A built-up roof membrane consists of alternating layers of bitumen and fabric, called roofing felts. The bitumen used in such a roof is typically a coal tar, asphalt, or adhesive. Once applied, the layers are usually topped with a gravel or stone in order to keep the underlying roofing system in place. Sheet metal flashings serve to protect areas where the stone cannot, such as vertical points of contact.
Out of every current roofing solution on the market, built-up roofs have been around the longest. While the technology behind them has improved over time, they are generally chosen for price rather than functionality.
Built-up roofs are primarily used for their low cost in both materials and installation labor. If immediate affordability is the top priority, a built-up roof may provide exactly what a business needs.
The tar and gravel typically used on the surface of such a roof are excellent for withstanding foot traffic and equipment placement, which makes a built-up roof a viable option when machinery and technicians will frequently find themselves on the roof.
Having multiple layers makes a built-up roof extremely well insulated against both heat and cold, although each layer adds significant weight to the roof of your building. Should the construction easily support the load, then there is no disadvantage to using a built-up roof, although it will be a potential barrier to use altogether when the weight is over building limits.
A built-up roof is installed by placing down layers of bitumen and reinforced fabric, which then become saturated with or coated in the bitumen. The number of layers can be modified, with more layers performing better. The initial layer, or base sheet, can either be mechanically fastened to the roof or fully adhered by simply laying the bitumen and fabric on top of the insulation or roof deck.
Proper installation of a built-up roof is absolutely vital, as the materials are not fully fabricated off site like with single-ply roofing membranes. However, with experienced labor and sufficient oversight, a built-up roof should provide reliable and lasting results.
The surface layer of stone in a built-up roof serves to protect the underlying bitumen and fabric from UV rays, temperature extremes, and wind damage, thus giving the roofing system a decent lifespan so long as the stone is not disturbed. Having a gravel surface may also be aesthetically appealing, as it lends itself to a more organic and less industrial appearance.
Once the basic layers are installed, a liquid-applied coating or a cap sheet may also be used on top of the fabric and bitumen layers in order to protect them.