CARCASE ROOFINO, that which supports the covering by a grated frame of timber-work, consisting of three tiers of timber, parallel to each other, and to the sloping surface of the covering. The most general disposition of the tim bers is the following : the first tier and support is a row of timbers, inclined to the pitch of the roof, supported at various points by other timbers, which, with the inclined timbers, form as many vertical frames, perpendicular to the sides of the building as there are inclined timbers: each frame is called a truss : the inclined timbers in the upper part of the truss are called principal rafters : the principal rafters support a set of horizontal timbers transversely, and parallel to each other, called purlins : the purlins support the third and last tier of timbers of the frame, transversely or parallel to the principal rafters : the timbers of the last tier are called bridging, or common rafters. The upper surfitces of the principals, those of the purlins, and those of the common rafters, are sometimes framed flush with each other, or in the same inclined plane, in order to save room, or to conceal more of the roof: in this way the purlins mint be tenoned, and the principals mortised to receive them ; the small rafters and purlins are also tenoned and mortised together. But the
best and strongest mode of carcase framing is, to make the purlins bridge over the principals, and the common rafters over the purlins. The principals rest upon a horizontal piece of timber, on the wall head, called the raising, or : when the purlins bridge over the principals, and the small rafters over the purlins, the small rafters rest at the bottom upon a piece of timber called a The manner of joining the timbers in carcase roofing and flooring may be seen in the article CARPENTRY; other particulars relative to roofing, may be seen under ROOFING, TIMIS'', and BOARDING.
Sometimes the covering is only supported by purlins rest ing upon the principal rafters ; in this case, the length of the boards is disposed parallel to the principal rafters ; but this position does not give so great strength to the roof as that which is horizontal.