In roofing a church with a nave and side aisles, the continuity of the tie-beams may be dispensed with, intermediate support being obtained from columns. It is however necessary to guard carefully against any lateral strain to the columns.
Many of the high-pitched roofs of old Gothic churches and halls are very ingeniously contrived, but they often throw great pressure on the walls, owing to the absence or elevated position of the ties; thereby rendering very solid walls and buttresses necessary. The Norman roof is an ingenious but complicated contrivance for the construction of roofs of large span with small pieces of wood. Fig. 21 shows this represents the truss of a truncated roof strengthened by a curved rib, the suspended pieces being, when the rib is formed in the manner first described, placed at each joint, and each consisting of two pieces, one on each side of the rib, notched to it and the beam, and fastened by bolts and straps.
One of the advantages of this mode of construction is that the tie beams may be suspended from any ntunber of points, which is im portant in large spans, where the have to be formed of several pieces scarfed together. [Seaturiso.] Diagonal braces, though un necessary with parabolic curves, may be added to meet accidental strains, as shown by the dotted lines in the cut. This principle of construction, with an arc composed of several pieces of timber, was followed in one of the largest roofs over built—that erected in 1791 over a riding-house at Moscow. The span of this roof, which has been said to be" the most extensive wooden roof in the world, is stated by Tredgold at 235 feet, the slope being about 19', and the external dimensions of the building 1920 by 310 feet. Ile states that it had sunk so much that it was proposed to add a second curve for additional strength.
A simple and economical roof, invented by Mr. A. 11 llehlaworth, and rewarded by the Society of Arta in 1S20, is supported by curved ribs of timber applied in a different manner. A detailed description is given in the 38th volume of the Society's ' Transactions; ' but fig. 19 will sufficiently explain the principle of its construction : a is a beam serving as n tie-beam, and also to support the upper floor of the build ing; bb are curved ribs, formed in a similar manner to those just described, the lower ends of which are firmly secured to the tie-beam 41. The principal rafters rest on these ribs, and their lower ends bear upon short timbers resting on the walls, these pieces being fastened by strong iron straps to the curved ribs, to counteract the outward thrust of the rafters. By this arrangement the whole of the interior of the roof, which is usually encumbered with king-posts, queen-posts, braces, kc., is rendered available for useful purposes, in addition to which it
effects a considerable saving of timber.
Wrought-iron straps of various forms are very useful, when judi ciously applied, in strengthening the joints of a roof. They should be fixed with regard to the unavoidable tendency of the timbers to shrinking, so that while they may, in some cases, counteract or lumen its effect, they may so far yield to it as to prevent a strain which should come upon a timber, being entirely thrown, by its alteration of form, upon the strap. Tie-beams are often suspended to the trussing arrangement, in which all the rafters abut on joggled king-posts, of which there are several, their relative position being maintained by diagonal braces. The timbers of this kind of roof are often left visible, being so carved as to have an ornamental effect. Such a roof may be made to exert very little injurious pressure on the walls.
When the space covered in is of an irregular shape, it is best to arrange the inclined planes of the roof in a similar manner to those of a rectangular building, leaving a level platform in the centre, corre sponding to the plan of the inclosed space. Where the space covered is circular, elliptical, or polygonal, although the construction of the roof may appear more complicated to the eye, it is. in fact, simpler and easier than that of a quadrangular building, the strain of the roof being more equally distributed. The nearer a roof approaches to a circle in plan, the stronger it will be, the parts deriving that mutual support from each other which forms the distinguishing character of the dome. Domes of wood, of great size, have been made without trussing, simply by forming the timbers into curved ribs abutting on the wall-plates, which then form n circle, and are kept in their proper positions by hori zontal circles framed with them at intervals. As the ribs approach the upper part of the dome, the intervals between them diminish in width, to allow for which every second or third rib is discontinued at intervals, the ends of the ribs thus discontinued being received by the horizontal circles, which may be compared to purlina, the ribs taking the place of rafter& The wooden dome formerly existing at the Halle. aux Blu5s, at Paris, was a remarkably bold example of this kind, being 200 feat in diameter, and having a large opening in the centre. It waa built at the suggestion of 31. Moulineau, and, having been destroyed by fire, has been replaced by i1 similar structure of iron, but of smaller dimensions.