249. The proportions to be given to the timbers of a roof depend so much on the design of the framing that it would be difficult to furnish rules to apply in all cases. Therefore those cases only are taken which are most frequently to be met with in practice. But with the aid of the principles already given in this work, and of such data as practice alone can supply, the carpenter will find no difficulty in ascertaining the proportions to be adopted in other designs.

In roofs as in floors the constant number in the rules has been derived from a comparison of examples already executed, and which have been known to stand.

* Rondelet, ' L'Art de Batir.' The roof of the church of Santa Maria Maggiore at Rome is similar, the span of which is nearly 56 feet, and the tie-beams 15 inches by 10 1/2 inches. See Kraft's ' Recueil de Charpente.'