This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.

Fig. 284.

The feet of the principal rafters are inserted in cast-iron shoes, which are fixed to stone templates.

When the span exceeds 30 feet a queen post truss is the most suitable wooden truss. It may be used for spans up to 40 feet. Plate VI. illustrates clearly the construction of this type of roof. It will be noticed that each post receives a thrust both at the head and foot, from one side only, and must be supported upon the other side, for which purpose Straining Sills and Straining-Beams are used.

The purlins are spaced so as to divide the common rafters into three equal parts, and the upper one is supported upon a cleat fixed to the straining beam.

The construction of all the joints are clearly shown in the Plate.

By multiplying the number of posts and struts, roofs may be constructed over very great spans. At one time this was commonly done, but such large roofs are now almost invariably made of iron, and will be therefore considered in a later Volume.

When it is desired to have a room in the roof it is either necessary to have a very steep pitch to the roof, or to form it as shown in Fig. 285. Such a roof is called a Mansard roof, after its inventor, a French architect of the seventeenth century. It is formed by supporting a king post truss upon a queen post truss, the tie beam of the former forming the straining beam between the queen posts. The advantage of this type of roof lies in the saving of the unnecessary space that would be required for a steep pitched roof giving the same sized room between its rafters.

The method of constructing a hipped roof is clearly shown in Fig. 286, where a half-truss is fixed at right angles to the last truss of a series, to support the centres of the purlins of the hipped end. The tie of the half-truss is notched over a fillet fixed to the side of the last truss, to which it is secured by a bolt. The principal rafter of the half-truss abuts against the head of the king post of the last truss, and is securely bolted to it as shown in the detail. Stout rafters, called Hip Rafters, form the angles of the hips, the upper ends being fixed to the head of the last king post by means of bolts, as shown in detail, and the lower ends tied in by means of a Dragon Tie, as shown in Fig. 286.

Fig. 285.

The following table will serve as a guide when proportioning roof timbers, and may be followed absolutely when the roof is hidden; but when an open roof is required the timbers should all be of the same width, so that the faces of all the timbers may be in one plane, as shown in all the illustrations of trussed roofs: -

Fig. 286.

Fig. 287.

Trusses, 10 feet apart; Pitch, 27 degrees; Slate covering-; Timber, Baltic fir.

Span in Feet. | Tie Beam. | King Post. | Queen Post. | Small Queens. | Principal Rafters. | Straining Beams. | Purlins. | Common Rafters. | |||||||||||||||||||

Inches. | Inches. | Inches. | Inches. | Inches. | Inches. | Inches. | Inches. | Inches. | |||||||||||||||||||

20 | 9 1/2 | x | 4 | 4 | x | 3 | .. | .. | 4 | x | 4 | .. | 3 1/2 | x | 2 | 8 | x | 4 3/4 | 3 1/2 | x | 2 | ||||||

22 | 9 1/2 | x | 5 | 5 | x | 3 | .. | .. | 5 | x | 3 | .. | 3 3/4 | x | 2 1/4 | 8 1/4 | x | 5 | 3 3/4 | x | 2 | ||||||

24 | 10 1/2 | x | 5 | 5 | x | 3 1/2 | .. | .. | 5 | x | 3 1/2 | .. | 4 | x | 2 1/2 | 8 1/2 | x | 5 | 4 | x | 2 | ||||||

26 | 11 1/2 | x | 5 | 5 | x | 4 | .. | .. | 5 | x | 4 1/4 | .. | 4 1/4 | x | 2 1/2 | 8 3/4 | x | 5 | 4 1/4 | x | 2 | ||||||

28 | 11 1/2 | x | 6 | 6 | x | 4 | .. | .. | 6 | x | 3 1/2 | .. | 4 1/2 | x | 2 3/4 | 8 3/4 | x | 5 1/4 | 4 1/2 | x | 2 | ||||||

30 | 12 | x | 6 | 6 | x | 4 1/2 | .. | .. | 6 | x | 4 | .. | 4 3/4 | x | 3 | 9 | x | 5 1/2 | 4 3/4 | x | 2 | ||||||

32 | 10 | x | 4 1/2 | .. | 4 1/2 | x | 4 | .. | 5 | x | 4 1/2 | 6 3/4 | x | 4 1/2 | 3 3/4 | x | 2 1/4 | 8 | x | 4 3/4 | 3 1/2 | x | 2 | ||||

34 | 10 | x | 5 | .. | 5 | x | 3 1/2 | .. | 5 | x | 5 | 6 3/4 | x | 5 | 4 | x | 2 1/2 | 8 1/4 | x | 5 | 3 3/4 | x | 2 | ||||

36 | 10 1/2 | x | 5 | .. | 5 | x | 4 | .. | 5 | x | 5 3/4 | 7 | x | 5 | 4 1/4 | x | 4 | 8 1/2 | x | 5 | 4 | x | 2 | ||||

38 | 10 | x | 6 | .. | 6 | x | 3 3/4 | .. | 6 | x | 6 | 7 1/4 | x | 6 | 4 1/2 | x | 2 1/2 | 8 1/2 | x | 5 | 4 | x | 2 | ||||

40 | 11 | x | 6 | .. | 6 | x | 4 | .. | 6 | x | 6 | 8 | x | 6 | 4 1/2 | x | 4 | 8 3/4 | x | 5 | 4 1/4 | x | 2 | ||||

42 | 11 1/2 | x | 6 | .. | 6 | x | 4 1/2 | .. | 6 1/4 | x | 6 | 8 1/4 | x | 6 | 4 1/2 | x | 2 3/4 | 8 3/4 | x | 5 1/4 | 4 1/2 | x | 2 | ||||

44 | 12 | x | 6 | .. | 6 | x | 5 | .. | 6 1/2 | x | 6 | 8 1/2 | x | 6 | 4 1/2 | x | 3 | 9 | x | 5 | 4 3/4 | x | 2 | ||||

46 | 12 | x | 6 1/2 | .. | 6 | x | 5 1/2 | .. | 7 1/2 | x | 6 | 9 | x | 6 | 4 3/4 | x | 3 | 9 | x | 5 1/4 | 5 | x | 2 | ||||

48 | 11 1/2 | x | 6 | .. | 6 | x | 5 3/4 | 6 | x | 2 1/4 | 7 1/2 | x | 6 | 8 1/4 | x | 6 | 4 1/2 | x | 2 3/4 | 8 1/2 | x | 5 | 4 | x | 2 | ||

50 | 12 | x | 6 | .. | 6 | x | 6 1/4 | 6 | x | 2 1/2 | 8 1/3 | x | 6 | 8 1/2 | x | 6 | 4 1/2 | x | 2 3/4 | 8 3/4 | x | 5 1/4 | 4 1/4 | x | 2 | ||

52 | 12 | x | 6 1/2 | .. | 6 | x | 6 3/4 | 6 | x | 2 3/4 | 9 1/4 | x | 6 | 8 3/4 | x | 6 | 4 3/4 | x | 2 3/4 | 8 3/4 | x | 5 1/4 | 4 1/4 | x | 2 | ||

54 | 12 | x | 7 | .. | 7 | x | 6 1/4 | 7 | x | 2 1/4 | 6 1/3 | x | 7 | 9 | x | 6 | 4 3/4 | x | 2 3/4 | 8 3/4 | x | 5 1/4 | 4 1/2 | x | 2 | ||

56 | 12 | x | 8 | .. | 7 | x | 6 3/4 | 7 | x | 2 1/2 | 7 1/2 | x | 7 | 9 1/4 | x | 6 | 5 | x | 2 3/4 | 8 3/4 | x | 5 1/4 | 4 1/2 | x | 2 | ||

58 | 12 | x | 8 1/2 | .. | 7 | x | 7 1/4 | 7 | x | 2 3/4 | 8 1/4 | x | 7 | 9 1/2 | x | 7 | 5 | x | 2 3/4 | 9 | x | 5 1/4 | 4 3/4 | x | 2 | ||

60 | 12 | x | 9 | .. | 7 1/2 | x | 7 | 7 | x | 3 | 9 | x | 7 | 10 | x | 7 | 5 | x | 3 | 9 | x | 5 1/2 | 4 3/4 | x | 2 |

Beam or Wind Filling is the name given to the brickwork carried up between the rafters in the roof boarding to exclude the wind and rain, as shown in Figs. 283 to 285.

Roofs with curved Surfaces are formed as shown in Fig. 287, the rafters being either cut away or firred up to the required curve. By firring up it is meant that pieces of timber, the same width as the rafters, are cut to the desired curve and spiked to them.

192.

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