Page:EB1911 - Volume 23.djvu/732

framing, usually of rough northern pine or spruce, is generally hidden from view by the ceilings. The spans usually are not great, and extra support is obtained at various points from partitions and cross walls. Where the span is large, that is, above 20 ft. without intermediate support, it is necessary to employ roofs with "principals" and "purlins," sometimes called "double rafter roofs." Principals are strong trusses of timber rigidly framed together and placed at intervals of about 10 ft. to support the weight of the roof covering. Purlins—stout timbers running longitudinally—are fixed on the principal rafters with intervals of about 8 ft., and on these the common rafters are fastened. Principals, or "roof trusses" as they are more often called, are framed together in various ways, and the members may be entirely of wood or reinforced by ties of iron rods or bars; the latter are called "composite trusses."

The "king-post truss" may be used for spans up to 30 ft. and is constructed as shown in figs. 1 and 2. It has a central post sustaining the "tie-beam" in the centre with struts projecting from its base to support the principal rafters at their centres at a point where the weight of the purlins renders strutting necessary. The members are connected by wrought iron straps and bolts; the strap connects the king-post and tie-beam and is often fitted with a gib-and-cotter arrangement (really a pair of iron folding wedges) which allows the whole truss to be tightened up should any settlement or shrinkage occur. "Queen-post trusses" have, in place of the king-post dividing the tie-beam into two, two queen-posts supporting it at two points (fig. 3). The joints between the members are made in a similar manner to those of the king-post principal with wrought-iron straps. The purlins are two in number on each slope, one supported at the top of each "queen," the other half-way between that point and the wall-plate and resting upon the principal rafter at a point where strutted from the base of the queen-post. A stout straining beam connects the heads of the queens. In fig. 4, a and b are details at the foot of the queen-post, and c at the head. Trusses of this type are suitable for spans up to 45 ft. In roofs of a larger span than this and up to 60 ft. the tie-beam requires to be upheld at more than two points, and additional posts called "princesses" are introduced for this purpose. This also entails extra struts and purlins.

In such large spans the straining beam often becomes of such a length as to require support and this is effected by continuing the principal rafters up to the ridge and introducing a short king-post to sustain the beam in the middle of its length. Open timber roofs of various types but principally of "hammer beam" construction were used in the middle ages where stone vaulting was not employed. Many of these old roofs still exist in good preservation and exhibit the great skill of the medieval carpenters who designed and erected them. Such forms are still used, chiefly for ecclesiastical buildings and the roofs over large halls. In the best periods of Gothic architecture the pitch of these roofs was made very steep, sometimes as much as 60° with the horizon. In the hammer-beam type of roof the tie-beam at the foot of the rafters is omitted, a collar being thrown across connecting the principal rafters at a point about half-way in their length, the lower portion of the principal consisting of a number of struts and braces rigidly connected in such a manner as to throw as little thrust as possible upon the walls serving as abutments. There are two kinds of hammer beams, the arched and the bracketed; the chief examples are Westminster Hall and Middle Temple Hall (Plate I. figs. 24 and 25). The "hammer beam" projects from the top of the wall and is bracketed from a corbel projecting from the wall some distance below. This form of roof has a style and dignity of its own, and gives greater height in the upper part of the building as well as being more ornamental and lighter in effect than tie-beam trusses, which have a rather heavy effect.

Fig. 4.—a. Detail of queen-post truss at b.
b. Vertical section through queen-post.
c. Detail of queen-post truss at head; purlin and wrought-iron straps are omitted for the sake of clearness.