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Community and Q&A

What is the proper way to flash a deck ledger?

josefchalat | Posted in General Questions on

Peter Yost’s details show a ledger spaced out from the sheathing and flashing arranged to allow water to flow between the ledger and the building. The July issue of Fine Homebuilding shows a ledger attached tight to the building with a flashing that directs water over the top of the ledger, and potentially on to the joist hangers and the ends of the joists. Does GBA have a preference for one detail or the other?

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  1. GBA Editor
    Martin Holladay | | #1

    You can do it either way. I like the air space -- or, better yet, an independent deck foundation so that no ledger board is required.

    This month's Fine Homebuilding includes a sidebar that answers your question. It reads, "Space or no space? Spacing the ledger off the wall minimizes moisture damage because water can flow freely between the ledger and the wall flashing. The code permits a 1⁄2-in. stack of washers as a spacer, or you can use alternatives such as Deck-2-Wall Spacers ( To ensure that the wall behind the ledger is watertight, I apply a strip of aluminum or PVC coil stock wide enough to cover the top lap on the siding (or foundation) below the ledger and to extend at least 4 in. above the top of the ledger to lap under the siding above. Because the code limits the distance between the back of the ledger and the face of the rim joist to 1 in., you need to reduce the space between the ledger and the sheathing if the sheathing is thicker than 1⁄2 in. Once the distance from rim joist to ledger exceeds 1 in., the code table doesn’t apply, and you’ll need an engineered ledger attachment or an approved alternative system."

    Below is the illustration from Fine Homebuilding; it is consistent with the GBA detail.

  2. Expert Member

    If you are going to attach the ledger directly to the building I suggest you set your hangers so that the joists are 3/4" above the ledger. This provides a drainage space so that water can travel under the deck boards on the flashing that caps the ledger and is not trapped against the house.

  3. GBA Editor
    Martin Holladay | | #3

    Here's the GBA detail.

  4. Expert Member
    ARMANDO COBO | | #4

    I spec rigid foam for outsulation and a WRB on all wall assemblies, so the use of washers is not appropriate as it compresses the washers to the rigid foam. A better application is fiber cement vertical siding strips to avoid the compression problem.

  5. dickrussell | | #5

    I still wonder about the long-term durability of any ledger attachment method that involves a bolt running from outside to inside and which doesn't lend itself to good sealing around that bolt against water migrating inward. That's why I chose to use the Maine Deck Bracket for my deck. The bolt heads are easily flashed with peel/stick membrane and then covered with siding, leaving only the aluminum web protruding.

    Of the two diagrams posted above, showing a ledger spaced out from the house, I would think that the second one, with the nuts on the outside, would be preferred. If some time down the road the ledger and deck have to be replaced, those nuts are accessible. With the nuts on the inside, and the room inside finished, the rim area is likely to be inaccessible.

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