DESIGN OF STRUCTURES 205 
- when the distance between the rafters does not exceed 8 feet, or 
2 in. x 2 in, x ,'; in. when that distance is increased to 10 feet. 
Tiles of great variety are used for roof coverings. They are sup- 
ported in the same way as slates. Tiles are heavy, and they require the 
roof to be of high pitch. 
Glass is largely used as a roof covering. Many roofs have glass sky- 
lights, while some are entirely covered by glass. The shorter slope of a 
saw-tooth roof and many railway station roofs have glass coverings. The 
Fig. 291. 
glass may be laid in sash bars of wood or tee-iron, with putty. Iron sash 
bars, however, expand and contract more than the glass with changes of 
temperature, and the putty is liable to crack. Hence many systems of 
glazing without putty have been introduced. 
Glass sheets suitable for roofing vary in width from 12 to 20 inches, 
and in thickness from 3 inch to } inch. They are made in lengths up to 
6 feet. About 3 inches of lap should be allowed between two sheets. 
Tee-iron sash bars vary from 1 inch to 2 inches in depth, and from } to 
ys inch in thickness. 
187. Details of Roof Trusses.—The various parts of roof trusses 
should be made as plain and simple as possible. Forging and welding 
