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Community and Q&A

Locking two buildings together; old, and new build

jklingel | Posted in Mechanicals on

Full circle? We are looking, again, at adding on instead of building new. I’d appreciate any links to prior discussions on this issue, here or other sites, books, etc. PROBLEM: How do you lock a new and an old foundation/footer together? Presently, we have a 2-story, daylight basement, stick. We are considering adding two stories on the daylight end, essentially just expanding horizontally. This will get everything my wife wants on one floor and leave a huge dungeon for me. I envision drilling holes into the present footers/foundation wall, “gluing” in rebar that sticks out X”, then encasing said rebar in concrete when we pour the base for the new building. I don’t think the sticks part will be too complex to figure out, but locking the bases together seems critical. Either that, or leave 4′ between the two buildings and have a second floor passageway between them; that would be easy to do, but then I guess you’d have to hope the two building “move” together, or, better yet, not at all (HIGHLY preferred). Any links or suggestions on caveats to watch for, think about, etc? Pluses and minuses to building snug-up vs 4′ apart? PS: This is in Fairbanks, AK, and all the ground where the new addition would go is undisturbed.

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  1. GBA Editor
    Martin Holladay | | #1

    There are too many variables for this question to be answered over the internet. You need to talk to an engineer.

    Plenty of successful additions are built on independent foundations that are not locked into the existing foundations. But in some circumstances, the technique you describe — drilling the existing footings in order to insert and epoxy rebar protrusions — is necessary.

    Warning to any readers eager to leap forward with advice: nowhere in the country are soils more squirrely than in Fairbanks, Alaska, where variations in permafrost leave roads looking like roller coasters every spring.

  2. jklingel | | #2

    Martin: Thanks. I figured that this was way to big to be answered here, but thought maybe someone had some references to a "how to" manual. Not that something like this is "typical", but more likely "custom" every time it is done. As for the soils, you got it; in some places. Down where there is permafrost, it is a nightmare for road builders and home builders. Fortunately, I am up in the hills where we have glacial till over fractured shist and good stability. Whatever I find out, I will surely check w/ an engineer, but I wanted to be informed enough beforehand to have an intelligent conversation. john

  3. Riversong | | #3


    Ditto on consulting a local soil or structural engineer. But such additions are common practice. Will the footings be at the same level and of the same type (full frost-wall vs shallow frost-protected)?

    Often it's best to leave existing footings undisturbed and pour independent footings a few feet away. Then the foundation walls can be drilled and pinned with rebar to a connecting wall and to each other to maintain integrity between old and new.

    Building the addition tight to the existing house reduces the exterior surface area and consequent heat loss, integrates the two spaces better than a passageway, and makes it easier to share mechanicals.

  4. jklingel | | #4

    Robert: Footings will be at the same ht, same style. Foundation wall will likely be ICFs; old foundation wall is concrete blocks, filled w/ concrete and rebar, 5 courses high. I did not really enjoy laying the blocks, so will likely go ICF this time. I would prefer to build "snug up" for the reasons you mentioned, and my wife thinks the walkway would look painful. I imagine it would be best to open the sheet rock on the common wall enough to remove the vast majority of the vapor barrier, too. Not a big deal to do, and the more I read (esp here) the more sensitive I am about having visqueen in a wall. Thanks for the help. john

  5. jklingel | | #5

    Spoke w/ an engineer. Will pin together w/ #5 rebar. Two in each footer, 3 in foundation wall, embedded 8" in old, 22" in new. I assume we will also pin along the length of the common wall, which will be poured against the old foundation wall. If that does not make sense to anyone curious about this operation, email me at [email protected] and I'll zap you some drawings. john

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