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Can I waterproof a concrete deck/roof with either of these strategies?

user-2310254 | Posted in Green Building Techniques on

I have an exposed concrete front porch. The porch also is the roof for a storage room located underneath. Because of a builder error, a waterproof membrane was not installed under the concrete and tied into the exterior drain plain. As a consequence, water always intrudes into the basement (conditioned space). (Before anyone asks, the builder will not honor his warranty and suing him is not a practical.)

I’ve researched several options for fixing this issue short of breaking up the concrete and starting over. The two most promising (and affordable) approaches are as follows:

– Option one. Remove several rows of cement siding next to the slab. Cut the strapping and use a hot knife to slice through the one-inch rigid foam. Slit the house wrap and apply a peal and stick window gasket material to the slab and the sheathing. Cover the gasket with galvanized flashing and anchor to sheathing and slab. Reinstall everything.

– Option two involves the same deconstruction steps for exposing the sheathing. But a vendor would adhere a vinyl decking material to the entire slab. The vinyl would be tied into the drain plain.

Both strategies are intended to bridge a small gap between the slab and the exterior that allows water to migrate into the conditioned space. I’m leaning toward option two, but the contractor is concerned that moisture will accumulate in the slab pan and eventually rust it out. I suspect that the vinyl will greatly reduce the amount of moisture that is held by the concrete and that the inner edge of the concrete will still be able to release moisture into the conditioned space in the storage room where my hvac system can dehumidify the air.

I’d really appreciate some input from the experts on this site since I need an effective plan to solve this problem. In an ideal world, I would bust out the slab and just start over, but that approach would be messy and expensive (not that either of my proposed fixes are exactly cheap).

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  1. GBA Editor
    Martin Holladay | | #1

    Option two sounds better, as long as the vinyl decking material is really waterproof and durable. Ask the vinyl decking contractor about the warranty.

    If you end up going with option one, remember that concrete is alkaline, and can react with galvanized flashing and cause the flashing to corrode. It's best to have a layer of peel-and-stick flashing between the metal flashing and the concrete.

    The problem with option one is that concrete is not waterproof. Even when a fairly waterproof mix is attempted, concrete can develop cracks. That's why concrete is rarely used as a roofing material (except when formed into small pieces and make into concrete roofing tiles -- and even then, it's really the roofing underlayment that is the truly waterproof layer).

  2. user-2310254 | | #2

    Thanks, Martin. The concrete is already cracking where it was jointed, so I also like the idea of applying waterproofing over everything to reduce my leak risk. I think I will have to talk them into doing this. The company's technical resource in Canada wanted to install strapping and exterior plywood over the deck to create an airspace. I thought that was unnecessary (and awkward if you are stepping up to the porch).

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