Buyer beware and the devil is in the details are pertinent warnings for building owners looking for quality, cost-efficient membrane roofing materials for their facilities. How do you decide which set of data is most relevant – performance history or material data sheets?
While ASTM International is a globally recognized leader in the development and delivery of international voluntary consensus standards, the problem with this type of consensus standard setting is the time it takes to go through the consensual process. The best way to evaluate membrane roofing is to balance material properties with performance data.
PVC (polyvinyl chloride) is one of the oldest thermoplastic membrane options. The advantage of this thermoplastic membrane type is the ability to fuse the sheet laps with heat to make a true fusion of joined materials, similar to a structural steel weld. The ASTM material standard is for PVC roof material is D4434.
Many companies are developing different chemical formulations for roof membranes. There are also numerous PVC membranes mixed with a PVC resin and ketone ethylene ester (KEE), commonly available under the brand DuPont Elvaloy. These copolymers strengthen flexibility, toughness, UV resistance, and weatherability. They can also be used alone or compounded with other resins.
The volume of this chemistry provided by different manufacturers is problematic because consensus standards of the materials cannot keep up with the number of innovations. Some manufacturers claim their material has superior properties to their competitor, but this data should be checked against the ASTM standards.
A typical material data sheet (MDS) outlines a general description of the product, compositions, material properties, code approvals of the material, and general application/installation methods. The material data sheet for the tri-polymer alloy, for example, will highlight its chemical composition, strengths, and performance.
For example, a specification for a public bid project includes a”fabric reinforced and fleece backed thermoplastic tri-polymer alloy (TPA) sheet: ASTM D4434, Type IV, flexible sheet, internally fabric or scrim reinforced.”
Per the standard, a Type IV sheet is one that “is internally reinforced with a fabric and which may also have a fabric backing with a minimum thickness of 0.036 inch (36 mils).”
However, there is no current ASTM standard for this combination of chemical formulations for the TPA sheet. The data sheet for the TPA material notes the materials “exceed the performance requirements of ASTM D4434-04, Type IV.”
Physical performance characteristics per the data sheet characteristics include tensile strength per the ASTM test method listed in D4434 to be 350 lbf (pound-force), but this is incongruous with ASTM D4434-04 and 4434-11, which state the minimum requirement for a PVC Type IV sheet is 400 lbf. This is also true of the latest version, D4434-11.
Other requirements noted in the data sheet include elongation at break at 35% at the machine direction and 33% at the cross machine direction. These exceed the ASTM D4434 minimum requirement of 25%. Interestingly, a couple of physical characteristics that are important in a thermoplastic single ply are not noted on the data sheet. These properties are test data for seam strength and accelerated weathering. It is unknown why these are not shown, as these should be strong points as membrane properties.
That's why you should look at more than the laboratory test data for the product when choosing an appropriate roof membrane,. One of the most important considerations for choosing a roof membrane is its actual performance history.
If the chemical formulation has been made to provide better than average lab test results and the material is touted as superior due to these test results, the building owner should beware. The true test is a successful history of actual in-service time for any roof membrane.
Darrell L. Smith, PE, is Principal at VJ Engineering. He is a licensed professional engineer and a Registered Roof Consultant. He is a member of ASTM Committee D.08 Roofing and Waterproofing and sub-committee D.08.20 Roofing Membrane Systems. Reach him at [email protected].