CHAPTER XIII 
DESIGN OF STRUCTURES—ROOFS 
183. Roofs and Roof Trusses.—The function of a roof or upper 
- eovering to a building is to protect the interior from wind and weather. 
_ It consists of a weather-proof covering supported on a suitable framework. 
This covering may be made either flat, sloping, or curved, the pitch or 
_ slope depending largely upon the nature of the material of which it is 
composed. The framework consists of (1) the roof trusses, or principals, 
which span from support to support and carry the roof structure ; (2) 
_ the purlins, longitudinal 
beams which run from 
truss to truss along the 0, T 
/ 
roof; (3) the rafters, sash 1/1, Y) P 
bars, ete., which rest | i] a, ve y 
_ upon the purlins, and to Vif, 7 y LY W. P 
which the covering pro- / 7), jj Ly 
"per is fixed ; (4) the wind yy $ 
_ ties, which prevent longi- jy/ 11, Wy) 
tudinal distortion of the 1449 Wy 
roof by the wind. Yip, , 
In Fig, 277, which 47/77 
shows, in oblique projec- fyi, y, "4 1. p W 
tion, the framing of a Up 
; roof, TT a = are the A- 
: _ the 77 
_ main trusses or princi- 
PP... the pur- Fia. 277. 
ns, WW the wind ties, and C the roof covering. RR is called the 
ridge of the roof, and the lower longitudinal edges are called the eaves. 
184, Iron Roof Trusses.—Roof trusses may be made of wood, but 
__ iron or steel principals are superior in nearly every respect for spans of 
_ any considerable size. Figs. 278-290 represent the more common types 
_ of iron or steel roof trusses. They are all composed of the following 
members or parts: (1) the principal rafters, which are the members, 
_ either straight or curved, running from the ridge to the abutments or 
_ Supports, and carrying the purlins ; (2) the tze rod, which may be straight 
_ or cambered, whose function it is to tie the two feet of the principal 
_tafters together, and thus relieve the abutments of the horizontal thrust, 
_ Which would otherwise come upon them; (3) secondary bracing, which 
_ divides the principal rafter into panels, and thus supports it both as a 
_ strut and as a beam. The upper ends of these secondary members 
_ Should come directly under the purlins, and thus relieve the principal 
rafter of transverse bending actions, The axes of any three adjacent 
. 201 
