The first is to protect the underlying layer of roofing materials on built up roofs burs and the second is to secure or weigh down the roofing material as is the case with single ply flat roofs.
Roof gravel weight.
To protect the underlying layer of roofing materials as is the case with built up roofs burs.
There are different types and weights of gravel used on flat roofs for these purposes.
Felt tar and gravel roofing 4 ply 5 3 lb m ft 2 25 9 kg m 2 felt tar and gravel roofing 5 ply 6 1 lb m ft 2 29 8 kg m 2 felt tar and slag roofing 3 ply 4 5 lb m ft 2 22 kg m 2.
The single ply membrane and other roof components including thermal insulation are loose laid.
To weigh down or secure the roofing material as is the case with single ply flat roofs.
Gravel is used on flat roofs for two reasons.
Contractors and builders use gravel on flat roofing for two primary reasons.
The gravel ballast secures the roof waterproofing against wind uplift forces.
A built up roof for example will typically include the built up membrane itself with or without gravel as well as some kind of insulation and cover board which will vary in thickness from roof to roof.
The buildup is ballasted with a gravel layer of at least 50 mm and 80 kg m2.
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Generally the weight for a gravel coated roof must be within 10 25 pounds per square foot.