The Viaduct Works' Handbook

63 with an open gable. Therefore it will be necessary to add 15 per cent, to the price of the roof for the extent of the hip, a regular hip being the width of the roof by half the width; or for two hips, the square of the span gives the area to be charged with this addi- tional per centago. If, instead of « hipping,” the ends of the roof are filled in ver- tically with galvanized corrugated iron, the additional cost, in ordi- nary cases, may be taken at about 1s. the square foot. The hip may be considered preferable in many cases, as it offers less surface to the wind, and adds to the strength of the roof, by acting as bracing to the whole structure, and is better in point of effect and compactness. It may be well to state, that in giving prices of roofs above, the details are based upon the supposition, that the wind may possibly produce a stress of at least 30 lbs. on the square foot, and 23 lbs. per square foot is allowed for the weight of framing, covering, and congregated snow, making 53 lbs. per square foot, and in no case is the iron strained above 5 tons per square inch of section. The following table of the weight of different kinds of covering may be found useful:— Lead covering weighs about 7 cwt. per square of 100 superficial feet. Zinc „ » 11 » » Corrugated iron „ 3% » » Slates. . . . 6 cwt. to 9 ewt. Tiles . . . about 12 cwt. » Boarding, 11 thick „ 41 cwt. » Slate and tile roofs require a slope of from 25° to 30° as the minimum ; the greater the slope the less liability of rain being driven under them by the force of the wind. The accompanying sketch illustrates an example of a somewhat larger span roof made at Crumlin, and erected by Messrs. Kennard, for a railway station in Spain; it is 80 feet span, 232 feet long, covered with corrugated iron, and has a skylight of glass about 14 feet wide on each side the entire length; the ends are filled up with an ornamental arrangement of plain and coloured glass. The total cost of such a roof may be estimated at about £2,000.