254 ROOF COVERINGS 



Care should be used in laying sheet steel roofing to see that it does 

 not come in contact -with materials containing acids, and it should be 

 kept well painted. The weight of flat steel of different gages is given 

 in Table XV. Nos. 26 and 28 gage sheets are commonly used for sheet 

 steel roofing. No. 26 black sheet steel was quoted in 1903 at about 

 $3.20 per 100 pounds, and No. 26 galvanized sheet steel at about $4.00 

 per loo pounds in small lots. Sheet steel roofing can be laid at a 

 somewhat less cost than tin roofing. 



Gravel Roofing. Gravel roofing is made by laying and firmly 

 nailing several layers of roofing felt on sheathing so as to break joints 

 from 9 to 12 inches ; the laps are mopped and cemented together with 

 roofing cement or tar, and finally the entire surface is covered with a 

 good coating of hot cement or tar. The cement or tar should not be 

 hot enough to injure the fibre of the felt. While the cement or tar 

 is still hot the surface of the roof is covered with a layer of clean gravel 

 that has been screened through a ^-inch mesh. It requires from 8 to 

 10 gallons of tar or cement and about V(> of a yard of gravel per square 

 of 100 square feet of roof. When the roof is to be subjected to the 

 action of corrosive gases it should be flashed with copper or composi- 

 tion, or the flashing may be made of felt. The number of layers of felt 

 varies with the conditions, but should never be less than four (4-ply). 



The details of laying gravel roofs differ and it is impossible to 

 do more than give a few standard specifications. The following specifi- 

 cations are about standard in the West. In writing specifications for 

 four-ply gravel roofing omit one layer of roofing felt in the specifications 

 for five-ply gravel roofing. Three-ply roofing is sometimes used for 

 temporary structures. 



Five (5) Ply Wool Felt, Composition and Gravel Roof. First 

 cover the sheathing boards with one (i) layer of dry felt and over 

 this put four (4) thicknesses of wool roofing felt, weighing not less than 

 fifteen (15) pounds (single thickness) to the square of one hundred 

 (100) feet. This felt to be smoothly and evenly laid and well cemented 

 together the full width of the lap, not less than nine (9) inches between 

 each layer, with best roofing cement or refined tar, using not less than 



