The simplest method of trussing a roof where the span is not over 35 feet is shown in Fig 119, the truss being of the type shown by Fig. 32, with its principals set flush with the rafters. The trusses should be placed about 6 feet apart, so that the timbers need not be large, and to distribute the weight more evenly on the walls. Such a roof is best adapted to sheds, pavilions and unfinished lofts where no ceiling is required.
For pavilions, platform roofs, etc., of from 35 to 40 feet in width, where a row of posts down the center is not objectionable, the rafters may be economically supported in the manner shown by Fig 120, a purlin being let into the underside of the rafters about 1 inch and supported by braces placed above the posts, the tie-beam being bolted to the plate, which also holds the ends of the rafters
When the span is greater than 35 feet, or where there is to be a ceiling, the common method of construction is to use parallel trusses, spaced from 12 to 18 feet apart, with their ends resting on the side walls, and with purlins spanning from truss to truss as shown in Fig. 121, the ceiling being supported by the tie-beam either as shown at A or B.
For wooden roofs of this kind the types of trusses shown by Figs 8, 10, 12 or 17 (Chapter I (Foundations On Firm Soils. Staking Out The Building)) are best adapted, the number of braces or panels being such that the principals, or truss rafters, will be supported at distances of from 8 to 10 feet, measured on the slant.
If it is desired to have a room in the attic, a truss like that shown by Fig. 19 may be used. Where a ceiling is not desired, trusses similar to Figs. 39 and 41 present a neat appearance, while at the same time being economical and effective.
In laying out a roof like that shown by Fig. 121, it will generally be found most economical to divide the trusses so that the purlins (which should always be placed at or near a joint) will not be greater than 12 feet apart, and to space the trusses not more than 18 feet apart for shingle roofs, or 16 feet apart for slate or tile roofs; this will permit of using 2x6 rafters for shingle roofs and 2x8 rafters for slate roofs and a single beam for the purlins.
While a truss, particularly if it has a horizontal tie-beam, acts in general in the same way as a beam, yet it differs from a beam in that it cannot, as a rule, be strengthened by introducing a support between the end bearings.
Thus, if we should put a post under the truss shown in Fig. 122 at the point indicated, the only aid that it would afford the truss the span, and would reduce the strains accordingly. Wherever an intermediate support is introduced, however, the space on each side should be treated as an independent span and two trusses used instead of one.
Fig. 122. - Example of Bad Construction.
Would be that it would take up the vertical component of the force acting in B or about one-half of the load on the lower purlin. If the post was introduced between the two joints it would do no good at all, and might be a source of danger in case the end bearings settled so as to produce a transverse strain in the tie-beam.
If the post was placed under the centre of a truss of the type shown it would divide the truss into two trusses, each of one-half'
Fig. 123. - Proper Construction.
Thus, if we had a roof to support of the shape shown in Figs. 122 and 123, and it was desired to introduce a post at about one-third of the span, the truss should be arranged as shown in Fig. 123, there being in reality two trusses, one on each side of the post, with their tops connected by the beam a b, the load on the purlin P being directly carried by the posts. Although the tie-beam is shown in one piece there would be no strain over the post, and it could be jointed there if necessary. The framework, 1, 2, 3, should be calculated as an independent truss. The strains in such a roof would be considerably less than if there were no post, but where the width is less than 60 feet the saving in the sizes of the truss members would be about offset by the cost of the post unless the post was placed at the centre. For greater widths an intermediate support will effect a considerable saving, and in any case would materially reduce the weights coming on the walls.