With the growth of single ply, sprayed-in-place polyurethane foam, metal, and modified bituminous systems for low slope commercial roofing, why should we be interested in asphalt at all?
How Asphalt Works
Asphalt is a dark-brown to black cementitious material created from bitumens that occur in nature or from petroleum processing. It’s used in roofing, waterproofing, and paving as both a waterproofer and a construction adhesive.
Multiple-ply hot built-up roofing (BUR) has been around for more than a century. A typical construction used a factory-coated base sheet that was mechanically fastened to the substrate, followed by two or three ply sheets that were embedded in hot asphalt and a flood coat of hot bitumen into which aggregate surfacing was embedded. The ply sheets could be asphalt-saturated organic felt, asbestos felt, or more recently, glass fiber mats.
Because of the aggregate surfacing, these roof systems generally qualified for Class A fire resistance. Variations of these basic systems include factory-applied mineral surfacing (cap sheets) and smooth-surfaced systems that received a field-applied coating as a final step.
Asphalt serves as a hot-melt adhesive, gluing the ply sheets to the substrate as well as to each other. It can be melted in a kettle at the job site or delivered as a hot melt liquid by tank truck.
Since most asphalt starts out as a byproduct of petroleum processing, it is fully compatible with mineral spirits (which are also a byproduct of petroleum) to make cold mastics for flashing or cold-applied membrane systems. Asphalt can also be emulsified as a water dispersion for surfacing BUR systems.
Where Can I Use Asphalt?
Asphalt is used in all our modified bituminous (MB) roof systems as a waterproofing coating for the reinforcements and also serves as the adhesive to bond the systems to the substrate. MB systems can use field-applied hot asphalt or utilize a torch to soften and fuse the sheets together. The modifiers can be classified as plastic-modified (atactic polypropylene) or elastomer-modified (sequenced butadiene-styrene or SBS).
More recently, many single-ply systems now offer fleece-backed products that can be embedded in hot asphalt, such as in re-cover applications over old BUR. Even tile roofs frequently use bituminous cap sheets or multiple layers of BUR as an underlay for the tile.
Asphalt should be applied when it is heated to a recommended application temperature. For asphalt, these temperatures apply:
For mop application: the temperature at which the asphalt’s apparent viscosity (equiviscous temperature, or EVT) is 125 cP, plus or minus 25 degrees F.
For mechanical spreader: the temperature at which the asphalt’s apparent viscosity is 75 cP, plus or minus 25 degrees F.