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Porch Ledger Attachment with Exterior Rigid Foam

Ray Sebold | Posted in Green Building Techniques on

I’m wondering what techniques others have come up with to attach porch deck and roof ledgers to exteriors walls with 3″ to 4″ of rigid exterior foam. I suspect this is a question that begs for an engineer’s stamp but I would appreciate an overview of methods others have used to both attach a porch roof, for example, without compromising the thermal break provided by the 3″ to 4″ of foam. Perhaps one can through-bolt the ledger through the foam directly to the interior stud wall. I’m presuming the ledger would be directly against the exterior of the foam in the same plane as the 1×3 furring (rain screen). I’m guessing the frequency and method of through-bolting would reflect the attached load. Thanks for any responses.

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Replies

  1. John Brooks | | #1

    You might get some ideas looking in this manual
    http://www.cchrc.org/docs/best_practices/REMOTE_Manual.pdf

  2. Corian Johnston | | #2

    Yes you should get an engineer as there are also new lateral tie requirements in the standard residential building code (IBC). To answer your question on foam, it will be very difficult to make this work with any foam in place as the bolts would want to twist downward when loaded. The amount of load, and therefore twisting, is a function of how big a deck you have and how many people are dancing on it at the same time. The capacity of the bolts, even with solid blocking, will be drastically reduced depending on the distance from the ledger to the supporting member. Go with at least the solid blocking noted in the earlier reference and give serious consideration to an engineered solution.

  3. Expert Member
    Armando Cobo | | #3

    IMHO, you maybe better off installing 4 footings and posts detached from the house. It should be less cost and more structurally sound.

  4. James Morgan | | #4

    Agree wholeheartedly with Armando. Resolving vertical loads through 3-4" of foam is going to be way more trouble than it's worth if posting down to ground is an available option.

  5. Chris Koehn | | #5

    There's a good article on the subject in the May 2009 "Structure" (available free on-line). Solutions almost always use blocking to minimize shear at the connector.
    The answer will also vary depending on where you are building. Here in earthquake land attachment requirements are quite a bit more stringent than elsewhere.
    Chris Koehn
    TimberGuides Design & Build

  6. Chris Koehn | | #6

    Sorry, that's May 2009. For some reason I can't post the link without being tagged as spam.

  7. Ray Sebold | | #7

    Thank you all for your responses. I understand the economy to include the load of a first floor deck in the foundation. Roofs and second floor decks are another story. John, your link to the CCHRC guide is great! I can't believe I missed this one.

    I am building in Western MA. No immanent earthquake, I hope...

  8. John Brooks | | #8

    Chris, STRUCTURE looks like a good resource
    I will attempt to post the link
    http://www.structuremag.org/issue.aspx?Archive=5/1/2009
    which article are you talking about?

  9. Dick Russell | | #9

    If the foam were only 2", you might look at the Maine Deck Bracket. It's like a section of aluminum I-beam, but with unequal size flanges. The large face bolts directly to the rim board, and the ledger to the smaller face. The faces are 4-1/8" apart IIRC, so inside the flanges is around 3-1/2". From that you'd have to subtract thickness of sheathing and siding, plus thickness of the heads on the bolts attaching the ledger to the outer flange. That's why you'd be down to around 2" of foam that would fit into the remaining space.

    To accommodate 3-4" of foam, you'd have to engineer your own standoffs, if you want to use the deck bracket concept. Likely it would have to be taller than the MDB, given the added torque due to the ledger being out farther from the rim. That also would raise questions about the attachment of the rim board to the floor structure and even required thickness of the rim board. Clearly an engineer would have to provide input.

    I agree with Armando's thought that you'd be better off if you can make the deck free-standing. Proper bracing of it would be traded off against the engineering of attachment to the house and the matter of sealing against water intrusion.

  10. James Morgan | | #10

    Perhaps an important takeaway from this discussion is that adding several inches of nonstructural material to the outside of a wall may have unintended consequences. A single-level deck is not too much of a problem but reliably attaching two-story roofed porch structures begins to get way complicated. I'd question the basic premise, if it's not too late, and look at ways to achieve the desired enclosure performance with a slimmer layer of exterior insulation.

  11. John Brooks | | #11

    Ray,
    James has a good point.
    Thorsten Chlupp has decided to abandon the REMOTE wall for many reasons.
    His new Arctic Wall much like Riversong's wall is extremely Not-So-Foamy and it can dry in either direction.

    The free standing deck IS a good idea
    perhaps you can engineer a "free-standing" covered deck
    I think I have seen it done on some German projects

  12. Rob Harrison CPHc | | #12

    I recommend hiring a structural engineer rather than speculating. What will work depends, of course. :) Intuitively and in general I agree with those who suggest a separate support for the deck outside of the thermal envelope. We've done that on a permitted Passive House project in a challenging climate where the distance from structural wall to face of exterior wall was large enough that bolting back to the structure was impractical. If you look at the very first Passive House project, Wolfgang Feist's 1992 townhouse in Darmstadt, you can see a similar approach.

    However, on our first permitted Passive House project we used 4" of rigid on the exterior, and supported ledgers for both deck and roof overhangs with bolts through blocking, a detail developed by our structural engineer. The blocking was 2x perpendicular to the rim joist. There were two bolts in each block, top and bottom. We are in Seismic Zone 3, btw. Of course they have to be properly sized and spaced for shear. He explained how it worked thusly: In order to fail, the bolts would have to elongate, which they can't do.

    Edit: Pictures of the latter detail right in first post in this thread, John Brooks link to the REMOTE manual!

  13. Ray Sebold | | #13

    Thank you all for your thoughtful responses. I've worked with structural engineers before and find that they, like many folks, vary quite a bit in their approaches. Some are very "rigid" and have their best and unyielding solution to your situation. Others are more willing to look at your proposed ideas and work with you to figure out a workable solution. I prefer working with the latter and, hence, have solicited your ideas such that I can offer possible solutions and initiate a discussion of a best solution for my situation.

    That said, I do like the idea of a small bump out to the foundation to carry the load at the house. I can see this being done perhaps more cheaply with a second pour or CMUs (pinned to foundation) on a footing bump our rather than spending the cost of achieving this in the original forming. BTW, I am strongly considering using Thermomass for my foundation insulation. It is an internal layer of EPS within the foundation wall - a 4" wythe, 2" EPS, 4" wythe for a total of 10". Anyone use their product before?

  14. Expert Member
    Armando Cobo | | #14

    A good building for its clients is not only aesthetically pleasant and functionally adequate for its clients, but structurally sound and cost effective. Anything is possible if you have enough in you budget, see: http://www.buildinggreen.com/live/index.cfm/2011/6/30/Structural-Thermal-Breaks-for-Steel-Framing.
    The more this project rumbles in my head, the more I think a free-standing deck and roof is better; two-story and all.

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