Roofs for Farm-Bidldinfjs. 127 



for application ; tliis is better shown by a practical instance. For 

 example, in Fig. 24, Sheet 5, a roof of 50 feet span shows a 

 timber strut 4 feet long. This member contains 1152 cubic 

 inches, weighs 23 lbs,, and would cost when worked and fixed 

 2s. 2d. ; a strut of cast iron of equal strength, but containing 

 only 366 cubic inches, would weigh 96 lbs., and would cost, 

 taking the lowest price of casting with fixing, 12^. Hence, it 

 follows that economy is better displayed by the use of wood than 

 cast iron, though so much more material is used. If wc consider, 

 too, that the risk of an accident from an imperfect casting or a 

 sudden change in the temperature, which may compromise the 

 whole structure, is much greater than if wood were used, we 

 show sufficient reasons why the employment of cast iron should 

 not supersede that of wood for struts in the framework of roofs 

 which do not exceed the ordinary limits of agricultural buildings. 

 For the smaller parts and adjuncts of a truss, however, such as 

 rafter-shoes and junctions, which are many times used in the 

 same form in any considerable length of roofing, and which are 

 better moulded and more economically made of cast-iron than 

 of any other material, the use of that metal is unquestionably 

 desirable ; but the preference of cast iron ends with these minor 

 parts of the roof. 



Wrought iron, though not economically admissible in small- 

 spanned roofs, has much to recommend it in connection with 

 timber in roofs of larger spans than 30 feet, for those members of 

 a truss which serve to resist tensile strain, cast iron being used in the 

 minor parts before enumerated, and timber for those members in 

 which rigidity is essential. The advantage of using wrought iron 

 instead of timber for tie and suspending rods will be understood 

 when it is stated, that while Baltic fir will only bear a straining 

 force of 2500 lbs. per inch, wrought iron may be safely strained 

 by 50,000 lbs. per inch, as stated by Tredgold ; or by 60,000 lbs., 

 according to Fairbairn, which shows the superior strength of iron 

 to be from 20 to 24 to 1 as compared with wood. Hence, where 

 rigidity is not required, and the width of the span is sufficient to 

 admit of its adoption with economy, wrought iron may take the 

 place of wood in proportion of 1 for 20 in size ; and as iron is 

 only 13^ times heavier, and only 9 times dearer than wood of 

 equal bulk, the advantage both of weight and cost is in favour 

 of iron : but these figures apply to the tension-rods only. When 

 the rafter-shoes and junctions are taken into consideration, the 

 amounts of weight and cost are altered. An examination of 

 the examples given will show the actual cost of different forms. 



The illustrations given on Sheet 4, of timber and iron roofs 

 for 20 feet spans, outside measurement, are four in number, three 



