Chap. XXII.
TIMBER ROOFS.
1331
Queen Post Trusses have two suspending pieces, and are used when the span is above30 feet up to 45 feet; there is an advantage in this species of framing which is not obtainedwhere the king-post is introduced, viz. a passage or space available for many usefulpurposes: where queen-posts are used each is so placed that it bears one-half the span:to find the scantling of one of these suspending pieces of timber, multiply its length infeet by the length of the tie-beam it holds up in feet; and then multiply the product sofound by 0*7 for fir and 0*32 for oak, to obtain the sectional area in inches of the post:
Fig. 2131.
QUEEN POST ROOF.
dividing this area by the thickness in inches, the breadth will be obtained: these rulesapply to the smaller portions of the posts; the heads must be made as small in addition aspossible, as there is then less inconvenience from their shrinking. Carpenters are generallynot disposed to use much iron for their trusses, and to dispense as much as possible withstraps and stirrup-irons : the most ancient method adopted was to wedge them up, andwhere oak was employed, and that well seasoned, it seems fully to have answered thepurpose. The straining pieces between the heads of the queens, in order that its strengthmay be as great as possible, its depth should be to its thickness as 10 to 7 : by mul-
Fig. 2132.
QUEEN POST ROOP WITH STRUTS.
tiplying the square root of the span in feet by the length of the straining piece in feet, andthen extracting the square root of the product, and multiplying it by 0-9 for fir, the depthin inches may be found: to find the thickness, multiply the depth by 0-7.
The Roof of the Teatro Argentino> at Rome , has a span of about 80 feet, and its slope isabout 24°: a short king-post without any side braces receives the upper ends of theprincipals, and by means of an iron suspends or holds up a collar beneath it, which abutsand frames into the heads of two queens : by this arrangement the tie-beam, which isformed of three pieces of timber, is held up at four places. The whole roof is of fir, andadmirably executed. The heads of the queens are strutted on to the ends of the tie-beams,which rest on other timbers laid like corbels on the walls ; the whole is sufficiently strongto hold up any scenes that are required to be suspended to them : there are twelve purlins,which carry the common rafters on each side, and the covering is tile, of considerable weight.
The Roof of the Odeon is of oak, and similar in design to the above : its span is about78 feet, and its angle about 34°, so that the stirrup-irons which hold up the tie-beamsare considerably longer.
The scantling for the principal timbers of queen-post roofs may be thus stated: —
Span.
T.e-beams.
Queens.
Principals.
Struts.
Straining piece.
is. In.
in. in.
in. in.
in. in.
in. in.
35
11 by 4
4 bv 4
5 by 4
4 bv 2
7 by 4
40
12 by 5
5 by 5
5 by 5
5 by 2^
7 by 5
45
13 by 6
6 by 6
6 bv 5
5 by 3
7 by 6
50
13 bv 8
8 by 8
8 by 6
5 bv 3
9 by 9
55
14 by 9
9 bv 8
8 by 7
5.1 bv 3
10 by 6
GO
15 by 10
10 by 8
8 by 8
6 by 3
11 by 6
4 q 2