PITCH OF ROOF 191 



monitor ventilator and skylights in the roof shown in (a), is a favorite 

 tvpe for shops ; truss (b) with double monitor ventilator is especially 

 adapted to round house construction ; trusses (c) and (e) are adapted 

 to shop and factory construction where a large amount of light is de- 

 sired, ventilation being obtained by means of circular ventilators ; truss 

 (d) is similar to (c) and (e), but allows of better ventilation; truss (f) 

 has skylights in the roof and has circular ventilators placed along the 

 ridge of the roof; truss (g) is the type in common use for 

 blacksmith shops, boiler houses, and roofs of small span. The "silk 

 mill" roof shown in (h) was used by the Klots Throwing Co. in their 

 silk mill at Carbondale, Pa. The spans of the three trusses are 48' 8" 

 each, with a clerestory of 13' 9" in the monitor ventilators, which are 

 glazed with glass H' o" high. The monitors face east and west, al- 

 lowing a maximum amount of direct sunlight in the morning and 

 evening, and none at midday. This roof has given very satisfactory 

 results, however, it would seem to the author that it would be necessary 

 to use shades, and that there would be shadows in the building. The 

 trusses in this building are spaced 10' 6" apart and support the 

 plank sheathing which carries the roof, no purlins being used. The 

 shafting to run the machinery in this building is placed in a sub-base- 

 rnent ; a method much more economical and convenient than the com- 

 mon one of suspending the shafting from the trusses. 



Pitch of Roof. The pitch of a roof is given in terms of the center 

 height divided by the span; for example a 6o-ft. span truss with % 

 pitch will have a center height, of 15 ft. The minimum pitch allow- 

 able in a roof will depend upon the character of the roof covering, and 

 upon the kind of sheathing used. For corrugated steel laid directly on 

 purlins, the pitch should preferably be not less than ^4 (6" in 12"), and 

 the minimum pitch, unless the joints are cemented, not less than l /$. 

 Slate and tile should not be used on a less slope than y^ and preferably 

 not less than J/. The lap of the slate and tile should te greater for the 

 less pitch. Gravel should never be used on a roof with a greater pitch 

 than about J^ , and even then the composition is very liable to run. As- 



