepted. Any proposed panel must, of course, be supported by full engineering calculations or by suitable load tests.

A further deterrent to use of stressed skin panels and sandwich panels is the fact that they tend to be quite flexible, though strong. The common rather stiff

deflection limit of 1/360 of the span

means that such panels have to be 5 to 6 in. thick to span 12 ft under a 50

P.S.F. snow load, although 3 to 4 in. is sufficient for adequate strength alone. The increasing use of a limit of 1/240

of the span for non-plastered ceilings, as in the current Housing Standards, allows the less bulky and less expensive 4-in. thick panels.

In Canada, stressed skin panels with their light weight, low cost and high in- sulating ability have been used success- fully in northern housing; many hun- dreds of stressed skin units, some up to

13 years old, are scattered throughout the north. Some of these make good use of roofceiling panels very similar to the arrangement discussed here. The gable roof that is very popular in Cana- dian housing can easily be adopted for a variety of house plans. Thus, several builders in an area can use a few simple panel designs from one fabrica- tor, obtained in the volume necessary for him to compete with conventional frame construction.

The several large shops now fabri- cating such panels in Canada usually use steel jigs with casein o r urea ad- hesives for bonding skins to webs, and nailing or power-stapling for gluing pres-

sure. Such companies estimate that the

sample panel shown in Fig. 1 can be

produced in volume t o sell for about

$36.00 per panel. or about 70 to 75

cents per sq ft, with prime painting in- cluded. Such panels can be quickly mounted and lag screwed to the side walls and to a centre ridge beam of box beam or lumber design, or to a centre partition. Such partitions or beams must be founded directly on footings, according to best practice, since panel systems can be subject to leakage if supports move.

The bottom skin can be patterned so that the joints are a part of the design. Such close fitting is practicable with ac- curate jig-built panels, or the joints may be covered with narrow battens. The plywood may be stained o r painted in shop or field. The sample panel in Fig. 1 weighs only 190 lb.

/

Closed stressed skin panels with full thick insulation cannot be vented effec- tively so that questions of internal con- densation may arise. Northern stressed skin buildings up to 13 years old indi- cate, however, that such closed, glued panels present no condensation problems if electrical fixtures and any other open- ings are kept out of the bottom skins.

Builders will recognize immediately one obvious disadvantage of such roof- ceiling panels: the interior partitions must be fitted to the slope, unless of course a flat roof is used. As with any component, trial uses by individual builders are necessary to find the over- all cost and time factors in the operation.

Builders may obtain designs for any spans through competent engineers and fabricators. The Plywood Manufacturers Association of British Columbia can as- sist in obtaining these services.

-

t

/

$7

DETAIL

A