Membrane is defined in Means Illustrated Construction Dictionary as: The impervious layer or layers of material used in constructing a flat roof. This definition allows for the inclusion of the many different materials and installation methodologies used in today’s low slope roofing solutions.
Built-up roofing, consisting of layers of organic or inorganic felts saturated with liquid asphalt or coal tar and then applied to a roof surface with more liquid asphalt or coal tar, has been providing reliable protection against the elements for many years. Installed in shingle fashion, the layers of felts, called plies, can be two, three, four or more layers thick. The uppermost surface is often topped with gravel or small stones to provide protection from physical damage and UV degradation.
Modified bitumen membrane roofing is essentially a pre-manufactured built-up roofing system. Thick layers of reinforcing felts and liquid bitumen are bound together in the manufacturing facility and shipped to the job site in roll form. Chemical modifiers enhance the attributes of the liquid bitumen to make it suitable for the job conditions. Modified bitumen roofing can be applied in a method similar to built-up roofing, by using hot liquid bitumen that is mopped in place, or it can be installed by torching it down. As the roof membrane is being unrolled, a worker uses a torch to melt the leading edge of the roll into a liquid that acts as a binder to hold the material down.
Other types of membrane roofing such as chlorosufonated polyethylene (CSPE), ethylene-propylene-diene-monomer (EPDM), polyisobutylene (PIB), polyvinyl-chloride (PVC) and thermoplastic polyolefin (TPO) consist of large sheets of material that are joined together using various chemical or thermal methods. Primers and adhesives can be used along overlapping edges to form an impervious joint between sheets, or heat welding can also be utilized to turn all the pieces into one large roof covering. The large sheets are then applied to the roof surface by using several methods. A ballasted roof uses the weight of stones or blocks of concrete similar to pavers to hold the roofing in place. Strips of metal fastened to the structure at all edges and seams can also be used. Another popular method is to use an adhesive that is similar to contact cement to hold the sheets down.
Spray polyurethane foam roofing (SPF) uses two chemicals that when mixed together in the proper proportions cause a chemical reaction resulting in a rapidly expanding foam that cures to a solid very quickly. Skilled installers using special pumping equipment and a regulated spray nozzle can install the foam to a specified thickness. The foam can be installed around roof interruptions and eliminates the need for special flashings. Once cured, the foam is covered with a UV inhibiting silicone layer.
What Will It Cost?
Architects and engineers will make a selection of the type of membrane roofing based on the requirements of the job and the attributes of the material. According to Means Building Construction Cost Data 2009, costs for membrane roofing can range from $125 per square (100 square feet) to over $350 per square. It is important to remember that a roofing system consists of not only the membrane, but also insulation, flashing, roof accessories and sealants.