Its stability depends solely on the permanence of its joints., and the safeguarding against decay, especially in the hammer-beams and the wall-posts. The huge cornice and the hammer-beams are, in reality, the only tensional members; the others are in compression. As an example of clever construction on the part of the fifteenth-century carpenters this roof of Needham Market Church is a truly astonishing achievement.
The great curved rib, as in Fig. 44, No. 24, when used in conjunction with the hammer-beam, marks the zenith of timber-roof construction in England. The view of Gainsburgh Great Hall, illustrated in Chapter VII (English Lacquer Work) of this volume, shows the rib as a great moulded arch-brace, springing from the wall-corbels to the collar. This Hall is in a timber building, and the stress of the entire roof is carried on great posts from the ground, tenoned into the ends of the principal rafters. These posts appear, on the outside of the Hall, as great timber buttresses; on their inside faces is a solid abutment, - probably a branch growth on the original tree itself, which was especially selected for the purpose, - on which the continuation of the arch-rib to the corbel is moulded. Above this springing, the arch-rib rises, in two sections, to its apex, where it is tenoned into the collar. At the point of junction of the two sections of the rib with the principal rafter, they are housed into it with long slotted tenons, secured by wooden pegs. With the solid abutments to this arch-rib, it will be seen that the corbels have no function other than an ornamental one, and even this latter is questionable when it is remembered that the original carved corbels have disappeared and have been replaced by others of cast iron in the ornamental style of a modern girder railway bridge. Surely even cast iron was never put to more ignoble use.
Fig. 83. Needham Market, Suffolk, Roof Of Nave. - Double-aisled hammer-beam type, with clerestory. - 29 ft 9 ins. span. 17 ft. between hammer-posts. 59 ft. long over all. - Built about 1460.
It has been pointed out, at the outset of this chapter, that the chronological arrangement of timber roofs does not show their progressive development. Of the three remaining examples of the English timber roof still to be considered, Westminster Hall (1395) is the earliest. The roof of the Exeter Law Library (the date of which is obscure, but which is certainly later) and Gainsburgh Hall, completed in 1484, would follow in order, but to adopt this method would involve taking the most complicated and the largest timber roof in existence and to descend from this to the comparatively simple type of Gainsburgh Hall. The latter, also, is a timber-framed building, and problems of roof construction can be solved by means not possible in the case of walls of stone or brick.
The roof of the Exeter Law Library, Figs. 87, 88 and 89, has every appearance of being copied from Hugh Herland's great roof in Westminster Hall. Similar winged struts is covered by a boss carved with the representation of a human head. From behind this sub-principal, which is in the form of a large flattened arch-brace (see Fig. 89), two other braces, with traceried spandrels, carry down from the great purlin to the hammer-posts, at some distance from the hammer-beam, joining others which rise to the apex of the great arch-rib (see Fig. 87).
Fig. 84. Sectional Diagram Of Needham Market Roof With Stresses Indicated. - Ernest R. Gribble, Delt. 91
Fig. 86. Needham Market, Suffolk. - View showing details of hammer-beams, hammer-posts, tie-beams and ashlaring, and carved cornice.
Although obviously designed in imitation of Westminster Hall, this Exeter roof differs largely in its construction from its model. It is framed in a very solid and rather clumsy manner, with heavy baulks of timber, and lacks the grace and scientific devising of the Westminster original.
The roof is carried, mainly, on the huge piece of timber, which contains, in the one piece, the wall-post and the lower section of the inner or large arch-rib. This is tenoned into the principal, and has a solid abutment from which the upper sections of the rib continue. The principal rafter is tenoned into the hammer-post at its upper extremity and at the other end into an extension of the hammer-beam on the wall side of the arch-rib. The hammer-beam proper, being tenoned into the arch-rib on its inner face, has no definite connection with this extension piece, which is fixed by being mortised on to the upper end of the wall-post, held firmly to its tenon by pegs. This false hammer-beam extension piece takes the thrust from the principal rafter. The real hammer-beam is tenoned into the lower section of the arch-rib or the wall-post, - which are here the same, as both are contained in the one solid timber, - and is supported by the lower internal rib-brace, which is tenoned into the hammer-beam at its one end, and into the wall-post at the other.
The main collar-beam - which bridges the hammer-posts at their upper extremities - the upper section of the arch-rib, and the upper rib-braces with their solid abutments are all framed together with tenon-and-mortise joints. The main arch-rib is further reinforced by moulded laminations, with butt joints arranged so as to overlap well those of the rib itself. These laminations are secured to the rib by wooden pegs. Both the common rafters and the ashlaring are concealed behind the plastering between the bays. Above the collar is the typical Western form of waggon ceiling which has already been described.