It’s easy to be confused when evaluating roofing material solutions for the flat or low slope roof. By understanding something about the source, benefits and drawbacks of each type of material, you can make a choice better suited to your needs and sift through the often confusing stack of proposals.
Manufacturers place their own proprietary names on their roofing materials to promote them. To compare and understand what is being offered, let’s begin with the basic categories of commercial low slope or flat roofing systems.
Single Ply Roofing and Multiple Plies
We will look at TPO, PVC, BUR, modified, cold process and EPDM.
Built Up Roof
The oldest form of commercial roofing is the asphalt built up roof composed of multiple plies. Today this tends to be the lower cost material because the asphalt is relatively inexpensive compared to polymer blends. Fiberglass felts are used between mop applied layers of hot asphalt, the surface is protected with a mineral granule surfaced cap sheet, or a layer of gravel or pumice slag to protect from UV and aid water dispersion and evaporation.
The primary limitation of BUR is that oxidized asphalt is an inherently brittle material. You can see this when the roofers use an ax to break the keg into chunks to fit into the kettle for heating. The temperature is critical and if applied too hot or too cool the roof life is compromised. Three ply is the economy grade, four ply is better. Lead flashings at extra cost, and polymer modified flashing cements or membranes help add flexibility at roof details, pipe flashings and drains.
Modified Membrane Roof
Modified membranes address the inherent inflexibility of hot asphalt by adding polymer modifiers in the manufacturing process. The two main types are plastic or rubber, called APP and SBS. Polyester reinforcement allows the materials to be bent and formed into flashings. APP membranes are heat welded, SBS usually adhered with hot asphalt. SBS performs better in freeze thaw cycles. APP better in hotter areas having higher UV resistance.
Cold Process Cool Roofing
Cold process roofing can use a water based colloidal suspension of asphalt mixed with clay to form a waterproof layer that remains flexible because it is not heated during manufacture or installation. These can be surfaced with reflective polymers to achieve a cool roof rating.
PVC or poly vinyl chloride is a plastic roof that relies on plasticizers to create a flexible material. Plasticizers can migrate out of the material over time and cause it to revert to the unprocessed brittle state. After approximately 30 years of development, these materials are relatively stable today. The thickness of the material measured in mils or thousandths of an inch will be a factor in longevity so an 80 mil sheet will last longer and cost more than a 40 mil.
TPO is a newer answer to PVC and is manufactured with no plasticizers. As a result the material tends to be somewhat less flexible initially but has no risk of plasticizer migration. Both PVC and TPO are thermoplastics which means they may use heat welded seams where the material can be softened and welded with hot air and pressure. Additives are needed to impart fire resistance and UV resistance. Sometimes lower cost manufacturers can skimp on formulation.
The EPDM Rubber Roofing
EPDM is the original rubber roof and is chemically resistant. EPDM is a thermoset meaning it cannot use welded seams. Seams must be glued with adhesive or butyl seam tape. The material can sometimes shrink slightly with age so it is important to securely fasten the perimeter flashings. To get good seam adhesion everything must be kept clean and free of contaminants.
By understanding the variety of flat roof materials and solutions, you can better evaluate the best roofing material for your project.
Now recognize, this brief introduction is a broad over view and a number of conditions will affect your decision. You’ll need to weigh mechanically fastened against fully adhered systems, insulated against non insulated, determine the best way to handle flashing details and special conditions of your building which may influence cost of transporting materials to the roof work area and design solutions to address incomplete drainage or other issues specific to your buildings.