38 BULLETIl!^ 801, U. S. DEPAKTMENT OF AGKICULTXJKE. 



ROOFING MATERIAL. 



Roof -covering material for tlie roof slab should be suitable for 

 application T^'ithout nails and- for application b}' 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 roof of the Avarehouse 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 worlv and inspection service at inter^^als 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.s 



parts of sand in order to provide ay even surface for application of tlie roofing 

 material. In some very dry climates it sTiould be satisfactory to depend on tlie 

 concrete slab to afford protection from Avater, but for ordinary conditions tlais 

 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. 



" 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 pitch 

 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 gi'avel, slag, or crushed stone 

 should be washed free from dirt and shotild be screened to a fairly uniform 

 size, which should be from one-fourth to five-eighths inch in diameter. The 

 value of such a ro6f 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 metliod 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 roofers 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. 



