HEAVY TIMBER 



of girders to be used. In many cases this dimension is regu- 

 lated by the local building ordinances, and an inspection of 

 such ordinances will show that sizes vary in different communi- 

 ties. From the standpoint of fire protection, the smallest roof 

 girder that should be used is at least 6 inches in either dimen- 

 sion, but the common building regulation is that such girders 

 should be at least 6 inches thick and have a cross-section area 

 of at least 72 square inches. This same regulation in regard 

 to minimum sizes should apply to beams in semi-mill construc- 

 tion, as well as to the girders in standard construction. 



The size of the bays in the roof will be determined by the 

 size of the bays of the floors below. 



In many cases a roof of semi-mill construction type is 

 placed on a building in which all of the floors are of standard 

 mill construction, or of laminated type. This design is not 

 necessary, since a certain minimum thickness of roof plank is 

 demanded regardless of the load to be carried, and the thick- 

 ness of 3 inches commonly specified is sufficient to support 

 ordinary roof loads with the lengths of span between girders 

 commonly found. A simple calculation will show this point to 

 be true, especially where a good grade of lumber is used. Roof 

 plank 3 inches in thickness may be tongued and grooved, but 

 with a thickness of 4 inches and over splined plank should be 

 used. 



Both roof timbers and planks should be self-releasing at 

 walls, and have exposed surfaces planed. 



Roof One of the most satisfactory types of roofing is of 



Covering the built-up variety, where layers of felt saturated 

 with tar are nailed down, and by the use of asphalt 

 or pitch protected and cemented together. 



The following specification quoted from a Report of the 

 American Railway Engineering Association gives what is con- 

 sidered to be good practice in the construction of a flat built- 

 up roofing on wood sheathing: 



SPECIFICATION FOR FELT, PITCH AND GRAVEL OR SLAG 

 ROOFING OVER BOARDS 



INCLINE. This specification should not be used where roof incline 

 exceeds three (3) inches to one (1) foot* For steeper inclines modified 

 specifications are required. 



* Modern practice tends to reduce this slope to 2 inches to 1 foot. 

 Page thirty-two 



