237. Roofs of less span and rise might be constructed in a similar manner, at a comparatively small expense. But in these, instead of forming the rib of short pieces, it might be bent by a method somewhat similar to that used for bending ship-timber.

If the depth of a piece of timber does not exceed about a hundred and twentieth part of its length, it may be bent into a curve that will rise about one-eighth of the span without impairing its elastic force. And if two such pieces be laid one upon the other, and then bent together by means of a rope fixed at the ends, they may be easily bent to the form of the required curve, by twisting the rope as a stone-sawyer tightens his saw, or as a common bow-saw is tightened. The pieces may then be bolted together; and if this operation be performed in a workman-like manner, the pieces will spring very little when the rope is gently slacked; and it is advisable to do it gradually, that the parts may take their proper bearing without crippling.

Otherwise, a piece of about one-sixtieth part of the span in thickness may be sawn along the middle of its depth, with a thin saw, from each end towards the middle of the length, leaving a part of about 8 feet in the middle of the length uncut. The pieces may then be bent to the proper curve, and bolted as before.

In either case the rise of the ribs should be half the height of the roof; and they should be bent about one-fourth more, to allow for the springing back when the rope is taken off. A roof of this kind for a 30-feet span is shown by Plate VIII. The suspending pieces are notched on each side, in pairs, and bolted or strapped together, as shown by Fig. 2, Plate VII.

Plate VIII.

ROOFS.

The advantages of this roof consist in the small number of joints in the truss, in being able to support the tie-beam at any number of points, in admitting of a firm and simple connection with the tie-beam, and in avoiding the ill effects attending the shrinking of king or queen posts. The scantlings are given in Table No. 11 at the end of the volume.

238. In the construction of modern roofs a continued tie at the foot of the rafters or some other means of relieving the thrust is almost always necessary, though sometimes it has been omitted, for in general the lightness of the walls renders them incapable of sustaining much lateral pressure; and this pressure is entirely removed by a tie-beam or an iron tie-rod.

As leaving out the tie-beam gains only a very small space in height, which might generally be obtained without injury to the external appearance of the building, by raising the walls a little higher, we will endeavour to show the defects of roofs without rods or tie-beams.

Referring to the roof described in Art. 228, the whole weight of such a roof is sustained by the parts of the rafters A C and B c (Fig. 75), and when the roof has the weight of the covering upon it, it will settle in proportion to this weight, in consequence of the lower parts of the rafters bending at C c, which will tend to press out the walls. The reader will readily see that a pressure against the walls in this mode of construction cannot be altogether avoided, though it may be lessened, by making the rafters very strong at the lower part. Failures have often been observed from adopting this form of roof, which should never be used except for very short spans, and then the precaution should be taken not to cut into the rafters at the points C c, where the ends of the collar-beam are fixed, but simply to nail them. The nails should be driven near to the back edges of the rafters, which being in compression from the cross strains are not weakened.

239. In wider spans another mode of construction has been employed, which, though better, is not a good one, from the powerful strains that are caused by the oblique disposition of the beams. To show the nature of these strains Plate IX. is taken from Price's work,* where muck is said in praise of it, Price probably not being capable of investigating it's construction according to the principles of meckanics. The essential parts of this roof are contained in Fig. 9, page 12; and by comparing the strains produced by the weight in that figure with the strains when C A is in a horizontal position, it will be found the strains are more than doubled by the oblique position of C A. Returning again to the section of the roof in Plate IX. Let the vertical line a E be drawn, and let a b upon this line represent the weight of half the roof; also draw c b parallel to A C, and c a parallel to A D. Then the weight and pressures will be measured by ab,b c, and a c. But if there had been a tie-beam A B, the pressures produced by the same weight would have been only b d and a d; hence it appears that they are nearly doubled, while the space gained in the middle in height amounts only to about one-ninth of the span. To gain this small advantage, we encounter the difficulty of making a firm connection of the ties at C, with the certainty of a considerable degree of settlement from the number of the joints and the magnitude of the strains. It also must be remembered, that the same degree of settlement will produce a greater effect in thrusting out the walls in proportion as C E is greater. Having thus pointed out the defects of this kind of roof, we shall leave the reader to judge for himself as to the propriety of adopting it.

Plate IX.

ROOFS.