248 ROOF COVERINGS 



earthy appearance will absorb water and is liable to be destroyed by 

 the frost. 



Roofing slates are usually made from J/ to % inches thick; 3-16- 

 inch being a very common thickness. Slates vary in size from 6" x 12" 

 to 24" x 44"; the sizes varying from 6" x 12" to 12" x 18" being the 

 most common. 



Slates are laid like shingles as shown in Fig. 130. The lap most 

 commonly used is 3 inches ; where less than the minimum pitch of y^ is 

 used the lap should be increased. 



The number of slates of different sizes required for one square of 

 100 square feet of roof for a 3-inch lap are given in Table XIX. 



The weight of slates of the various lengths and thicknesses required 

 for one square of roofing, using a 3-inch lap is given in Table XX. 

 The weight of slate is about 174 pounds per cubic foot* 



The weight of slate per superficial square foot for different thick- 

 nesses is given in Table XXI. 



The minimum pitch recommended for a slate roof is l / ; but even 

 with steeper slopes the rain and snow may be driven under the edges 

 of the slates by the wind. This can be prevented by laying the slates 

 in slater's cement. Cemented joints should always be used around eaves, 

 ridges and chimneys. 



Slates are commonly laid on plank sheathing. The sheathing should 

 be strong enough to prevent deflections that will break the slate, and 

 should be tongued and grooved, or shiplapped, and dressed on the upper 

 surface. Concrete sheathing reinforced with wire lath or expanded 

 metal is now being used quite extensively for slate and tile roofs, and 

 makes a fireproof roof. Tar roofing felt laid between the slates and 

 the sheathing assists materially in making the roof waterproof, and 

 prevents breakage when the roof is walked on. The use of rubber- 

 soled shoes by the workmen will materially reduce the breakage caused 

 by walking on the roof. Roofing slates may also be supported directly 

 on laths or sub-purlins. The details of this method are practically 

 the same as for tile roofing, which see. 



