Recent Roofing News e-newsletters have followed the U.S. Corps of Engineers’ Commentary on Roofing Systems, including sections on bituminous, modified bituminous (MB), and EPDM roofing systems. This column will cover weldable single-ply systems, including thermoplastic polyolefin (TPO), polyvinyl chloride (PVC), ketone ethylene ester (KEE), chlorinated polyethylene (CPE), chlorosulfonated polyethylene (Hypalon® or CSPE), and polyisobutylene (PIB).
All of these thermoplastic materials can be fused together by the application of heat, although self-adhesive and solvent-welded sheets have been used as well. As compared to the EPDM materials mentioned last month, these sheets do not form permanent cross-links (through vulcanization). In the absence of cross-links, thermoplastic roof membranes are subject to creep or cold-flow under stress. Most products rely upon internal reinforcement such as woven scrim while others use a non-woven fleece backing material.
Standards and Specifications for Thermoplastic Membranes
Since the issuance of the Corps’ technical instruction in 1999, a number of new and upgraded ASTM specifications have appeared:
- D4434 for PVC materials.
- D5019 for CSPE.
- D6754 for KEE.
- D6878 for TPO.
- D7067 for white reinforced PIB.
ASTM has also published a number of recommended practices for these systems:
- D5036 application of fully adhered PVC.
- D5082 application of intermittently attached (mechanically fastened) PVC systems.
In addition to the ASTM specifications, a number of important documents have recently been issued:
- A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs (IRC-47652E), National Research Council Canada’s Institute for Research in Construction.
- FM Global Property Loss Prevention Data Sheet 1-29, Roof Deck Securement and Above-Deck Roof Components (updated August 2007).
Advantages of Thermoplastic Roof Systems
With the exception of PIB, all these materials can be heat-welded in the field. These are true welds, with strength equal to or greater than the sheets themselves. As a result, seam failures are rare. With the advent of automatic heat welders, seam formation is not especially labor intensive, although skilled operators are required. For many mechanically fastened systems, double welds are now recommended, as the wind up-lift resistance of these systems is significantly better than the single welds of previous generations. (See Fig. 1 for an example of a double-weld cutaway.)
Light-colored sheets are used both for aesthetics and to optimize longevity. The internal reinforcement helps reduce thermal contraction or shrinkage of the sheets, as well as to improve tear resistance. While ballasted systems have been used in the past, they are rarely used today since the fully adhered and mechanically fastened systems are much lighter in weight. Ballast was first eliminated from PVC systems when it was discovered that the ballasted roofs were becoming brittle much quicker than with exposed membranes. This was attributed to extraction of plasticizer through hydrolysis (much like kitty litter absorbs liquids).
For fully adhered systems, the nature and condition of the substrate is very important. If solvent-based field adhesives are to be used, the thermal insulation boards must be resistant to solvent fumes. This generally eliminates all polystyrene boards unless a suitable cover board is used. In addition, boards with low peel strength (such as perlite) should also be avoided.
For mechanically fastened systems, fasteners are typically placed within the seam area, using stress plates to distribute forces. Although it is technically possible to use rigid anchor bars in the overlap, it is not recommended to use the bars in the seam area, unless a double weld is to be used. For extreme wind conditions, bars and cover battens can be used in the field of the sheets. FM data sheet 1-29 illustrates some of these options.
If roofing rolls have been stored under cold conditions, they should be unrolled and allowed to reach ambient temperatures. Instructions suggest allowing the sheets time to “relax,” as otherwise they may contract due to built-in stresses (memory). TPO sheets have been reported to prematurely crack when the applicator has bent the sheet back 180 degrees while installing fasteners or wiping the seam area.
MSDS labels should be read and understood, and many of these systems use flammable solvents. Fumes may be objectionable to occupants. This could require shutting down air-intake equipment or constructing temporary elevated intake ducts to avoid problems. Solvents may extract oils from the workers’ skin; gloves may be required.
Most of the single-ply systems incorporate energy-efficient, lightweight thermal insulations such a polystyrene or polyisocyanurate. Attention is needed in mechanically fastened systems to incorporating air retarders, as air leakage can be an important source of wasted energy. In the absence of an effective air retarder, mechanically fastened systems billow and allow lateral airflow. Some states (Massachusetts, Minnesota, etc.) actually mandate the use of an air retarder in their energy and/or building codes.
Some of these membrane systems are conducive to photovoltaic cells being bonded directly to the surface.
Since most of these membranes are light-colored and non-metallic, they generally meet ENERGY STAR® and governmental requirements for air-conditioned buildings. Dirt accumulation may be a factor, especially on poor-draining roofs. While washing the roofs may be possible, it may not provide a positive payback over time.