The tar-like material used to cover these roofs is called bitumen by professional Denver roofing contractors. The thick, black material must be heated for the roofers to spread it on the roof. Almost like asphalt, the layers of thick bitumen hold the gravel in place. The process is cheaper than other roofing choices because all the raw materials can be hauled to the job site to be mixed and poured. If you look at an asphalt shingle, you will realize that a bitumen roof is really just a handmade version.
A composition shingle consists of a felt base with tar and small pebble grains. Well a built up roof uses felt roofing, bitumen and and gravel on a larger scale. The felt material serves to reinforce the roof and stop the bitumen from spreading. Called “plies” built up flat roofs built by Denver roofing contractors measure the thickness of the roof by the number of plies. So a 3 ply roof has three layers of roofing material inside the tar membrane.
The way that Denver roofing contractors get the tar or asphalt bitumen up to a melting point is with a special heated tanker. The tanker then releases liquid tar on the roof to be mopped around by roofers. Needless to say, this is not a roofer’s favorite part of the job. There are some tar alternatives that do not need heat to apply. However this means they probably use some organic adhesive that could be just as bad for you.
The mix-in product does not have to be gravel. Any kind of slag, glass fibers or mineral grains can be mixed in to create a built up roof. Some roofers opt out of this all together and just leave the roof covered in tar. Gravel is popular however because it makes the roof stronger, more resistant to the sun (imagine walking on tar in a sweltering day), can reduce the cost of total roofing materials, and also looks more attractive. Another alternative for sun control is to paint the roof white.