Chap. XXII.

TIMBER ROOFS.

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limited iron must be employed ; a cast-iron girder is much more simple, and less expensive,than iron framing introduced into timber as the means of strengthening it ; but as thiscannot always be readily obtained, it will be necessary to describe other methods that havebeen adopted in trussing girders : one of the most simple and best methods is by cuttingthe girder, for a considerable distance, in the direction of its length, and wedging theupper portions upon the middle, and then hooping it round with iron straps.

Another method is to place two pieces of timber like rafters on the upper part of thegirder, and which has abutments cut to receive their ends; a stirrup iron passes over thecentre, or one is placed on each side at a little distance from it, and these ties are placedsquare with the rafters: another method is a kind of queen truss; two rafters abut againsta straining piece, and are let in, as well as hooped round by stirrup iroys ; this plan isvaried by omitting the straining piece, and introducing an oak block to form the upperabutment.

As timber resists more when pressed longitudinally, or in the direction of its fibre, thanwhen it is acted upon laterally, it is ad\isable to substitute for the piece of oak a plate ofiron or lead, or some other incompressible metal ; the rafters can then be adjusted, to forma common joint in the middle, and a complete truss; the girder should he suspended inthe middle, whilst the iron ties at the extremities are tightened by this means: the girdermay then be made to acquire greater stiffness, by having a curvature given it, fitted upwith other pieces of timber cut to the camber, and made to fill up the voids betweenthe rafters and the girder. It has been satisfactorily proved that a piece of timber curvedon its upper side, and its extremities secured, to prevent its springing, when placed on twosupports, and loaded in the middle with a weight sufficient to make it bend one-thirdof its thickness, if strengthened by another piece of timber, so as to preserve its curveand form an arc, will sustain a weight more than double without yielding. Formingfloors with joists of small thickness, where camber timbers are united with straight,so that they are not permitted to spring, is perhaps the most simple method of makingthem strong ; thus for floors where the joists have a bearing of 24 feet in the clear, thedepths are made in two, the lower 7 inches in thickness, and the upper, 13 inches, are madeup, after cutting the superior surface in a curved form, by laying on them others bent tothe curve, by means of iron straps 3 or 4 feet apart, taking care to give to thiscurve half the difference between 7 and 13, viz. 3 inches, so that the lower joists will be indepth 7 inches in the middle and 4 at each end; we shall thus have joists sufficiently strongto bear any ordinary weight that can be put upon them. Girders may be formed in thisway, by uniting several such joists side by side, so that for a bearing of 24 feet, where theyought to have a depth given to them of an eighteenth of this width, or 16 inches, it may hethus done: supposing the under joists to be 7 inches in depth, 9 inches more is required,the half of which is 4£ inches, which is the amount of curvature to be given to the upperjoists ; instead of cutting away any portion of the lower joists, the curved top may hegiven by furring out the 4\ inches in the middle and letting it fall oft' to nothing at theextremities: on these joists so prepared, with a curvature on the upper sides, the upperjoists are laid, and then bent to the lower ones, and secured by means of iron straps : ascries of such joists bound together or united forms a guider.

Some builders have taken advantage of the difficulty of crushing timber in the directionof its length, and applied it very ingeniously for obtaining strength in a girder : a sawcut is made across the middle of the girder to the depth of one-third its vertical thickness ;it is then bent upwards, and when the cut has opened sufficiently, a wedge is introducedinto it, thus making it camber, and consequently stiffer, whence it can bear a greater weight;Parent has proved that joists so cut and prepared are one-sixth stronger than those ofthe same scantling unsawn.

Another method of building up a girder, or strengthening a beam, was adopted bySmeaton, which was by bending a piece into a curve, and securing it from bending backby straps and bolts ; the additional stiffness so obtained is considerable, particularly if thepieces are properly bolted, or prevented from sliding on each other; the thickness of thebent pieces should be made about the fiftieth part of the bearing, and as many shouldbe added as will increase them to the depth required; when the whole depth of thecurved pieces exceeds half the depth of the guider, then straight pieces must be addedto the under side, so as to make the entire depth of the straight part exceed that of thecurved part: care must be taken, when pieces cannot be found of sufficient length, that thejoints do not come in the middle of the length of the lower half of the girder.

Hoofs, or the Coverings of Buildings, which protect them from the weather, serve also tobind them more firmly together, and a knowledge of the strains to which the varioustimbers are subjected, as well as what should constitute their relative strength, is one ofthe most important studies of mechanical carpentry: the various inclinations that have beengiven to them is perhaps dependent upon the nature of the climate where they are used.In northern latitudes, where there is more rain, and the cold is greater, the pitch is madehigher ; this decreases as we approach warm and dry climates; in Egypt , the covering of

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