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I have a walk-out basement with two garage doors. I'd like to remove these garage doors and fill in the gaps in the foundation, and then add one or two service doors in their place.

I'm wondering if I should somehow tie the new walls I'm going to put in to the existing poured foundation. My initial thought was to lay cinderblock with rebar every other row that ties into the existing foundation. I'm also planning to fill this wall in with concrete to make a solid wall. I'm not sure if that's overkill or if it's not sufficient enough. It would probably be fairly comparable, cost-wise, to have concrete poured instead, but I'd rather do the work myself and that means cinderblock.

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  • What's the header situation? It matters whether you're mortaring two sides or three.
    – isherwood
    Commented Jun 13, 2023 at 18:34
  • If the basement is specified for use as a shelter, you might have to check with an architect or structural engineer to ensure the quality of the final product. Also, the construction details.
    – r13
    Commented Jun 13, 2023 at 18:38

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Where filling an opening with block in a non-structural situation it's usually sufficient to simply mortar the block to the existing block or concrete. This is common when eliminating windows, for example. You may want to strip any existing finishes or coatings to allow a good bond.

If this bit of wall is replacing a header over the door there's more to it, and you'd be wise to consult a local expert or at least use some steel reinforcement between the old and new walls. You may also cut a channel into the existing opening to allow the mortar to engage with the old wall.

One other consideration is the weight of the new wall. Your slab may be unsupported in the door opening. If there's no footing under it the weight may cause settling, which would obviously be problematic. You'll want to know what that situation is.

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    I presume there is a perimeter concrete footer beam under the slab where this wall would go. Would this have to be verified? In the original design this would have the function of supporting the edge of the garage slab that vehicles would drive over. Would such a beam be able to support a CMU wall with concrete poured into it? And should holes be drilled to fit vertical rebar to stabilize the connection of the CMU wall to the slab? Finally, at the top should framing (top plates and cripple studs) or shims be used to transfer some of the header load to the CMU wall? Commented Jun 13, 2023 at 20:22
  • You make a good point about the footing. I missed that. Block walls aren't usually anchored to footings in my part of the world. The mortar bond and friction are adequate. Earthquake-prone regions may have other requirements. I'm not sure why you'd want to try and support an existing header beam. Presumably it's up to the task by itself.
    – isherwood
    Commented Jun 13, 2023 at 20:25
  • The existing garage door header and and load bearing post between the doors make possible replacing the garage doors with stud framed walls (with exterior siding or 4x8' sheets) or even superinsulated factory prefab wall units, unless of course the OP plans to bury the wall with soil. The replacement walls would not be load bearing and this would not add significant load to the slab in the current doorways. The OP could frame it however he decides. There seems to be no need for CMU walls. Commented Jun 13, 2023 at 21:57
  • Thanks. There is a concrete header above the existing doors, so I presume I'm safe there as that's bearing the load of the house above. I had thought of doing a stud wall to close in this space, however a greenhouse is going on the other side of the wall and I thought concrete blocks would be better, in the winter it would act as a thermal mass to radiate the sun's heat through the night and in the summer it'd insulate the rest of the house from the greenhouse's high temps. Commented Jun 14, 2023 at 14:34

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