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Under One Roof

Canadian Roofing Reference Manual

13.2.4 Thermoplastic Membranes

Thermoplastic membranes are composed of polymers which soften when heated and harden when cooled. Thermoplastic membranes can usually be hot air welded throughout their service life. This section contains general repair procedures for both reinforced and unreinforced thermoplastic roof membranes.

Typical Thermoplastic Membrane Repair Procedure

The following patching procedure is utilized in several repairs which require installing a patch to return the membrane to a watertight condition.

  • It is essential to prepare the surface by removing debris, contaminants and ballast from the area of the membrane or flashing to be repaired.
  • The area prepared should extend beyond the perimeter of the patch to provide an ample clean work area. Scrub the repair area with a solution of detergent and water.
  • Use warm (if available) water and a stiff bristle brush to scrub the membrane. Rinse thoroughly with clean water and allow the membrane to dry.
  • If the existing membrane surface is excessively contaminated or degraded in the repair location, carefully enlarge the hole (make round or round any ends/corners) to allow the insertion of the new patch material under the existing membrane so that welding of the patch may be accomplished to the underside of the existing membrane.
  • Cut a patch from a piece of new membrane material large enough to extend 100 mm (4 in) beyond any part of the defect. Round all corners of the patch to limit peeling of square corners.
  • Wipe the area of the existing membrane to receive the patch and the underside of the patch material with a clean, absorbent, lint-free cloth dampened with a surface cleaner as currently recommended by the membrane manufacturer. Do not pour the cleaner directly on to the membrane. Allow the surface of the membrane and patch to air dry.
  • Pre-weld the patch by inserting the hot-air nozzle under the patch and forming a continuous pre-weld approximately 50 mm (2 in) from the edge of the patch. This pre-weld will prevent heat leakage during the final welding.
  • Follow the welder nozzle closely with a rubber-faced hand roller to provide pressure to adhere the back of the weld area. Final weld the outer 50 mm (2 in) by concentrating the heat on the remaining unadhered outer portion of the patch until the membrane and patch have reached the welding temperature. The hot-air welder should be moved quickly enough to avoid scorching the membrane, yet slowly enough to achieve a complete weld. Immediately behind the welder, roll the material to be bonded with the rubber faced hand roller, rolling parallel to the outside edge of the welder nozzle and pressing firmly to adhere and mate the materials.
  • After the welded area has cooled, check seams for voids with a rounded tip probe such as a screwdriver or seam probe tool. Seam seal the outer perimeter with seam sealant or caulking paste compatible with the membrane.
  • If the ballast was removed, redistribute the ballast over the exposed area.


If the membrane is not damaged, no repair is necessary.

If the membrane is creased, cracked, or damaged then repair is necessary.

  • Cut the ridge or buckle to lay the material flat after preparing the surface and then install a patch over the cut.
  • Complete the repair as described in the Typical Thermoplastic Membrane Repair Procedure above.

Laps and Splices

  • Pull back the material in the unbonded lap until an area is reached where a strong bond exists. If water leakage has occurred, open the membrane and inspect the insulation and deck for damage. Remove and replace any wet or damaged insulation, vapour retarder and/or roof deck as required.
  • Clean and reweld the open lap as best as possible to adhere the seam.
  • Strip in the rewelded seam with a nominal 200 mm (8 in) wide patch installed as described in the Typical Thermoplastic Membrane Repair Procedure above.

Ballast Scour

If ballast scour is discovered, inspect the membrane for damage and repair if necessary.

  • Redistribute the ballast being careful to not damage the membrane.

Penetration Pockets (Pitch Pans)

If the filler material has cracked, pulled away from the side(s) of the pocket, or is below the level of the top of the pan, remove the loose or damaged sealant material.

  • Wire brush and clean the penetration extending through the existing metal pan.
  • Thoroughly mix and pour in the new two-component sealant.
  • Crown the sealer above the top of the penetration pocket to provide drainage away from the penetration.