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Is water migration through deck grade beams into slab a concern?

Owenmd | Posted in General Questions on

Hi my name is Owen,

We are working on a project in SF bay area, zone 3 and  we are in the process of digging grade beams for a slab on grade foundation. I have read articles on GBA and seen details for water proofing a slab on grade with drainage and vapor barrier wrapped up side walls of slab.

We have exterior uncovered deck grade beams that extend from our houses’ mat slab and will support a wood deck.  I am not clear if I need to be concerned about water migration from these deck grade beams in the winter when it rains. We have wood flooring inside so we don’t want a moist slab.

We are on soil that is subject to settlement so the grade beams of deck are 18’x18″ and there are around 20 connections to the house mat slab, to give a sense of the area that deck grade beams are connecting with house slab.

Is this situation a concern?

Also There are a few small slab extensions to the exterior that are covered in stone, or by decking, should these have some membrane or sealant over them?


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  1. GBA Editor
    Martin Holladay | | #1

    I have never before heard of a foundation like the one you describe. It sounds as if your slab has finger-like extensions -- extensions designed to support a deck.

    This means that the "fingers" -- the grade beams designed to support your deck -- are (a) thermal bridges, and (b) potential highways for moisture (as you correctly surmise).

    In your relatively mild climate, you may be able to ignore the thermal bridging problem. But I think you need to address the potential moisture issues.

    The standard solution to a damp slab is to install a layer of 6-mil polyethylene above the slab, followed by a continuous layer of rigid foam and OSB or plywood subflooring. The OSB or plywood is attached to the concrete slab with long Tapcons that extend through the rigid foam.

  2. Owenmd | | #2

    Martin thanks,

    I attached a foundation plan, and detail of slab/deck grade beams. yes grade beams extend from the foundation to support the deck so that if there is settlement the deck is still level with house, this is engineers design based on weak soil that came from geotech.

    We also have a high water table with very little slope for sub drains to drain (like 1/2%) so I was worried about moisture and wanting to continue polyethylene up footings, is this still a good idea given water migrating from deck grade beams and possibly stoping water with some layer above slab? the contractor says he has never seen grade beams/footings having poly wrap up sides and thinks it a waste of time/money.

    We do have interior insulation but it is a fiberboard, plywood and then hardwood floors, so need to protect floor assembly from water if there is a danger. Is Polyethylene best in this situation. Do I need to think about any mechanism for slab to dry? seems like that has been something I have read about at GBA. Would a WRB prevent water but allow moisture to escape from slab or is this not a good idea? Not sure if the little vapor that came through slab would cause mold issues or just dry out given or floor coverings are vapor open?

    One other issue I was struggling with on floor assembly was whether to screw down with Tapcons or float with 2 layers of 3/8" ply glued and screwed. My contractor thought Tapcons, but didn't know if there was a strong case for floating given thermal bridging or if there were any moisture issues screwing through which ever membrane we put over slab.

    Many thanks!

  3. onslow | | #3

    Owen D,

    I couldn't really see the supplied drawings with clarity, so I can't really see how you intend to tie the slab to the grade beams. However, if you have a very high water table then it might be worth looking into a product called Xypex.

    It comes in at least two forms, one of which is mixed directly into the concrete. Primarily used in commercial work, like parking garage decks and such, it has also been used for foundations near bodies of water. I have heard of at least one locality that will waive exterior waterproof coatings on foundations that use directly incorporated Xypex. The rep in your area should be able to better describe the properties and differences between admixture and applied types.

    I used the admixture form to protect the footings against water levels and then decided to go all in and do all the wall pours as well. I have a very dry basement, though some might dispute why. The water paths for my situation are unlike yours.

    Water is very patient and persistent so the idea of relying on plastic wrap around the beams to protect against ground water is a risky option. The chances of the plastic even being put down without holes getting poked in it are less than a winning lottery ticket. Surviving the forms stripping would not likely have much better odds. You mention 20 locations of contact between the beams and these might more successfully be managed with care. Still, you would want to weigh the odds on perfect work there, too.

    Since I am also not too clear on the slab to floor profile, I can only say that you might want to search the GBA site for other mentions of putting flooring down on basement slabs. Lots of opinions and tactical ideas. Tapcons are not well regarded by many including myself. A floating floor has its own issues, but the thought of how many fasteners would be needed in the plan shown is daunting. I would look into applied coatings for sealing the slab regardless of which floor method you go with. After all, in San Francisco moisture problems will come in on little cat feet, right?

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