The above prices are figured on the tile being laid on wooden sheathing; if laid on book tile or cement add 20 per cent.
If copper nails are used, care must be taken in figuring the number of nails, as well as their length and gauge, for the special forms of tile specified. Fluctuating values of copper make this an item of much importance.
Ridges, hip rolls, barge tile, and finials are charged as extras and due allowance must be made for cutting at valleys and hips.
63. Gravel Roofs.—In gravel roofing, the cost per square depends on the number of thicknesses of tarred felt and the quantity of pitch used per square. A value of 4 cents per square foot for four thicknesses may be considered an average.
ROOF MENSURATION
64. While a knowledge of how to apply the ordinary principles of mensuration is all that is necessary to calculate any roof area, yet the modern house, with its numerous gables and irregular surfaces, introduces complications that render some further explanation of roof measurement desirable. The most common error made in figuring roofs—and one that should be carefully guarded against—is that of using the apparent length of slopes, as shown by the plan or side elevations, instead of the true length, as obtained from the end elevations.
Fig. 6
65. The area of a plain gable roof, as shown in end and side elevations in [Fig. 6], is found by multiplying the length g j by the slope length b d, and further multiplying by 2, for both sides. The area of each gable is found by multiplying the width of the gable a d by the altitude c b, and dividing by 2.
