38 BULLETIN 801, TJ. S. DEPARTMENT OF AGRICULTURE. 
ROOFING MATERIAL. 
Roof-covering material for the roof slab should be suitable for 
application without nails and for application by roofing cement 
directly to the surface. The material should be enduring under 
exposure to the weather and not inflammable. Two types of roofing 
material which may be mentioned as illustrating these qualities and 
being well suited to the desirable flat roc f of the warehouse are " tar 
and gravel '* and asbestos composition built-up roofings. Materials 
for tar and gravel roofing are manufactured by a number -of con- 
cerns, some of which, at a small charge, will supply some supervision 
of the work and inspection service at intervals with a guarantee 
against leakage for 20 years. Where such an arrangement is not 
made, the owner should require the roofing contractor to guarantee 
the roof against leakage for a period of 10 years. This roof is prob- 
ably the most economical for the warehouse. Its use is limited to 
comparatively flat surfaces, the slope of which in no case should ex- 
ceed 3 inches per foot and may be practically flat. The best slope 
for the warehouse roof from every standpoint is one-half inch per 
foot. 3 
parts of sand in order to provide an. even surface for application of the roofing 
material. In some very dry climates it slionld be satisfactory to depend on the 
concrete slab to afford protection from water, but for ordinary conditions this 
is not desirable, as it is difficult to prevent such cracks and openings as will 
result from contraction and expansion and from poor workmanship. Great care 
necessary to overcome this would usually be more expensive than the use of 
roofing material. 
3 The tar and gravel roofing is composed of several layers, preferably five, of 
roofing felt (a fibrous material formed into a paper-like sheet by the use of a 
tar or bituminous binder), well covered on the top surface with tar riitch 
applied hot and overlaid with gravel. The first layer of paper should be 
cemented to the concrete surface of the roof by hot tar pitch while successive 
layers should be bound together in a similar manner. The final surface of tar- 
pitch mixture poured on roof binds the gravel covering to the surface, the gravel 
being spread in place while the tar is hot. The gravel serves as a protection 
to the tar against the heat of the sun. The gravel, slag, or crushed stone 
should be washed free from dirt and should be screened to a fairly uniform 
size, which should be from one-fourth to five-eighths inch in diameter. The 
value of such a roof depends very largely upon the reliability of the workman, 
as this is a case where workmanship counts as much as material. 
Asbestos composition roofings also are suitable for cementing directly to the 
roof slab. They are laid in several cemented plies similar to the method for 
tar and gravel roof, but no protecting gravel covering is required. The slope 
of the roof may be flat or as great as desired. Here also the personal element 
of workmanship is a vital factor, some manufacturers allowing their product to 
be used only where it is to be applied by roofeis licensed or employed by them. 
Such roofing is ordinarily more expensive than the tar and gravel roofing. The 
asbestos roofing also may be purchased under a guarantee for service. 
