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Polymer-Modified Bituminous Roofing Systems
In Issues 62, 63, and 64 of Buildings.com’s Roofing News, I introduced the U.S. Corps of Engineers’ Commentary on Roofing Systems, some general criteria for system selection, aspects of protected membrane and vegetated roof systems, and traditional bituminous built-up roofing (BUR). This column will look at “high-tech” bituminous roof systems: ones that have been modified by the addition of polymeric materials (or modified bitumen [MB]).
If BUR is so wonderful, why modify it?
As indicated in last month’s column, MB systems have distinct advantages over traditional BUR where roof stresses are high, such as at flashings. They have served to prolong the life of the BUR market, as indicated in the Corps of Engineers Document TI-809-53:
As contrasted to BURs, which rely on field application of bitumen to make them waterproof, polymer-modified bituminous sheets (MB) use reinforcing sheets that have been factory coated with rubberized bitumen. The sheets themselves are inherently waterproof. The use of polymers mixed with the coating bitumen improves flexibility, toughness, and low temperature properties. A major advantage of MB systems over BUR is that they generally use fewer layers and are, therefore, less labor and material intensive.
Categories of MB systems include:
- Heat-fused, where a propane torch or hot-air welder is used to re-melt the factory-applied bitumen to serve as an adhesive.
- Mopped-in-place, using conventional hot BUR techniques.
- Liquid solvent-based adhesives.
- Self-adhesive (peel and stick)
In most cases, two plies of reinforcing sheets are used to construct the membrane. The base layer could be a modified sheet, a conventional asphaltic base sheet, or several layers of BUR ply sheets laid in hot asphalt. (The latter system is sometimes referred to as a “hybrid system.”)
Both thermoplastic modifiers and elastomeric materials are used. Thermoplastics include atactic polypropylene (APP), alpha-polyolefins (APO), isotactic polypropylene (IPP), and ethylene-vinyl acetate (EVA). Elastomerics include styrene-butadiene-styrene (SBS), and styrene-butadiene rubber (SBR).
The choice of polymer modifier has a bearing on the application method. In general, the thermoplastic sheets (APP types) are heat-fused to the substrate, as the melting point of these sheets is too high to be reliably adhered with hot asphalt. The SBS-modified sheets can be either torched or mopped. The higher softening point of APP sheets sometimes makes them favored in very hot climates, while the better low-temperature flexibility of SBS sheets is preferred in cold climates.
Glass fiber, polyester, and combinations are commonly used, although plastic core materials have been used in some waterproofing and shingle underlayment applications. The reinforcements carry the sheet during manufacture and provide tensile strength and stability to the finished membrane.
Many BUR systems use a flood-coat of hot bitumen and aggregate as a top surfacing. While this is durable and water- and fire-resistant, the surfacing is heavy (4 to 6 psf) as well as labor- and material-intensive. Since MB sheets are factory coated, most times roofing granules or metallic foils are factory applied as well. With recent attention toward achieving more highly reflective roof surfacings, a white coating can be factory applied over the granules or even over the polymeric sheet itself. In other cases, the cap sheet is installed unsurfaced, relying upon the glass mat internal reinforcement to provide weather resistance, or is field coated with a colored or asphalt-aluminum coating. Flood coat and gravel is rarely, if ever, used with MB systems.
Coated MB sheets are stiff in cold weather, and rolls should be stored at temperatures above freezing for 24 hours before unrolling. In some solvent-adhesive systems, the body of the sheet is embedded in the adhesive, but the side and end laps may be torched to get a quick and reliable seal. Self-adhering sheets generally require temperatures above 50-degrees F. to achieve adhesion, and many systems supplement the self-adhesion at T-joints by using mastic adhesive.
Use of Propane Torches
Torch application is appealing, because the heat of the torch makes the MB sheets more conformable. Heat is required only at the point of application, eliminating the logistics of kettle, luggers, and mops. In addition, the heat helps evaporate moisture from the substrate, minimizing the potential for blistering. On the other hand, new safety issues arise. Fire districts may require daily reporting when propane is being used, and may prohibit the use of torches altogether in occupied buildings. A fire watch should be specified for at least an hour after the last torch is extinguished. A certified torch applicator should also be required. The Certified Torch Applicator (CERTA) program is offered by both the National Roofing Contractors Association (NRCA) and the Midwest Roofing Contractors Association (MRCA).
Both polymer-modified and unmodified bituminous adhesives have been used. Concerns with volatile organic compounds (VOCs) have led to some low-volatility, high-solids contents adhesives. Water-based adhesives cannot be used because the MB sheets are impermeable and the water cannot escape. Slow setting is a detriment, especially in new construction where it is hard to control the traffic of other trades. Side and end laps may curl up before the adhesive sets up. Some systems torch or heat-seal the side and end laps to solve the curling problem.
Roof Decks and Vapor Retarders
Requirements are similar to BUR systems except that torching should be avoided over most insulation boards and combustible decks. A number of fire-retarded underlayments, such as gypsum board, can be installed over cellular insulations as a cover board. Cover board is also recommended when hot-mopping to foam insulations.
Vertical flashings for most MB systems require cant strips to reduce the angle at the base of the wall or curb. (Fire-retarded cants are needed for torch systems.) MB flashing materials have been so good that it is tempting to eliminate the need for a backer sheet; however, for best waterproofing and durability, a backer sheet is still recommended.
Since the Corps of Engineers document was published in 1999, a number of additional MB specifications have appeared.
ASTM D5147 – Sampling and testing MB sheet material
ASTM D5849 – Evaluating resistance of MB roofing to cyclic displacement
ASTM D6135 – Application of self-adhering MB waterproofing
ASTM D6162 – SBS sheets with combination of polyester and glass fiber reinforcements
ASTM D6163 – SBS sheets with glass fiber reinforcement
ASTM D6164 – SBS sheet with polyester reinforcement
ASTM D6222 – APP sheets with polyester reinforcement
ASTM D6223 – APP sheets with glass fiber reinforcement
ASTM D6298 – Glass reinforced SBS sheets with factory-applied metal surface
ASTM D6509 – APP base sheet materials with glass reinforcement
ASTM D6769 – Application of cold-applied MB waterproofing
ASTM D6950 – Application of APP waterproofing systems
A minimum design slope of 0.25 inches per foot is recommended for all membrane roof systems. Major codes now require “positive drainage” rather than a specified minimum slope. Plumbing codes also require at least two means of draining each roof area in case the primary drain becomes plugged.
United Facilities Criteria: Commentary on Roofing Systems
National Roofing Contractors Association
Single Ply Roofing Industry
National Research Council of Canada’s Institute for Research in Construction
Asphalt Roofing Manufacturers Association