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Under One Roof

Canadian Roofing Reference Manual

7.2.5 Deck Preparation

All decks to receive thermoset membrane must be smooth, clean, dry and of sufficient strength to support the roofing system.

Concrete Deck

Concrete decks must be fully cured and smooth. This may require the grinding of the concrete surface to remove fins and protrusions prior to the installation of the membrane or the installation of a protection mat over rough surfaces. Precast concrete must provide a smooth surface and proper drainage. In fully adhered systems joints should be grouted to prevent seepage of the adhesive.


Thermoset membranes can be adhered directly to plywood that has a minimum thickness of 15.9 mm (5/8 in). Wood planking must be covered with plywood or a suitable insulation securely fastened to the deck.


All decks must be clean and dry prior to the installation of the membrane. Thermoset membranes cannot be installed directly over steel decks. A levelling layer of a suitable insulation, plywood or gypsum board must be installed first.


It is important that only insulations approved by the membrane manufacturer be used in loose-laid, adhered or mechanically fastened systems. In adhered and mechanically fastened systems the insulation is secured to the deck by screws and plates. The number and spacing of the screws is critical and must be installed in accordance with manufacturer’s instructions. In adhered systems the type of facer on the insulation is important in preventing delamination. Manufacturers print lists of acceptable insulations in their specifications. Never use an insulation that has not been approved by the membrane manufacturer.