312 ARITHMETIC 



Rule. To find the number of shingles required to cover a roof, 

 compute the total area of the roof in square inches, and divide 

 this area by the product of the average width of the shingles and 

 the length that is exposed to the weather. 



EXAMPLE. What will it cost to shingle a roof, each 

 side measuring 40 ft. x 16 ft., if the shingles cost $4.50 

 per M? 



SOLUTION. Since the size of the exposed portion is not 

 stated, it will be assumed as 4 in.X4 in. Then, for one side, 



40 X1 ^* 144 = 5.760 shingles will be required, and for both 



4X4 



sides, 5,760X2 = 11,520 shingles. Therefore, the cost will be 

 11.52X34.50 = $51.84. 



Multiply by 144 in order to reduce the square feet (40X16) 

 to square inches. Allowance should also be made for waste. 



MASONRY 



In estimating the cubical contents of stone walls, the perch 

 of 24| cu. ft. is used. 



Rule. To find the number of perches of masonry in a wall, 

 divide the volume of the watt in cubic feet by 24 J. 



In estimating the contents of stone foundations for build- 

 ings, the length of the wall is measured on the outside, thus 

 counting each corner twice. If a wall 2 ft. thick measures 

 12 ft. X 20 ft. on the outside, and the corners are assumed to 

 be parts of the longer sides, there will be 2 walls each 20 ft. 

 long, and 2 walls each 8 ft. long. The actual length is there- 

 fore 2X20+2X8 = 56 ft. The length estimated on the out- 

 side is 2X20+2X12=64 ft. To find the actual length of 

 such a wall, subtract 4 times the thickness of the wall from 

 the length measured on the outside. Thus, in the above case, 

 actual length = 64 -4X2 = 56 ft. 



Usually, masons make no allowance for windows or doors 

 in estimating their work. In estimating the quantity of stone 

 required for the wall, such allowances should be made. 



EXAMPLE. (a) How many perches of stone will be re- 

 quired to build the walls of a church 60 ft. long by 



