The following are the scantlings of the various timbers : Inches Scantling.
AA, The tie-beam, 57 feet long, the span of the walls being 51 feet 14 x 12 B, Queen-posts 9 X 12 c, Braces 9 X 7 D, Straining-beam 10 X 7 E, Straining-piece 6 X 7 F, Principal rafters 10 X 7 o, Camber-beam, for the platform 9 X 7 n, An iron rod, supporting the tic-beam . . 2 x 2 The trusses are seven feet clear ; the platform is covered with lead, which is supported by horizontal beams six by four inches.
The timbers of this truss are well disposed, and perhaps contain less wood than most roofs of the same dimensions.
Figure 2 is the roof of St. Paul's, Covent - Garden, designed by Mr. Hardwick, and constructed by Mr. \Vapshot in 1796.
This roof, although of the same general construction as the last, varies from it in several particulars. The lower portion is precisely the same as the second class of truss, but in addi tion there is a second pair of principals, which are supported on the lower by studs, and the lower principals thus becoming only auxiliaries ; the queen-posts are continued up to the principals, and a king-post is carried from the apex to the straining-beam.
The scantlings are : Inches Scantling.
AA, The tie-beam, spanning 50 feet 2 inches . 16 X 12 B, Queen-posts 9 X 8 c, Straining beam 10 X D, King-post, 14 inches at the joggle . . 9 X 8 E, Strut 9 X S F,F, Auxiliary rafters, at bottom 10 x n,H, Principal rafters, at bottom 10 X 81 r, Studs supporting the principals . . . x This roof is much better constructed than the original one by Inigo Jones. A truss of the present design contains only 98 cubic feet of timber, whereas that by kip Jones had 273, and was very insufficient at the joggles, and had some of its timbers very ill disposed : the interior truss is well con trived for supporting the exterior, which reaches seven feet beyond the walls. The tie-beam has, perhaps, too much cam ber, being six inches ; for since it acts as a string, it will lengthen in the settling of the roof.
Figure 3 represents the roof of Drury-lane theatre, SO feet 3 inches span, and the trusses 15 feet apart ; constructed by Mr. Edward Gray Saunders. This was destroyed by fire in 1S09.
This is rather a curious form of roof ; the principal truss is simply one of the second class with queen-posts and strain ing-beam, but above this, and partially resting upon it, are three other smaller trusses, which finan the outer-roof.
These are simple trusses of the first class with king-post and struts, the central one having a continuous bearing on the straining-bcain of the main truss, and the side ones resting at one end on the straining-beam, and at the other on the wall.
The principal beams are trussed in the middle space with oak braces, five inches square. This was requisite on account of its width, which is 32 feet, that the floors might carry the work-shops necessary for the use of the theatre. This truss, which is most admirably constructed, is hardly to be equalled for strength, stiffness, and lightness, and will safely bear a load of nearly 300 tons, which is four times more than ever it is likely to be loaded with.
Plate VII. Figure 1, exhibits the roof of Covent Garden theatre. The tie-beam in this case is supported by five sus penders, a king-post in the centre, and two queen-posts on either side ; between the two innermost queen-posts is a straining-beam, and on the other sides of them are auxiliary rafters, which abut against a shoulder near their heads. A strut is carried from the foot of the inner queen-post to the head of the outer. The straining-beam is suspended by two queen-posts, between which is another straining-piece, and a strut from their head to the angle formed by the king-post and straining-beam below. The posts are all made double, and are shown in the Plate above the roof in their respec tive places.
Figure 2 represents the present roof of Drury-lane theatre. There are here both principal and auxiliary rafters, the tie beam being suspended at two points from the fbrmer, and at two from the latter, the two first queen-posts being the inner ones. These are kept apart by a straining-beam, against which they are pressed from the other side by the auxiliary rafters ; struts are placed between the feet of the principal and the head of the secondary queen-posts, and the bearing of the sub-rafters is still further reduced by a strut from the foot, and on the other side of the smaller queen-posts. The straining-beam is supported by a king-post from the apex of the principals, which in their turn are supported by struts from the foot of the king-post, the other portion having a continuous bearing on the auxiliary rafters.