The Viaduct Works' Handbook

59 better for roofing over furnaces. Galvanized iron forms a very light covering, and its lightness and portability render it very suitable for export, whilst its rigidity when corrugated imparts great strength to the framing when properly secured to it. No. 16, B. W. G., corrugated iron is the thickness most generally used, and weighs about 350 pounds per square. Galvanized iron and zinc should not be used in situations much exposed to air impregnated with the gases arising from combustion of coal, as, for example, on the roof over a retort house of a gas works; the galvanized iron will decay very rapidly in such a situation by the decomposition of the zinc. Painted cor- rugated iron is very much used because it is cheaper than galvanized iron, and, when in pure air and kept well painted, is found to form a tolerably good and durable covering, but should be used somewhat thicker than galvanized iron. The accompanying sketcli represents the form of framework or “truss ” generally used for ordinary roofs up to about 60 feet span, the pitch or rise for slates being about 2 to 1. In the fol- lowing table of approximate prices for roofs of this class, the price is given for a square of 100 superficial feet, measured up tlie slope of the rafter. In calculating the cost of a roof from this, without drawings, a near approximation may be obtained by taking the outside width of the building over the gutters and adding 10 per cent.; this multiplied by the length of building will give the number of superficial feet, and divided by 100 gives the number of squares. Example:—A building is to be covered which is 300 feet long, 40 feet wide, and the gutters project 6 inches each side; therefore 300 x 41 = 12,300, add 10 per cent. = 13,530, divide by 100 = 135’3 squares.