What is meant by the term single-ply roofing?

The class of commercial roofing membranes commonly described as "single-ply" are flexible sheets of compounded synthetic materials that are manufactured in a factory to strict quality control requirements.

Single ply roof systems provide strength, flexibility, and long-lasting durability. The inherent advantages of pre-fabricated sheets are the consistency of the quality of the products that are manufactured, the versatility in their attachment methods, and therefore, their broader applicability.

What kinds of products are considered single-ply?

SPRI, the Single Ply Roofing Industry, identifies three major categories of single ply membranes: thermosets, thermoplastics, and modified bitumens.

Thermoset membranes are compounded from rubber polymers. The most commonly used polymer is EPDM (often referred to as "rubber roofing"). Another thermoset material is neoprene, although this particular formulation is no longer widely used for roofing. Thermoset membranes are successful for use as roofing materials because of their proven ability to withstand the potentially damaging effects of sunlight and most common chemicals generally found on roofs.

Hypalon is a unique material because it is manufactured as a thermoplastic, but, because it cures over time, it becomes a thermoset. Hypalon materials are heat sealed at the seams.

Thermoplastic membranes are based on plastic polymers. The most common thermoplastic is PVC (polyvinyl chloride) which has been made flexible through the inclusion of certain ingredients called plasticizers. A number of different products in this category are available, each having its own unique formula.

Thermoplastic membranes are identified by seams that are formed using either heat or chemical welding. These seams are as strong or stronger than the membrane itself. Most thermoplastic membranes are manufactured to include a reinforcement layer, usually polyester or fiberglass, which provides increased strength and dimensional stability.

Modified bitumen membranes are interesting hybrids that incorporate the high tech formulation and prefabrication advantages of single-ply with some of the traditional installation techniques used in built-up roofing. These materials are factory-fabricated layers of asphalt, "modified" using a rubber or plastic ingredient for increased flexibility, and combined with a reinforcement for added strength and stability.

There are two primary modifiers used today: APP (atactic polypropylene) and SBS (styrene butadiene styrene). The type of modifier used may determine the method of sheet installation. Some are mopped down using hot asphalt and some use torches to melt the asphalt so that it flows onto the substrate. The seams are sealed by the same technique.

Which type of membrane should I use?

That depends on several factors, the most significant of which is cost. The decision, however, should not be made on the basis of cost alone. Other important considerations are building height, wind exposure, anticipated roof traffic, and aesthetics. In any case, the track record of the supplier and the reputation of the roofing contractor should be investigated.

Which type of the attachment system should I choose?

Again, that depends on your particular situation. If the structural part of the roof (the deck) is able to withstand the weight, a ballasted roof may be the best option. But if the slope of the roof is greater than 2" in 12", then this system may not be appropriate. There may be other limitations to the use of a ballasted system, such as roof height, proximity to shorelines and other high wind zones, and availability of ballast. A deck that accepts fasteners easily, such as steel or wood, makes a good substrate for a mechanically fastened membrane. These systems can be designed to provide the necessary resistance to known wind forces and are not subject to slope limitations.

Another alternative is the fully adhered system, in which the membrane is attached to the substrate using a specified adhesive. Depending on the membrane, the adhesive may be solvent- or water-based, or asphalt. The finished surface of an adhered roof is smooth. Colored membranes may be used which may make an attractive aesthetic contribution to the building's appearance.

Where should I go to get more information about these products?

If there is a particular product in which you are interested, it's always best to start with the manufacturer. They are ones who know the most about when and how their products should be used. Professional roofing contractors, design professionals, and roof consultants may also be of help.

SPRI is an excellent resource for objective information about various products and systems. We offer many valuable documents and publications to help educate contractors, architects, and building owner and maintenance personnel about roofing and the many options that are available today. SPRI publications range from generic technical guidelines for design and application to general information about roof maintenance and emergency repairs. Give us a call.


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