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Enclosing the space under a deck

Brad964C4 | Posted in General Questions on

I have an interesting situation I’m trying to resolve. The home is in southern NY & my client had a deck built about 5 years ago which is fully enclosed below with concrete block walls & stone veneer. The space underneath the deck is unfinished & has at least 8′-9′ of headroom & is just used for storage of outdoor furniture in the off season. The space has a few inches of gravel over earth.

Decking is mahogany…..not T & G, so water drains through the deck. The space suffers from high moisture levels & is wreaking havoc with the deck framing, as some have rotted & need to be replaced. Inside face of the concrete block walls are wet as well.

My proposed solution is to:
Remove the decking (saving for reinstallation. Is possible since hidden fasteners were used).
Install EPDM membrane or a deck drainage system that laps over the top of all joist & beams.
Pitch membrane to a gutter, tying into a leader which would extend to daylight outside the space.
Install a concrete slab over gravel & 10 mil poly VB to eliminate moisture from below.
Reinstall decking from above with screws.

This should keep the space dry, but my question is should we consider an exhaust fan that’s on a humidistat to exhaust moist air? I think we should not be considering any type of passive vents (like those used in a crawl space), as those seem to do more harm than good. Any suggestions or recommendations are welcome…

Brad DeMotte, RA

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  1. GBA Editor
    Martin Holladay | | #1

    If you can control all modes of moisture entry, this space will be dry. If it isn't dry enough after you have controlled all modes of moisture entry, the owners can install a dehumidifier in the space.

    To control rain entry from above, you need a roof. If you are a roofer or you can hire a good roofer, then you're all set. The deck goes above the roof, and the deck needs to be installed in such a way that it does not threaten the integrity of the roofing. People install decks on top of roofs all the time, so it can be done. These decks are often detailed poorly, however, so hire a roofer who knows what he or she is doing.

    Hopefully, the cladding on the walls was installed properly, in a way that manages moisture, with all necessary flashing.

    As far as the floor is concerned, you need a polyethylene vapor barrier, with a concrete slab above. Ideally, the top of the concrete slab will be above the exterior grade. If you can do all of that, you have controlled water entry into the space.

    Then let the space dry out. If it is still too damp, plug in your dehumidifier.

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