Chap. XXII.
TIMBER ROOFS.
1337
such an arrangement would no doubt retain the curved or bent timbers in their originalposition.
Fig. 2147.
In making the joints where struts and ties unite, great attention is requisite, particularlywhen they are intended to hold up a suspending rod or timber : the joint should always beat right angles with the timbers, and the head of the piece theyare to sustain cut to fit that arrangement, in order to preventit being pulled through; the irons or stirrups for holding upthe tie-beams or other large timbers are attached to a pin madeto pass through them, but there is no better method than thatwhich allows the iron to strap entirely around the timber it is tohold up.
Fig. 2148.
Philibert Delorme , in a work published in 1561, describes a method of constructing roofsand domes with timbers, or rather planks, of short lengths, and which is only objectiona leon account of the labour necessary in putting them together: one of these roofs coveredthe stables of the Tuilleries, at Paris , and after remaining perfectly sound and secure for234 years was demolished to make way for some improvements in the palace: the archeswere formed of two planks, about 9 inches in width, cut into convenient lengths forthe required form; this seldom exceeded 4 feet, and when applied to each other toform the rib, they were so laid that the ends of one set came in the middle of the lengthof the others: mortises in length double their width were then made in them, throughwhich were longitudinal ties, and these had a small wooden pin passed through them,on each side of the circular ribs, to keep them in their upright position, or preventtheir twisting. In some of the buildings executed by this celebrated architect, the spanof the roof exceeded 60 or 70 feet; two ribs were then used, so put together that theypartook of the character of hollow voussoirs, and were very strong; indeed there is scarcelyany span where, if properly proportioned, they might not be applied.
Designs are also shown for the construction of girders to carry floors upon this principle,which are well adapted for lofty rooms intended to have coved ceilings, and by placing theribs in pairs the whole might be covered in a very regular way : when this system isapplied to domes, we must commence by tracing out the true curve to be executed, andlaying on this mould the first range of planks; when it is intended for a simple roof,the under side of which is not to be formed into an arch, the planks must be placed belowthe curve line traced; if for a vault, above it, but always in such a manner that the linesmay be traced upon them; when the ribs are to be curved above and below, then the