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Understanding differences among low-slope roof membranes

by Scott Baxter

There are various types of low-slope roof membranes, and it can be helpful for facility personnel or building owners to understand the characteristics of the roof membranes on their buildings.

EPDM most commonly is a black synthetic rubber roof membrane. There are several methods of securing an EPDM roof membrane. One method uses large stones or concrete pavers, known as ballast, to cover the EPDM membrane. The weight of the ballast secures the above-deck roof insulation and roof membrane. If no ballast exists, the system is mechanically attached or adhered. A mechanically attached system has round plates aligned a few inches inside the laps in the membrane. If you drag a foot across the membrane and it wrinkles in front of the foot, the system likely is mechanically attached. An adhered system more readily will show the joints in the substrate boards (insulation or cover boards) and may have plates visible beneath the membrane but will not be aligned parallel with the lap. EPDM membranes typically are joined at the seams with two-sided adhesive tapes but also can be mated using butyl bonding adhesive.

TPO roof membranes typically are white and can be ballasted, mechanically attached or fully adhered; a majority are mechanically attached. TPO has become more common in the industry during the past several years. TPO membranes are thermoplastic, meaning the seams can be mated using hot air; the seams are welded together to provide a mostly homogeneous membrane.

PVC membranes are similar in appearance to TPO membranes and typically are white or off-white. They also are thermoplastic but are more pliant than TPO membranes and conform with substrate changes more readily.

If a roof system is not a single-ply roof membrane, it likely is a bituminous membrane—asphalt- or coal tar pitch-based. Bituminous membranes typically are built-up roof (BUR) membranes or polymer-modified bitumen membranes.

BUR membranes can be hot asphalt-applied or coal tar pitch membranes, as well as constructed using "cold process" adhesives. BUR roof systems are multi-ply roof systems—typically three or four plies—constructed on the roof by a roofing crew. BUR membranes often are surfaced with one of three surfacings: bitumen and gravel (not to be confused with ballast), mineral-surfaced cap sheets or a liquid-applied roof coating such as aluminum coating. The roof coating may provide reflectivity, ultraviolet protection and a tough weathering surface.

Most polymer-modified bitumen membranes consist of an unsurfaced base layer of polymer-modified bitumen membrane adhered to a substrate, along with a cap sheet, with or without protective surfacing. Three common installation methods for polymer-modified bitumen membrane application are hot-applied (using hot asphalt), cold-applied (using cold adhesives) or torch-applied. Cap sheets may be surfaced with minerals, granules or foil facers. Another option is to apply a protective coating, such as an aluminum coating, over the cap sheet. Most polymer-modified bitumen membranes are manufactured in 1-m widths.

Once you understand the differences among low-slope roof membranes, your knowledge will allow you to have an informed discussion with your roofing professional.

Scott Baxter is property management team commercial sales manager for Interstate Roofing Inc., Portland, Ore.

Date : 3/5/2015