124 



Roofs for Farm-Buildimis. 



more expensive, but still of moderate cost. Tliis form of truss 

 lias been much objected to on the ground that its construction 

 is contrary to scientific principles ; but its cheapness of cost, in 

 additit)!! to the fact that it gives head-room, has often led to 

 its adoption. This roof will be found to cost 4Z. 7^. 6(Z. per 

 square. 



The examples of roofs of 24 feet span, inside measurement, 

 will be found to cost as below : — 



As I have stated that all roofs of a span not exceeding 30 feet 

 in width are better constructed of timber than of iron, it might be 

 assumed that this rule applied to roofs of double-storied build- 

 ings, without exception ; but this is not exactly the case, as there 

 are instances in which iron suspending-rods may be advan- 

 tageously used in supporting the floor from the trusses of a 

 timber- framed roof. 



The following example of a granary, on the late Lord Pal- 

 merston's Broadlands estate, shows how this idea has been 

 carried out (see p. 125). 



Again, a timber roof, formed by arched trusses of laminated 

 deals, might be given as an exception, in which the use of iron 

 may be advantageously limited to two iron rods, which tie the 

 ends of the trusses, and at the same time support the floor. This 

 roof and floor, shown on Sheet 2, Fig. 9, by cross and longi- 

 tudinal sections, are specially applicable to granaries in which 

 head-room is required ; and may be compared with a granary 

 roof on the same sheet. Fig. 10, of a queen-post construction, 

 made wholly of wood, where the tie-beam also forms the floor, 

 which will be found to cost less than the former example by 

 about 10s. per square (10 x 10) of ground covered. 



Beyond a span of 30 feet, the requisite strength is obtained, 

 when timber only is used, by scantlings, v/hich not only give an 

 exceedingly heavy and objectionable appearance to the roof, but 

 are practically so much more heavy than wrought iron of equal 

 strength as to require walls of a greater thickness, and therel'ore 



