Roof

(roof.) [From A. S. hrof.] (Arch.) The timber or iron framework which sustains the slates, tiles, lead, ., that form the coverings of the tops of buildings. The elevation of a roof depends on the climate; the more northern the country the greater its pitch should be. In its simplest form a R. consists simply of two rafters, whose lateral thrust against the walls on which they rest is counteracted by tying them together with a collar, placed some way up the lengths; and if nece ary, by a tie-beam placed at their feet. Beyond certain lengths it is clear that the tie-beam itself will have a tendency to become depre ed in the middle; it must then be suspended at its centro, from the junctions of the rafters, by a king-post. If the rafters are long, they also have a tendency to sink in the middle; which must be prevented by struts, or oblique pieces, abutting against the sides of the lower extremity of the king post. The whole now would form a frame called a tru . Sometimes the upper ends of the rafters, instead of abutting against each other, abut against the ends ofa horizontal piece, called a collar, which is placed between | them to keep them apart. Instead of the king-post there are then two vertical pieces, called queen-posts, which hang from the extremities of this collar, and support, at their lower ends, struts tending in various directions, for the sustentation of various points. Purlins are strong pieces placed acro the principal rafters to steady them, and to support the common rafters, which are fixed at the ordinary small distances apart. D H E G F D B C B Fig. 568.- HAMMER-BEAM ROOF. A, Hammer-beam. E. Collar. B B. Pendant-post. F, Side-post. C,Hammer-brace. G, Collar-brace. DD, Rafter. H, Upper Collar. Some R. are very complicated and ingeniously designed; but the principles on which they are constructed can always be easily comprehended from the foregoing. The Hammer-beam roof (Fig. 568) contains most of the peculiarities of structure which distinguish the medieval from the modern roofs. [s. 777] IMG:p0822_0.jpg:Gravür