192 FRAMEWORK 



phalt is inclined to run and should not be used on a roof with a pitch 

 of more than, say, 2 inches to the foot. If the laps are carefully made 

 and cemented a gravel and tar or asphalt roof may be practically flat ; a 

 pitch of y^ to I inch to the foot is, however, usually preferred. Tin 

 may be used on a roof of any slope if the joints are properly soldered. 

 Most of the patent composition roofings give better satisfaction if laid 

 on a roof with a pitch of ^ to %. Shingles should not be used on a 

 roof with a pitch less than ^4, and preferably the pitch should be J<j 



to y 2 . 



Pitch of Truss. There is very little difference in the weight of 

 Fink trusses with horizontal bottom chords, in which the top chord 

 has a pitch of ^, J4, or J^. The difference in weight is quite notice- 

 able, however, when the lower chord is cambered; the truss with the 

 Yz pitch being then more economical than either the j or the % pitch. 

 Cambering the lower chord of a truss more than, say, 1-40 of the span 

 adds considerable to the weight. For example the computed weights 

 of a 6o-ft. Fink truss with a horizontal lower chord, and a 6o-ft. 

 Fink truss with a camber of 3 feet in the lower chord, showed that the 

 cambered truss weighed 40 per cent more for the l /\ pitch and 15 per 

 cent more for the Y$ pitch, than the truss having the same pitch with 

 horizontal lower chord. It is, however, desirable for appearance sake 

 to put a slight camber in the bottom chords of roof trusses, for the 

 reason that to the eye a horizontal lower chord will appear to sag if 

 viewed from one side. 



In deciding on the proper pitch, it should be noted that while the 

 Y$ pitch gives a better slope and has a less snow load than a roof with 

 l /4 or y$ pitch, it has a greater wind load and more roof surface. Tak- 

 ing all things into consideration ^4 pitch is probably the most econom- 

 ical pitch for a roof. A roof with y$ pitch is, however, very nearly as 

 economical, and should preferably be used where corrugated steel roof- 

 ing is used without sheathing, and where the snow load is large. 



Economic Spacing of Trusses. The weight of the trusses and 

 columns per square foot of area decreases as the spacing increases, while 



