Chap. XXII.

TIMBER ROOFS.

1341

in any layer, the whole truss loses its continuity, and becomes weak, and liable to fracturein that part : where iron bolts are passed through, not only do the holes bored to receivethem produce mischief by weakening the series of planks, but the bolts prevent that neces-sary play between each layer which is conducive to its strength : the manner in which theside-pieces are attached contributes very much to the stiffness of the whole, as, being unitedabove and below, there is no fear of the planks starting or changing their position : wheretangential timbers can be laid on the extrados of these arches, and united to the pendantside-pieces, a further security against any change of form is insured. The solid contentof an arch constructed of bent timber being ascertained, an estimate may be readily madeof its cost, and the contrast between it and the ordinarily framed roof easily made: a seriesof rafters laid longitudinally may carry the slate

or tarred paper covering, and all expensive fram-ing be dispensed with.

Fig. 2157.

Other roofs of far greater span have beenconstructed by the same engineer, who aftermany experiments suggested the employmentof a double curved rib having different radii:

these, held in at the springing, were in the spandrill space braced by timbers arranged likeSt. Andrew ’s crosses; iron bolts or timber binding-pieces pinned at the sides hold themasses together, and form a most effectual support to either tiles or slates that may be laidupon them.

Roofs of bent timbers may be formed in the same manner as adopted for the constructionof bridges by M. Wiebeking, who employed pieces of as great a length as he couldobtain, and then bent them to the form of the required curve : joints should, if possible, beentirely dispensed with; nothing should be introduced which in any way can weakenthe strength of the ribs, or render any part of them liable to decay. As timber shrinksbut little in the direction of its length, there is no inconvenience arising from thiscircumstance, when planks are bent one over the other: whatever change takes place iseither in the width or thickness of each layer, and this may occur to a considerableextent without disturbing the strength of the entire rib : but when the timber is not