Why Roof Trusses? They Cut Costs, Use Less Lumber, Speed the Job

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Why Roof Trusses? They Cut Costs, Use Less Lumber, Speed the Job

Hansen, A. T.

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National Research Council

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Division of Building Research

H O U S I N G N O T E N O . 5

WHY

ROOF TRUSSES? THEY CUT COSTS,

USE LESS LUMBER, SPEED THE JOB

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A.

T. Hansen

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Reprinted from Canadian

Builder,

Vol.

XII,

No.

8, August 1962, p. 38-39

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FIGURE 2

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SIMPLE JIG for making nailed trusses. l'russ in bockground i s loaded to failure

Why roof trusses? They cut costs,

use less lumber, speed the job

By A.

T.

HANSEN,

Housing Section,

Division of Building Research,

NRC,

Ottawa.

House framing methods have remain- ed basically unchanged for many years with one notable exception. This is in the field of roof framing. Within a relatively short time the use of wood trusses in residential cons:ruction has be- come an accepted and well-established construction method. Their almost spec- tacular increase in popularity is indica- tive of the many advantages that the system offers.

One of the advantages with trusses is the efficient use of lumber. A standard truss made with 2 in. x 4 in. members and spaced on 2-ft. centres requires only slightly more than half the amount of lumber required to frame the roof and ceiling in the conventional manner with

2 in. x 6 in. joists and rafters spaced 16 in. O.C.

Labor costs are fairly low, although these naturally vary with the design of the truss or the type of connections used. From a limited number of observa- tions made during the truss testing pro- y a m at the Division of Building Re- search, the total labor required to cut and assemble typical nailed trusses

(4/12 and 5/ 12 slope) designed for a 50- When partitions are installed after psf snow load area was less than one trusses are in place, they are usually manhour per truss. Trusses assembled made about 'h in. short of the ceiling

by presses with toothed plate connectors height so that they can be easily tilted usually require considerably less time. - . up in place. Once up, the partitions are

With trusses most of the work is done on the ground or in a shop under more satisfactory working conditions. In addition, unskilled labor may be utilized if :he work is carried out in a plant; this can further reduce labor costs.

Roof and ceiling framing can be com- pleted more quickly than with conventional framing. It has been reported that three men can install trusses for an average size house in from 45 to 60 minutes. The speed with which the trusses can be erected, plus the fact that the interior partitions do not have to be in place before the trusses are installed, means that the house can be brought very quickly to the stage where further construction can proceed independent of weather conditions.

Trusses support imposed loads without relying on additional support from the partitions beneath' them. This permits freedom to design the plan of the house independent of bearing partition loca- tion. In addition the finish flooring, ceiling, and perimeter wall cladding may be installed without having to contend with a maze of interior partition framing.

usually wedged to take up the %-in. gap. One builder in Newfoundland, howccr, prefers to cut the partitions full height. The partition is positioned at the top, and the bottom is forced into place, eliminating the necessity for wedging. It seems to be common practice in the U.S.A., and to some extent in Canada, to consider partitions located beneath trusses as non-loadbearing.

Although to the author's knowledge [his has not caused anv difficulties, lab- oratory tests indicate that trusses deflect- ing under load can exert considerable force on a partition restraining this de- flection. As the partition yields. the force is reduced. The amount of yielding to eliminate the load is small and depends upon the stiffness of the truss, the loca- tion of the partition and load on the truss.

There is a possibility, therefore, that i f openings in partitions are framed as non-loadbearing, some damage to the wall finish may result if the truss is subjected to substantial snow loads. There is no question of structural collapse, of course, since the load is reduced as the partition yields.

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One method to ensure that partitions will not be loaded is to build them about $5 in. short of ceiling height and cover the gap between the partitions and ceiling with a molding. In this case the partition would not be wedged.

Roof trusses for use in houses built under the requirements of the 1962

Housing Standards must support at least twice the design snow load plus the design ceiling load for 24 hours without failure and must not deflect more than

1/360 of the span when loaded with the design loads for one hour. Most roof trusses exceed these requirements. Few conventionally framed roofs could meet this standard of strength, especially in the higher snow load areas.

This greater strength not only ensures a sag-free roof but provides additional insurance against collapse if the roof is subjected to unusually high snow loads. Roof trusses vary in method of as- sembly and geometry. The most commonly used geometric shapes include the

" W Truss, the Howe Truss, and the

King Post Truss (Fig. 1).

Of these, the " W Truss seems to be the most popular, although the Howe Truss is being increasingly used, pos- sibly because of its slightly better stiffness characteristics. The King Post, although the simplest, has limited strength and relatively poor stiffness characteristics and is therefore more suited to shorter spans and lower snow loads.

Nailed trusses offer a number of advantages, especially for the smaller volume builder who wishes to manufacture his own truss. Their fabrication requires little in the way of expensive equipment. The jig necessary to assemble the truss can be simply made on any floor or large bench (Fig. 2), by nailing a series of short wooden cleats to the working surface t o hold the truss members in the correct relative positions during as- sembly. Fabrication can be carried out in a shop o r on the site using the wood subfloor as the working surface.

A number of standard nailed truss designs have been prepared on the basis of tests carried out by the Division of Building Research and the Forest Prod- ucts Research Branch of the Dept. of Forestry covering spans from 24 t o 28 ft and slopes of 4/12 and 5/12. These designs have been reproduced by CMHC in their Builders Bulletin No. 109 and are available free of charge from the Corporation or from the Division of Building Research. Trusses made ac- cording to these plans may be used in houses built under the National Housing Act without further proof testing by the fabricator. Further truss tests have been undertaken by the Forest Products Re- search Branch and this Division to de- velop nailed trusses for spans up to 32

ft and slopes down to 3/12 for various

snow load areas. Truss designs based on this work will be available shortly.

The manufacture of glued trusses should not be attempted by those not familiar with gluing techniques or those who are not equipped with the necessary facilities since correct fabrication is essential for an acceptable finished prod- uct. When properly constructed, glued trusses are strong and usually considerably stiffer than other types, and in addition are quite competitive, economi- cally.

The glues commonly used with these trusses include casein, phenol resorcinol, and resorcinol. Clamping pressure to fasten the plywood gussets to the structural members is usually provided by nails or staples spaced 4 in. apart when good gap filling glues such as casein are used. Nails or staples must be closer to- gether for glues with poor gap-filling qualities.

The nails or staples are not designed to contribute any strength to glued joints so it is essential to ensure a good glue bond at all joints. To achieve this, the moisture content of the wood should be kept below 15% when the previously mentioned glues are used and the wood surfaces must be clean.

Lumber must be of uniform thickness and free of warping or cupping in order to achieve adequate surface contact with the plywood gussets. It is also important that the trusses are not disturbed while the glue is setting and that the tempera- ture is maintained sufficiently high to permit setting (e.g., over 50 deg. F. for casein, over 80 deg. F. for phenol resorcinol, and over 75 deg. F. for resorcinol). Furthermore, the trusses should be protected from the weather even after the glue has set unless a waterproof glue such as phenol resorcinol or resorci- no1 is used.

Although relatively unknown only a few years ago, the use of metal plate connectors in residential roof trusses has rapidly increased. There is a large selec- tion of metal plate connectors on the market and new ones appear regularly. The plates generally are of three basic types:

-The nailed-on type, consisting of predrilled sheet steel of about 20 gauge, is used in much the same manner as nailed-on plywood gussets. These plates, like the nailed plywood, are well suited to the smaller builder's operations since little in the way of expensive shop facilities are required.

-The second type consists of similar weight sheet steel, but with relatively short wedge shape projections punched out at right angles to the plate (Fig. 3). The plates, while designed to be pressed into the structural members, also as a rule, incorporate a limited amount of nailing in addition to the teeth to attach the plate.

HOWE TRUSS

SIMPLE KING POST TRUSS

FIGURE I COMMON TRUSS SHAPES

METAL TRUSS PLATE WlTH NAIL LlKE PROJECTIONS METAL TRUSS PLATE WlTH WEDGE LlKE PROJECTIONS

FIGURE 3

TYPICAL METAL TRUSS PLATES WlTH PROJECTING T E E T H

-The third type of plate is made from heavier gauge stock, usually about

14 gauge, with nail-like projections punched out at right angles to the plate

(Fig. 3). These plates must also be pressed into position and usually no additional nailing is necessary.

Plates that must be pressed into position are, of course, more suited to the larger volume truss manufacturer since relatively expensive jigs and presses are usually required. The time and labor savings that may accrue should be carefully weighed in the light of capital cost and expected volume of production to ascertain the most economical system. Most truss plate manufacturers have standard designs available for their par- ticular system and have conducted load- ing tests on these trusses. The metal plates in general provide strong, stiff trusses and their increased use would seem to be a good indication of their general economy.