46. The general arrangement or layout of the roof construction should be considered when making the preliminary studies, as in buildings with trussed roofs the manner in which the roofs are to be supported affects both the cost and the appearance of the building, and often the internal arrangement.

In deciding upon the "layout," the type of truss or trusses to be used, their approximate span and rise should be carefully considered and fixed, as upon these will often depend the shape of the ceiling, the arrangement of the supports and the height and pitch of the roof.

If these factors have been wisely determined, the consideration of the strength and details of the construction may be left until the general plans are completed, as within certain limits of span and rise any truss may be made of sufficient strength and rigidity, and the size of the truss members does not, as a rule, affect the rest of the building.

When designing a trussed roof it should be remembered that short spans and a level tie are in general the most economical, and if the span can be reduced by using posts and without interfering with the internal arrangements it is better to do so.

In roofing buildings having large rooms, with permanent walls between, advantage should always be taken of the walls by placing posts over them to support the purlins directly above.

The principal points to be fixed in laying out a trussed roof are: The type or types of trusses to be used, the pitch of the roof, the distance that the trusses shall be placed apart and the spacing of the purlins. As the settlement of these questions will be determined in a great measure by the character and style of the building and the purpose for which it is to be used, it seems best to take them up in connection with different classes of buildings, or forms of construction.