Chap. XXII.
TIMBER ROOFS
1S2S
The carpenter should avoid asmuch as possible the effect of allcross strains, or those which aretransverse; and in the ar-rangement of the timbers ofa roof, he should never em-ploy a very open angle ata point where a load is to .be supported, the obli-quity of the two pieces #//forming the an-gle requiringthem to exert agreat force, inorder to opposea much smallerone.
If the two braces forming the ob-tuse angle in the present figure con-sisted of wood cut across the grain,and the piece joining theirextremities were cut in theusual manner, the obliquepieces would contract morethan the others in drying,and the angle would be-come more obtuse, so that, the strain would
be increasedconsiderably be-yond what it wasat first.
ROOFS WITH TIE-BEAMS SUSPENDED.
At the Theatre of St. Martin, at Paris , which is of considerable span, the timbers arearranged for the convenience of the scenes, which are in many instances suspended from it;shores or supports are consequently necessary to discharge or bear the great weight to whichthe several floors are subject, and they are admirably placed for the purpose.
The angle of inclination of a roof varies much,and is dependent in a great degree upon the natureof the material with which it is covered. A steeproof is absolutely necessary in climates where thereare considerable falls of snow, and w r e must alwaysbear in mind, that the horizontal force exerted byroofs of all kinds is proportionate to the length of }a line perpendicular to therafter, descending from its ex-tremity till it meets anothersimilar line drawn from theojiposite rafter.
The transverse strain which a roof is required tobear, and which tends to break the rafters, is betterprovided against in one of low than in a high pitch,supposing the number of posts and braces equal ineach ; for when a weight is to be borne by a rafter,whether it be placed in the middle or equally dis-tributed throughout the entire length, the rafterneither gains nor loses force by being lengthened orraised higher, while the horizontal span continuesthe same.