204 APPLIED MECHANICS 
shown in Fig. 289 has curved rafters, and may have a span of from 20 
to 40 feet. The sickle-shaped truss, shown in Fig. 290, is of a type 
suitable for large spans of, say, from 40 to 100 feet. 
For very large spans, arched principals are used. The abutments 
must then be made strong enough to carry the thrust of the arch. The 
span may, however, be divided, and combinations of simple roof trusses 
used. 
186. Roof Coverings.—Zinc, in sheets, about ;', inch thick, laid upon 
boarding with wooden rolls, forms a light covering. The joints must be 
arranged to allow free expansion and contraction of the metal with 
changes of temperature, while still remaining water-tight. The sheets 
are 7 to 8 feet long and about 3 feet wide. 
Corrugated tron is much used as a roof covering. For small spans, 
not exceeding 10 feet, it may be simply arched, and used without any 
main trusses, the free ends being held in at intervals by tie rods, or 
simply screwed to timber wall plates. On larger spans curved angle or 
tee-irons are introduced as rafters to give support and stiffness. If, how- 
ever, the span exceeds 15 feet, a properly designed roof truss should be 
used; the sheets of corrugated iron are then laid upon purlins. The 
sheets of corrugated iron vary from 6 to 8 feet in length, 2 to 3 feet in 
width, and from 24 to 16 L.S.W.G. thick (0°022 to 0°064 inch). The 
corrugations vary in width from 3 to 6 inches centre to centre, the depth 
being about one-fourth of the width. 
The span of the sheets depends upon the depth of the corrugations, 
the thickness of the metal, and the weight per square foot to be carried. 
The following formula may be used. L=12 a =p? Where L=span in feet, 
¢ = thickness of metal in inches, d=depth of cntibices in inches, and 
w = weight per square foot to be supported. 
The sheets may be laid directly on the purlins, the corrugations 
following the slope of the roof. The lap of the horizontal joints should 
not be less than 6 inches, and these joints should come directly over the 
purlins. Where two sheets join along their sides, at least one complete 
corrugation should overlap, and the two sheets should be fastened to- 
gether by screw bolts or rivets pitched about 9 inches apart. 
If the purlins are of wood, the corrugated iron may be fastened to 
them directly by means of screws or by stirrup bolts, as shown at (0), 
Fig. 291. If angle iron purlins are employed, the sheets are best fixed 
by means of hook bolts, as shown at (a), Fig. 291. These hook bolts are 
commonly about 5%, inch in diameter. All bolts and rivets should pass 
through the ridges of the corrugations, and should be provided with 
washers to prevent leakage. A flat bar, or sometimes an angle bar, is 
often introduced at the eaves, running along the length of the roof, and 
held down by the. lowest row of bolts. This prevents the wind ‘from 
tearing the corrugated iron from the roof should it get beneath the 
sheets, This wind tie is shown at (6), Fig. 291. 
Large slates (Duchess, 24 inches x 12 inches, or other sizes) are often 
used for the covering of iron roofs, These may be laid upon boarding 
and nailed in the usual way, or light angle iron purlins may be fixed to the 
rafters, and on these the slates are laid and then wired on, as shown at 
(e), Fig. 291. These angle iron purlins may be about 14 in. x 14 in. x $ in. 
