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Single Ply Roof Membranes and Common Design and Warranty Considerations

July 16, 2013

Brief History and Membrane Types

Single ply membrane roofing was first introduced in the early 1960’s as an economical means to protect and water-proof low slope roofing without the cost and labor intensive heating methods associated with traditional asphalt and coal tar pitch roofing.  Single ply roofing can be secured to substrates through mechanical fastening, adhesives (full adherence), and /or ballasted with gravel, pavers, etc.  these roof membranes consist of three basic types of materials: EPDM (ethylene propylene diene monomer “rubber”), TPO (Thermoplastic Polyolefin), and PVC (polyvinyl chloride “plastic”).  EPDM, the first to be introduced to the market, is a vulcanized or cured rubber product and is often referred to as a thermoset.  EPDM sheets are produced in a variety of thickness from 30 to 90 mils and can also be produced in a variety of colors with the most common being black.  EPDM roofs are typically adhesively applied and seamed, and are often ballasted

TPO roofing membranes were introduced to the construction industry in the early 1990’s and like EPDM, TPO membranes can be either mechanically fastened or fully adhered to a substrate.  However, unlike EPDM, and TPO can never be applied as a ballasted system due to the chemical composition of the membrane.  During installation, the seams of TPO sheets are hot-air welded as opposed to glued or taped.  TPO membranes are produced in thicknesses ranging from 45 to 90 mils and have become a popular choice in recent years as they can be more energy efficient due to the reflective nature and associated energy efficiency of white membranes.  PVC membranes are also heat welded single ply membranes, and are typically produced as a white sheet similar to  TPO’s.  PVC offers several unique advantages when compared to other membrane types including durability, pliability, and recyclability. The chlorine in PVC membranes also offers superior chemical and fire resistance.

Design and Warranty Considerations

The first aspect of the overall single ply roof design to consider when selecting a membrane type is buildings height, geographic location, and wind exposure.  The International Building Code mandates that the roof be designed to withstand wind uplift pressures based upon the published wind speed maps, and ASCE (American Society of Civil Engineers) 7 calculations.  Many architects utilize Factory Mutual (FM) classifications as a means to establish a standard of quality and the design wind uplift pressure whether the building is truly FM insured or not.  While this is generally an accepted practice, it is important to understand that these specified wind uplift pressures do not correlate to the roofing systems warranty, and that any desired wind speed warranty must be addressed separately.  When selecting the appropriate membrane type for the building, the thickness of the membrane must also be selected, as thicker membranes offer significant increases in puncture resistance, thickness over scrim, weather ability, and seam strength, usually for a minimal increase in cost.

The substrate materials to be installed below the single ply membrane must also be carefully considered.  The overall integrity of the single ply membrane is only as good as that of materials installed below it. Extruded polystyrene (XPS) or expanded polystyrene (EPS) insulation boards have a long history of use under single ply roofing membranes.  In most cases these products can be fully adhered or mechanically fastened, and in some cases can be loose laid and ballasted over the single ply membrane.  Polyisocyanurate insulation is also a popular choice for roof insulation material for low slope and flat roofs.  Polyisocyanurate insulation offers several distinct advantages over XPS and EPS as it is more durable, dimensionally stable, and offers a higher “R” value per inch than that of XPS or EPS.  Polyisocyanurate insulation is approximately 6.0 R value per inch of thickness and therefore allows the architect or designer to more easily achieve the insulation requirements mandated by today’s stringent energy codes, as well as ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning Engineers) 90.1.  Polyisocyanurate insulation is also produced with a more durable coated fiberglass face on each side in place of the standard black paper facer, offering superior fire resistance, higher wind uplift performance, and greater mold and mildew resistance.  Whichever insulation board is ultimately selected for the project, it is important that the total thickness needed be comprised of at least two layers of the insulation material, such that the joints between the boards can be staggered, and reduce the possibility of thermal bridging.

The edge or perimeter condition where the roof membrane terminates at the parapet or wall must also be part of the overall system design.  All single ply roofing manufacturer’s produce a variety of copings, gravel stops and other metal edge products to address most roof conditions.  In fact, all single ply roofing manufacturers require the use of their proprietary edge systems to uphold their specified warranty.  More importantly, the use of manufactured edge metal is actually codified in Chapter 15 of the International Building Code.  Section 1504.5 Edge Securement for low-slope roofs states that “low-slope membrane roof system metal edge securement, except gutters, shall be designed and installed for wind loads in accordance with Chapter 16 and tested for resistance in accordance with ANSI/SPRI ES-1 test.” Without this knowledge, many contractors substitute the factory manufactured edge metal for coping or gravel stops that are fabricated in a local shop. This decision, often made in the interest of time or cost savings, ultimately compromises the roof warranty and doesn’t meet the building code requirements.

Lastly, the warranty for the overall roofing system must also be carefully considered. Simply selecting whether the building owner would prefer a 20 or 30 year warranty falls short of truly understanding what needs to be specified.  Warranties are often a source of confusion among architects and building owners. For example, an assumption that the roofing system was designed to withstand wind uplift pressures of a 90 mph wind and specifies FM Global standards improperly implies that the warranty must also be for up to a 90 mph wind.  However, the industry standard for wind speed warranties is 55 mph, unless otherwise specified as a specific requirement of wind speed to be provided by the manufacturer.  Most single ply roofing manufacturers do offer higher wind speed warranties; however, the granting of such a warranty is dependent upon the overall system design and other components utilized in the design.  Too often the designers, contractors, and building owner come to this realization during the roof installation, when it is simply too late to address the issue without significant cost increase and time delay. 

Conclusion

Single ply roofing membranes are often an appropriate choice for a variety of building types and can serve a building for many years.  They are often unique to each manufacturer and the design and wanted warranties can be complicated.  Architects, Designers, Contractors and Owners must understand all aspects of the specified system in order to avoid a myriad of issues that can arise both during construction and throughout the life of the building.