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STEEL ROOF TRUSSES AND MILL BUILDINGS. 



CHAP. I. 



Slates are laid like shingles as shown in Fig. 23. The lap most commonly used is 3 inches; 

 where less than the minimum pitch of \ is used the lap should be increased. The number of slates 

 of different sizes required for one square of 100 sq. ft. of roof for a 3-in. lap are given in Table II. 

 The weight of slates of the various lengths and thicknesses required for one square of roofing, 

 using a 3-in. lap is given in Table III. The weight of slate is about 174 Ib. per cu. ft. The weight 

 of slate per superficial sq. ft. for different thicknesses is given in Table IV. 



TABLE II. 

 NUMBER OF ROOFING SLATES REQUIRED TO LAY ONE SQUARE OF ROOF WITH 3-lN. LAP. 



TABLE III. 

 THE WEIGHT OF SLATE REQUIRED FOR ONE SQUARE OF ROOF. 



TABLE IV. 

 WEIGHT OF SLATE PER SQUARE FOOT. 



The minimum pitch recommended for a slate roof is J; 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 or grooved, or shiplapped, and 

 dressed on the upper surface. Concrete sheathing reinforced with wire mesh, expanded metal 

 or rods is now being used quite extensively for slate and tile roofs, and makes a fireproof roof, see 



