Roofing Systems

Built-Up Membrane Systems

The National Roofing Contractors Association (NRCA) reports that built-up roofing (BUR) systems have been in use for more than 100 years and account for roughly 20 percent of all new low-slope roofing construction applications and 21 percent of re-roofing. EPS has been used in this roofing application for more than 30 years. Performance properties for EPS thermal insulation can be found in ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. This standard covers the types, physical properties and dimensions of commercially available EPS insulation products.

BUR systems constructed with a concrete roof deck allows the roof insulation to be applied directly to the deck since there is no requirement for a thermal barrier due to the noncombustible concrete deck. The EPS foam may be adhered to the concrete with asphalt using a technique described as "mop and flop" by the roofing industry. Hot asphalt is applied to the concrete deck and allowed to cool slightly before the foam is permanently dropped into place. In this BUR system a coverboard is then "mopped and flopped" onto the EPS with the joints overlapping and taped. The BUR is then applied on top of the coverboard, which provides a uniform and consistent base for the weatherproofing system.

When using a steel deck the building code requires a thermal barrier with an index of 15, such as 1/2 inch gypsum board. The required barrier protects the foam from ignition as well as limiting the potential contribution to under-deck flame spread during a fire by the roofing system components. In BUR systems because of the thermoplastic properties of EPS it is necessary to place a coverboard above the foam to protect it from hot asphalt used to adhere and build up the waterproof protective roof covering materials. In a typical BUR system there will be three or four alternating layers of materials which will include bitumen, either asphalt or coal tar, and roofing felts, which can be surfaced with some type of exposed aggregate embedded in hot asphalt or a smooth coating or cap sheet.

EPS foam used in BUR systems can be installed as described above or may by delivered to the job site as a complete composite panel that would include the thermal barrier when necessary, the insulating foam and the coverboard. This composite panel can then be easily adhered or mechanically fastened to the concrete or steel deck saving on-site labor. The use of prefabricated composite panels for BUR roofing applications has been steadily growing because of the labor savings involved in installation.

Modified Bitumen Membranes

The NRCA reports that polymer-modified bitumen membrane systems have been used in the United States since the mid 1970's and account for approximately 18 percent of all new low-slope construction and 23 percent of re-roofing applications. These membranes are available in either asphalt or coal tar based systems that have been modified with a polymer and reinforced with a variety of materials including glass fiber, polyester or polyethylene. They can be installed with torches, in hot asphalt, cold adhesives and/or self-adhered and sometimes in combination with hot-air welders. Performance properties of the membrane are dependent upon the modifying polymer and the reinforcing material. EPS insulation used in modified bitumen roofing systems should be handled and installed similarly to BUR systems and will require thermal barriers and coverboards.

EPS should not be used in new construction applications when coal tar adhesives are present. In re-roofing applications where EPS is applied over an existing BUR that contains coal tar adhesives special precautions must be taken to protect the foam during extremely high temperature conditions. It is possible that vapor emissions from the coal tar used to seal or join the edges of the original roof membrane system will deteriorate the polystyrene foam. The use of a coverboard with tightly sealed taped joints is recommended.


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