flooring, as there are no principals, purlins, or even ridge. It is a defective form of roof, as the rafters have a tendency Fig. 59. to spread and thrust out the walls. In the example before us, this effect has been prevented by the insertion of tie-beams, from which the collars have been propped up (fig. 59), thus, in fact, balancing the roof on the centres of the collars, which are in consequence violently strained.

a post; d sill; cc struts.

120. After the introduction of the four centered arch, a great many church roofs of the construction just described were altered, as shown by the dotted lines in fig. 60, in order to obtain more light by

Fig. 60.

the introduction of clerestory windows over the nave arches. The flat roofs, which superseded the former ones, were often formed without any truss whatever, being simply an arrangement of main beams, purlins, and rafters, precisely similar to a double-framed floor, with the

difference only that the main beams, instead of being perfectly straight, were usually cut out of crooked timber so as to divide the roof into two inclined planes.

Fig. 61.

To throw the weight of the roof as low down as possible, the ends of the main beams are often supported on upright posts placed against the walls and resting on projecting corbels, the wall posts and beams being connected by struts in such a way that deflection in the centre of the beam cannot take place, unless the load be sufficient to force out the walls, as shown by the dotted lines in fig. 61

The struts are often cut out of stout plank, forming solid spandrils, the edges of which are moulded to suit the profile of the main beam (see fig. 62), which also shows the man

Fig. 62.

ner of securing the struts to the wall posts and to the beam with tongues and wooden pins.

121. Fig. 63 exhibits a construction often to be met with, which, in general appearance, resembles a trussed king post roof, but which is in reality very different, the tie beam being a strong girder supporting the king post, which, in

Fig. 63.

stead of serving to suspend the tie-beam from the principals, is a prop to the latter. In this and the previous example, any tending to deflection of the tie-beam is prevented by struts: the weight of the roof is thrown by means of wall posts considerably below the feet of the rafters, so that the weight of the upper part of the wall is made available to resist the thrust of the struts.

122. The roofs we have been describing are not to be recommended as displaying any great amount of constructive skill. Indeed, although they answer very well for small spans with timbers of large scantling and side walls of suffcient thickness to resist a considerable thrust, they are totally unsuited to large spans, and are in every way inferior to trussed roofs.

The above remarks do not apply to the high pitched roofs of the large halls of the fifteenth and sixteenth centuries, which, for the most part, are trussed in a very perfect manner, so as to exert no thrust upon the walls; although, in some instances, as at Westminster Hall, they depend upon the latter for support.

The general design of these roofs is shown in figs. 64 and

Fig 64.

ga.TENSION BEAM
bb. STRUT

65. The essential parts of each truss are, a pair of principals connected by a collar or wind beam, and two hammer beams, with queen posts over them, the whole forming three triangles, which, if not secured in their relative positions, otherwise than by the mere transverse strength of the principals, would turn on the points c c (fig. 65), the weight of the roof thrusting out the walls in the manner shown in the figure. There are two ways in which a truss of this kind

may be prevented from spreading. 1st, The ends of the hammer beams may be connected with the collar by tension pieces, a a (fig. 64), by which the thrust on the walls will be converted into a vertical pressure. 2d, The hammer beams may be kept in their places by struts, bb, the walls being made sufficiently strong by buttresses, or otherwise, to resist the thrust.

In existing examples, we find sometimes one and sometimes the other of these plans followed; and occasionally both methods are combined in such a manner that it is often difficult to say what parts are in a state of compression, and what are in a state of tension.

123. The roof of the great hall at Hampton Court (fig. 66) is very strong, and so securely tied, that were the

Fig. 66

bottom struts, bb, removed, there would be little danger of the principals thrusting out the walls; and, on the other hand, from the weight of the roof being carried down to a considerable distance below the hammer beams by the wall posts, the walls themselves offer so much resistance to side thrust, that there would be no injurious strain on them were the tension pieces, a a, removed.

124. The construction of the roof of the hall at Eltham Palace, Kent (fig. 67), differs very considerably from that

of the Hampton Court roof. The whole weight is thrown on the top of the wall, and the bottom pieces, bb, are merely ornamental, the tension pieces, a a, forming a complete tie.