How a tried and true roofing system remains a top contender
How a tried and true roofing system remains a top contender.
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.PageBreak
What Do the Standards Say?
There are three main standards governing the use of asphalt in roofing:
- D312: Asphalt Used in Roofing (originally issued in 1929)
- D449: Asphalt Used in Damp-Proofing and Waterproofing (originally issued in 1937)
- D7654: Asphalt Used in Roofing Measured by Dynamic Shear Rheometer (issued in 2010)
These older standards use a ring-and-ball method (ASTM D36) or Mettler cup and ball method (ASTM D3461) to define softening point of asphalt, combined with a needle penetration at differing temperatures to classify asphalts. Both the penetrometer and ring and ball test are fairly easy to use, and there has been little controversy over inter-laboratory test results.
However, slippage has been a problem with built-up roofing, and even more pronounced with MB systems. This is because the MB systems use all factory-coated sheets laid one ply on one, which is known to be more prone to membrane slippage than shingled BUR construction – to say nothing of the increased weight of the modified bitumen sheets that increases the downward load, adding to the potential for slipping membranes.
For various reasons, including bitumen migration when porous glass-fiber mats are used in BUR, the industry has pretty much abandoned the softer asphalt grades and adopted Type III asphalt exclusively, even for flood coats. In hotter climates such as the desert Southwest, Type IV asphalt is used. Unfortunately, even Type IV as it appears in D312 does not guarantee the roof will not slip.
The problem is that softening point is not a direct measure of viscosity, especially at rooftop temperatures. Many contractors, manufacturers, and consultants have found that a certain Type III asphalt from a specific source works fine, but another Type III with identical softening point results in slippage.
Both the paving and roofing industries have recognized this problem, and various viscosity test devices have been proposed to classify asphalt by viscosity units. However, after decades of trying, new viscosity devices called dynamic shear rheometers (DSR) seem to fit the bill.
Before settling on asphalt as your roofing material of choice, take these two factors into account:
Objectionable odor from the kettle during melting: Low-odor material is now available. Consider investigating these products to see if they fit your project better.
Low flash point for given grades: The minimum flash required by ASTM D312 is now 500 degrees F. In the past, D312 allowed some asphalt products with a minimum flash point of only 437 degrees. Today, however, some products possess a minimum flash of 525 degrees, providing an improved spread between the EVT temperature and the flash point.
With the versatility and proven performance of the family of asphalt products, as adhesives, coatings, BUR, and MB systems, the odds are it will be still be a roofing material of choice into the next century.
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