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Originally published in Interiors & Sources

01/02/2013

How to Preserve Thermoplastic Roofs

Patch a weldable membrane in 8 steps

By Richard L. Fricklas

 
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    Today’s thermoplastic membranes are heat-welded, a significant improvement over solvent-welded products.

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    Reinforcing fabric between the facer and bottom membrane layer controls PVC’s potential to shrink when exposed to weather.

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    Most modern thermoplastic systems incorporate mechanical fasteners placed in the seams of the membrane, allowing the membrane to form a watertight seal and cover the fasteners in a single step.

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    Mechanically fastened stress plates factory-faced with polymer can be placed below the membrane for a non-penetrating installation.

While the commercial roofing industry closely defines the polymers in manufacture of single ply roofing systems by their acronyms – such as EPDM, PIB, PVC, CSPE, TPO, and the like – from a functional point of view, these membranes are either weldable or not.

Successive generations of these polymers offer continual improvement, making today’s thermoplastic repair job relatively smooth. Ready to repair? Read on.

Innovations in Membrane Materials
PVC systems of the 1960s generally solvent-welded the seams together, using tetrahydrofurane (THF) as the welding agent. However, during application, the THF could absorb moisture from the atmosphere and contaminate the welding solution. THF also has a very low flash point. Today, heat welding has replaced solvent welding, with self-propelled hot air welding machines for field seams and hand-held welders for patching and detail work.

Another change from the first-generation PVC systems was the addition of reinforcing fabric between the facer and bottom membrane layer. This controlled the potential for membrane shrinkage upon weather exposure and improved stress distribution under wind loads. Manufacturers have since eliminated stone ballast to hold and protect the roof membrane. It was found that some ballast materials contained clay that could extract the plasticizer from the PVC compound, resulting in shortened roof life.

Mechanical Fasteners Streamline Installation
Today, you’ll likely find that most PVC and TPO membranes are mechanically fastened to the substrate. Fasteners could be placed in the seams of the membrane, so that the overlying membrane forms both the watertight seam and covers the fasteners in a single step.

Another recent innovation is to incorporate mechanically fastened stress plates that are factory-faced with polymer (TPO for TPO membranes, PVC for PVC membranes). As compared to the penetrating fasteners of earlier generations, these stress plates are placed beneath the membrane, allowing installation without penetration.

How to Patch a Thermoplastic Membrane
Need to repair your membrane? You’ll need hand or machine welders, a reliable source of power, a pressure roller and probe, and a compatible repair material. Once you’ve gathered the necessary supplies, these eight steps recommended by the National Roofing Contractors Association (NRCA) will help you patch the hole.

  1. Thoroughly clean the surface to be welded, extending the perimeter of the patch to provide an ample work area.
  2. Scrub the repair area with a solution of detergent and water.
  3. Rinse thoroughly with clean water and allow the membrane to dry.
  4. If the existing membrane surface is excessively contaminated, enlarge the hole and insert new patch material so that a weld can be made to the bottom side of the existing membrane.
  5. Wipe the area of the defect and the underside of the patch material with a clean, absorbent cloth dampened with a solvent such as acetone or MEK.
  6. Insert tip of welder into overlap area, heating and pressing the two surfaces together.
  7. After the welded area has cooled, check seams for voids with a rounded tip probe.
  8. Where patches are made with reinforced materials, seal the seams at the outer perimeter to prevent water from wicking through the exposed edge of the reinforcement.

Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems and continues to participate in seminars for the University of Wisconsin and RCI Inc., the Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, the William C. Correll award from RCI, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.

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