Building Waterproof Roof Decks

When a deck is being framed for a waterproof roof covering, solid blocking between joists is required to support plywood edges and prevent deflection.

Blocking should also be installed between wall studs to provide a continuous nail base for flashing.

This large rooftop deck features two center drains, and was framed to slope the sheathing toward the drains. Most roof-covering manufacturers require plywood rather than OSB sheathing as a substrate.

When wood posts must penetrate a roof deck, the flashing and roof covering are typically lapped up all four sides of the post and covered by a post sleeve.

Thunderbird’s 4-inch deck drain, designed for membrane-type roof coverings, is attached to 16-gauge copper flashing.

The copper stair flashing in the photo is just like deck flashing in miniature—when the cementitious deck coating is applied, it will completely cover the horizontal and vertical legs of the flashing.

Duradek’s copper scupper box is coated with PVC so that the deck membrane can be heat-welded to it, making the assembly watertight.

Before a membrane-type vinyl covering is installed, seams and fastener holes must be filled using the manufacturer’s patching compound.

Multiple pieces of vinyl membrane are needed to cover wide decks.

When sections of vinyl membrane are heat-welded together, the seams are clearly visible.

Cementitious coatings like Desert Crete are applied over metal lath stapled to the plywood sheathing. Chopped fiberglass mat is then glued to the base coat with acrylic resin, which acts as an anti-fracture and waterproofing membrane. After it dries, a protective screed coat is applied, with additional material helping to slope the surface for drainage, as required.

For a final finish, the deck can then be completed with tile or a spray-on texture.

Another option for cementitious coatings is to apply a stamped and stained finish to the deck, using heavy rubber templates laid into a wet skim coat of polymer concrete over the base coat to create the pattern.

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