210 APPLIED MECHANICS 
Several forms of joints for purlins are shown in Fig, 296, 
Fig. 296. 
es methods of introducing longitudinal wind bracing are shown in — 
Fig. 297. 4 
a) oo 
& 1 
| (tea) (Oe 
Fie. 297. 
188. Weight of Roof Coverings.—For the purpose of estimating — 
approximately the weight to be carried by a roof truss, the particulars 
given in the following table may be used. The weights given are in 
pounds per square foot of covered area. 
Sheet zinc . 3 : : . 14} Slates, large  . . 4 >, 10 
Corrugated iron % s Riri? * » medium. Seas as 
Tiles, plain : 2 : , » small. i i . 515 
Pantiles . s 3 : . 12 | Boarding, 1 inch thic. , .- ae 
189, Pitch and Slope of a Roof.—The ratio of the rise to the span 
is called the pztch of a roof. If the roof is symmetrical and the slope or 
inclination is denoted by 0, then tan 6=rise+half the span. The min- 
imum slope for a roof depends on the nature of the covering, and is roughly 
5° for zinc, 11° for corrugated iron, 22° for large slates, 26° for pantiles’ 
and medium sized slates, 30° for small slates, and 45° for plain tiles. 
190. Procedure in Designing a Roof.—The method of procedure 
in getting out the designs for a roof may now be briefly given. 
Decide upon the type of truss. This will depend upon the various 
