120 Roofs for Farm-Buildincjs. 



The objection to iron as a covering, however, does not apply 

 in the same degree to arched corrugated iron, where the strength 

 thus given to the material renders the employment of trusses 

 unnecessary. It has therefore been much recommended on the 

 score of lightness and cheapness. But experience has proved 

 that it is not durable, particularly when used as covering for 

 buildings where stock are confined ; in such cases the corrosion 

 which ensues is, no doubt, in a great measure caused by the 

 vapours which arise from all animals, and from their manure, 

 which influences even galvanism is powerless to resist. 



The weight that a roof has to support is of two kinds : one 

 constant, and the other variable. The first is the weight of the 

 roof itself, and the second consists of the external pressure of wind 

 and rain, and the weight of snow when it occurs. The allowance 

 due for wind is taken at about 7 or 8 lbs. per square foot, and 

 for snow at about 5 or 6 lbs. Altogether the weight of the roof, 

 together with the variable pressure of wind, rain, &c., varies 

 from about 23 lbs. as a minimum, to 40 lbs. as a maximum 

 per square foot, the latter is commonly taken as the basis of 

 calculation.* 



The scantlings of the timbers of roofs, sanctioned by the 

 Inclosure Commissioners for England and Wales have been 

 adopted in the several illustrations and comparisons given in 

 this Essay, and as they form the rule for all landowners, who as 

 tenants for life desire to erect farm-buildings, and charge their 

 estates with the cost, it has been considered well to give the 

 Commissioners' minutes verbatim. 



" In all cases where fir timber is used, that obtained from 

 Memel or Norway, and battens from Dram, St. Petersburg, 

 or other Norway or Baltic ports, is to be preferred. 



" All oak used to be of English growth. 



" No timber to be placed nearer to the inside of any flue than 

 one foot, 



" All timbers to be cut die square, and to hold the scantlings 

 specified when finished. 



* M. Mathieu, a French engineer, in the ' Annales de la Construction ' for 

 •1863, malces the following calculations on the same point : — 



'' If a roof be of slate, we may reckon that the maximum pressure under the 

 most unfavourable circumstances amounts to 125 kilos, per square metre, dis- 

 tributed as follows : — 



Kil. 



Weight of slates, laths, &c -ir) 



,, of framework of roof, &c. 45 



,, of snow 25 



Pressure (ordinary) of wind 20 



125." 



This is equal to nearly 28 lbs. per square foot. 



