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Fastening details – deck ledger to insulated slab

user-5293988 | Posted in Green Building Techniques on

I am designing an insulated Monolithic Slab. I plan to install 4″ of Roxul comfort board beneath and around the perimeter of the slab.

Does anyone have experience attaching a deck ledger to a slab (or stem wall) that is insulated with Roxul? Or is it advisable to place a higher density product beneath the ledger to avoid compression which may happen with the Roxul Comfortboard?

What type of fasteners are recommended for this application? Would a standard wedge type concrete fastener like a red head work?

Other solutions?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it's hard to imagine a situation where a deck ledger would be attached to the perimeter of a slab. It sounds like the deck would end up too close to grade.

    That said, your best solution is to support the deck on independent footings.

  2. user-5293988 | | #2


    My plan is to leave the top of the grade beam & slab at 10" above grade. This will give me room to have a 2x6 ledger.

    I am trying to stay away from Independent footings for two main reasons:

    1 - interfering with horizontal roxul insulation as per fpsf guidelines
    2 - detailing the water barrier atop the insulation sounds hairy (to ensure water drains toward the perimeter drain at the outer edge of this horizontal insulation)

    From what I observe, an attached deck or porch is a fairly common feature to many buildings and is a great way to blend inside/outside space.

  3. Expert Member

    If you want to connect the ledger through exterior insulation I'd suggest offset supports such as Maine Deck Brackets. One caveat is that they are made of aluminium so you would need to provide some barrier (a piece of membrane?) between them and the concrete, and select the anchor material carefully.

    When a deck is going to be that close to grade, I either make it a patio, or if wood is the preferred finish I construct the deck on a bed of gravel with no ledger, beams or posts, so that the whole deck is load bearing.

    1. CamWalker | | #10


      Sorry to hack such an old post, but I’ve read your approach several times but finding it hard to find your many posts again. I wonder if you could spare some more information regarding your deck on grade approach, especially when it sits next to a house built with a frost protected monolithic slab. I have just designed such a house, and your approach seems like the best way to build the wrap around decks as they are so close to the ground anyway, and budget is a concern at this point (halfway through the build ofcourse). Thank you kindly


      1. Expert Member
        MALCOLM TAYLOR | | #11


        Probably the best way to think about it is that the deck is a patio made of wood. So you prepare the substrate in much the same way. Compact underneath, and lay a layer of free draining stone (I use 3/4" clear-crushed). The deck is continuously supported on the stone base, so you can use pt 2"x4"s and top them with any decking you like. Although the substrate is initially clean rock and there isn't much chance of rot, as time goes on debris may accumulate underneath, so it's best to use ground contact rated pt for the framing.

        Here is a picture of my back patio/deck built that way, which is a mixture of concrete and wood.

        1. CamWalker | | #12

          Beautiful! And thank you for sharing. So a good 6” of washed 3/4 crushed stone On top of a good 12” or more of compact fill?

          1. Expert Member
            MALCOLM TAYLOR | | #13


            A lot depends on where you are and the existing soil.

            The more free draining rock, the less chance of frost heave. Here in the PNW I just used 2".

            If the soil is fairly free of organics and compacts well, you may not have to bring in any additional material. One of the beauties of the system is there are no point loads, so the base isn't that important - and much less so than if you were using pavers or a slab.

  4. user-5293988 | | #4


    Thanks for the tip about the deck brackets - that solution shows potential. I like the idea of keeping the framing and decking spaced away from the building to facilitate drying.

  5. Expert Member

    There are a few variations on the same idea. The Remote System uses blocks of PT wood.

    Good luck with your build!

  6. user-5293988 | | #6

    Thanks for the link Malcolm.

    There are many great photos and diagrams for attaching ledgers through thick foam to a building to a rim or band joist.

    Recently I came across this diagram of using anchor bolts to attach a ledger with an icf system..although the diagram doesn't have any insulation between the framing and the concrete. Maybe I could use the embedded anchor bolts and pass them through either some pt blocks (as in the remote system) or some high compression XPS or CompacFoam before going through the ledger.

  7. Expert Member

    Either might work. The only solution that will probably be accepted without an engineers stamp is the Maine Bracket. All the rest are going to need some professional input at some point, so it's a bit of a guess what they will be comfortable specifying.

  8. ethant | | #8

    I am very curious about Michael Iacona's CompacFoam ledger detail mentioned above. Is there any way to save it from the error code currently displayed?

  9. GBA Editor
    Martin Holladay | | #9

    I tried, but I can resurrect the image. It seems to have "illegal characters" (spaces) in its title.

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