ABSTRACT
Roof trusses, the outline of which was similar to the couple-close truss, figure 395, and the horizontal member of which was supported along its entire length by masonry or brickwork in its turn, supported by a stone arch, the abutments of which were the main walls ; and having one central, vertical member in the truss under compression, this member being known as a crown-post. The crowning moulding of the arch is simply an ornamental member. The tendency of an arch when loaded at the crown is to sink at the latter part and for the haunches to rise. The weight of the rafters and roof coverings is transmitted to the principal rafters by means of the purlins, the principal rafters transmit the weight to the arch through a number of vertical studs, the latter being tenoned to the principal rafters and the back of the arch.
