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Cantilevered decks – solve moisture problems on previously-installed ones?

HertzR | Posted in General Questions on

We have a small cantilevered deck off our 2nd floor bedroom, where the deck’s support members extend through the building envelope with no supports below. I recognize this is a less than ideal structure, but it was installed by the previous home owner 11 years ago.

We’ve noticed signs of moisture intrusion on the interior living space (to be expected with this deck set up). The joists are in good shape – just slight moisture-induced cracking/warping to interior drywall and hardwood flooring adjacent to the deck.

My question is: what’s the best way to address the moisture issues besides simply tearing down the deck or starting over?

My current plan of attack is:
1) re-insulate interior joist spacing behind deck with 6-8″ XPS foam board sealed with caulk;
2) re-flash deck joists using the approach described below in the FHB link; and
3) replace the deck flooring with Azek porch boards (which have minimal spacing between each board) to limit water drainage from the deck surface onto the joists. The deck joists are level, so installing the new porch boards would involve machining long angled shims to give the deck a slight pitch (am aiming for 1/8-1/4″ per linear ft).

Is this a wasted effort? As in, am I better off just tearing the thing down because there really isn’t a good solution? Or can I make something work effectively that will last 10+ years?

Many thanks in advance!

Flashing cantilevered joists

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  1. GBA Editor
    Martin Holladay | | #1

    Mike Guertin's flashing tips are good as far as they go -- in fact, the suggestions are probably the best approach available -- but there is still an opportunity for wicking through the wood fibers.

    I think that the solution involves a chainsaw or Sawzall, and starting from scratch.

  2. Expert Member

    The only sure-fire way to deal with cantilevered decks is to use a membrane as the decking. This involves sloping the joists and providing a plywood substrate. It also means removing the siding and re-flashing the deck/wall intersection. Martin's solution is usually an easier option.

  3. Jon_R | | #3

    I have a roof overhang that seems to work. So perhaps an awning.

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