Built-up roofing (BUR) is a seamless, semi-flexible membrane composed of 2-5 built-up layers, or plies, of coated or fiberglass-reinforced fabrics layered and finished with bitumen (asphalt, cold-applied adhesive or hot coal tar).
Popular in commercial or other flat roofing systems, BUR has been used in the U.S. since the mid-1800s, reaching the height of its popularity in the 1970s. Commonly referred to as “tar and gravel” roofs, BUR systems are the preferred roofing methods used on low-slope roofs due to their seamless, sealed surface. Modern BUR systems implement a rigid layer for insulation and energy efficiency.
Built-up roofing is typically made of three main components: ply sheets, bitumen and surfacing materials. The bitumen acts as the ‘glue’ holding the layers together and can be either a hot bitumen - meaning it is heated so that it liquefies during installation - or it can be cold, which is not heated and can be sprayed or applied with a squeegee. Hot bitumen tends to give off harmful fumes, while cold bitumen does not.
Surfacing materials, which make up the top layer of the BUR system, typically consist of gravel or ballasted asphalt. This is the finishing layer and provides the finished look and helps protect the layers below from harmful elements like UV rays or falling objects, as well as making the roof safer to walk on. A ballasted asphalt finishing layer uses larger stones (about 1.5” - 2.5” in diameter) that are not attached to the roof structure, using its weight to hold the roofing in place, while gravel-finished surfaces are typically sealed with a final layer of coating to keep stones in place.
There are three main types of BUR applications:
BUR systems are known for their many beneficial attributes. They include:
The installation of a built-up roof is done in four main steps; the execution of each step may vary slightly depending on materials used and how many layers are added.
One of the common drawbacks of a BUR system is that it is unable to handle the thermal shock that comes with the fluctuating weather in certain geographical areas. This problem was addressed in the 1960s and going forward by the development of more durable materials that can be used as an alternative to BUR systems.
A conventional alternative to BUR systems, this modified bitumen is a flexible roofing membrane containing asphalt and SBS (styrene-butadiene-styrene) polymers that can withstand temperature fluctuations and the expand-contract cycle that comes with said fluctuations. This material is considered in the roofing industry to be an evolution of traditional asphalt built-up roofing systems, due to its elastomeric properties as well as its inherent resistance to UV rays.
Atactic polypropylene bitumens, like SBS, create highly flexible membranes that are able to fluctuate with the temperatures of each season. APP, however, is considered a longer-lasting BUR alternative due to its ability to age more gracefully than other types of membranes. This hybrid bitumen membrane provides the benefits of high-tech formulation and combines them with the simpler single-ply application.
Single-ply membranes are meant to serve a single-layer installation and can be either fully adhered to the substrate, mechanically attached or ballasted. The seams are sealed by heat welding or adhesive. This alternative to BUR systems is not as popular as the modified bitumen systems because the exclusive compatibility of the products makes it more difficult to repair or maintain the system.
This waterproofing membrane is widely useful, from below grade to rooftops, and is applied, as its name implies, as liquid layers. The main draw of this method is the ability to apply it directly to the substrate, but the caveat is that it must be applied by a knowledgeable professional who can prepare the surface properly before application.