8 STANDARD SPECIFICATIONS 



24. Pitch of Roofs. The pitch must vary according to cir- 

 cumstances, but shall preferably be made as follows: 



For tar and gravel roofs from ^ to -J 



" tile, corrugated steel and 



shingled roofs " i to J 



" slate roofs " J and up 



25. Bracing. Roof trusses shall be braced in pairs; in the 

 plane of the top chord the bracing may be made adjustable, 

 whereas in the plane of the bottom chord the bracing must be 

 rigid. 



Bracing in planes of the sides and ends of buildings may also 

 be made adjustable. 



Knee-braces must be provided wherever practicable, for 

 instance, between trusses and columns and between crane 

 girders and columns. 



26. Purlins. Both steel and timber purlins shall be attached 

 to the roof trusses by means of lug angles. Steel purlins shall 

 be composed of single-rolled shapes (Z-bars, Us, I-beams and Ls) 

 whenever possible, and be of a depth 5 than one-thirtieth (^ ) 

 of the span. Otherwise plate girder or lattice girder purlins 

 shall be used, and no trussed purlins will be permitted. 



Purlins shall generally be spaced from 4 ft. to 5 ft. apart for 

 all forms of roof covering except reinforced concrete, where the 

 spacing may be increased to 12 ft. 



27. Girts. Girts shall be attached to columns by means of lug 

 angles and shall be composed of single rolled shapes. They shall 

 generally not be spaced more than 4 ft. apart. 



28. Beam Girders. Beams composed of single rolled shapes 

 (I-beams and Us) shall be proportioned by. their moment of 

 inertia. 



No beam shall be used whose flange width is less than one- 

 twentieth (-5^) of the span length, unless its compression flange 

 is properly supported sidewise against buckling. When more 

 than one beam is used to form a girder, they shall be connected 

 by bolts and separators, so as to bring the unsupported flange 

 width within the above ratio, spaced with a maximum limit of 

 5 ft. Beams used in floors shall have a depth of not less than 

 one-twentieth (-^) of the span length; they shall be riveted 



