It cannot be expected to give protection against workmen installing mechanical or other equipment over the roofing. Aggregate must be so graded as to permit proper nesting and filling of voids between individual particles and to provide a complete and continuous membrane. Complete coverage must be maintained over all parts of the roof if satisfactory service is to be expected. An aggregate should be hard enough to resist abrasion and frost action, but not so brittle that it encourages fracture and disintegration. The usual aggregates are crushed stone and gravel, or crushed air-cooled blast furnace slag.
Slag aggregate is opaque and angular.
Its alkaline surfaces have a greater affinity for bituminous material than for water, with resultant good adherence of the bitument to the aggregate. Methods of test to determine these physical properties are also specified to ensure that light gravel, which will easily blow off, is not used. Hardness is of importance if the aggregate is susceptible to dusting. The practice of using what is available rather than what is suitable should be discontinued. Various interpretations can be made from such references.