235. For spans that exceed 65 feet the truss adopted in the construction of the old Drury Lane Theatre, in 1793, is, in respect to form, perhaps one of the best of its kind that can be devised where a large open space is required. * Plate VI. shows a roof on the same principle, of which the scantlings may be obtained from the Table No. 10 at the end of the volume. One part of the principal truss is shown with a queen post, the other with suspending pieces, as described in Art. 254 and in the Section on Joints, etc. The middle part of the principal tie-beam is supposed to be built as a girder.

Plate VI.

ROOFS.

For large spans when a sloping roof is required, the best method is to use small king-post trusses, as in Plate VI., and to support them on a wrought-iron girder, which for a span of 80 feet need seldom be more than 6 feet deep, instead of 18 feet as the truss in Plate VI. Sometimes, however, deep wooden trusses may be used as partitions between the rooms, in which case they are not so objectionable as they otherwise would be.

236. There is much difficulty in executing a roof when the joints are numerous and the timbers of large dimensions; as the shrinkage of the king or queen posts often produces considerable derangements in the truss. It is obvious, that to make principal rafters in a continued series of pieces abutting end to end against one another would remedy these defects. These pieces would then form, a kind of curve, which might be made regular, or left with projecting angles, as shown by Fig. 76. They could either be bolted or mortised and put together with wooden keys, as represented in Fig. 77. The length of the pieces would be determined by the form of the curve; crooked timber where it can be procured would be preferable for the ribs, as the joints should be as few as possible, and they should be crossed like the joints in stone-work.

* A description of the roof of Drury Lane Theatre is given in Nicholson's 'Carpenter's Assistant, p. 60, Plate lxxi.

Fig. 76.

Fig. 77.

Plate VII., Fig. 1, shows a roof constructed in this manner. Each of the supports for the tie-beam marked S, S, etc, consists of two pieces, one put on each side of the rib, and notched both to the rib and to the tie-beam. The pieces are bolted together, as shown by a section to a larger scale, through one of these pairs of suspending pieces, in Fig. 2. This mode of construction admits of a much firmer connection with the tie-beam than is obtained by the ordinary mode, and the number of suspending pieces may be increased at pleasure. The best situation for the suspending pieces is at the joints of the curved rib.

Plate VII.

ROOFS.

Fig 1.

Fig: 2.

The weight of the roof being very nearly uniformly distributed, the form of the curved rib should be a parabola (see Sect. I., Art. 64); and as this curve is easily described with sufficient accuracy for the purpose, it is best to adopt it, because, in that case, the strain from the weight of the roof and ceiling will have no tendency whatever to derange the form of the rib; and its depth will always be sufficient to withstand any partial force to which a roof is likely to be exposed. Consequently, when the rib is of a parabolic form, diagonal braces will not be required; nevertheless they may be added if thought necessary, as shown by the lines in the figure, particularly as they will increase the strength to resist partial strains.

To construct the parabola, let A B, Fig. 78, be drawn for the upper side of the tie-beam, and AC, CB, for the under side of the common or small rafters. Then divide A C and C B each into the same number of equal parts (an even number is to be preferred), and join the points 1 and 1, 2 and 2, etc.; then the curve formed by these intersecting lines will be the parabola required.

Fig. 78.

But it will be found that this curve scarcely differs from a circular arc that rises half the height of the roof; therefore, either may be used.

If a lantern or other structure is to be raised on the top, a hyperbolic curve should be adopted; which admits of a considerable increase of pressure at the crown.

The scantlings of the curved ribs are given in Table No. 12 at the end of the volume. The tie-beam will require to be scarfed for large spans, and would be best made in two thicknesses, and joined so that the scarfs should not be opposite one another.