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We’ve got a situation that has elicited a number of interesting and…other ideas. Maybe someone here has done this before? We have an existing concrete slab floor and are replacing the floor adjacent to it (which also used to be concrete) with a wood joist floor. The new floor will need to have 18″ of crawl space beneath the joist, so we need to excavate approximately 30″ down immediately adjacent to the existing slab floor. Obviously we’re concerned with undermining the concrete slab floor or reducing the strength of its base by digging that deep right next to it. We also need to have some way of keeping the first joist (which will run parallel to the edge of the concrete slab) away from the soil at the edge of the concrete/joist floor edge. We’re thinking of putting in a concrete stemwall that would be in the crawlspace of the new floor but adjacent to the edge of the concrete slab–then backfilling the small gap between the stemwall and the soil directly beneath the slab. The subfloor would then sit directly on top of the stemwall, and the first joist would actually be attached to the side of the stemwall. Has anyone done this before with success? Is there a better way to approach it?
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How about excivating and then pouring a stemwall. If you only form the finished face of the wall, the concrete will fill in any undermining that might have occured. I think this would be easier and more solid than backfill.
I'm assuming that this is just a slab on grade with no footing?
*bengt- I have dome this before and did it the way Ryan suggested. We excavated plumb down from the edge of the slab with A backhoe and dug out, by shovel, under the slab approx. 6" , formed up only the inside face allowing the unex. soil to become the outside form. We filled to A level flush with the underside of the exsisting slab on the outside and above the slab on the inside, thus supporting the slab edge. It worked well and has'nt been A problem for three years and counting. If floor joist heighth is A concern, we ran our paralell wall higher than our perpendicular walls.
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Bengt, you fairly answered you own question: you provided the lead-in and Ryan got specific with the one detail that mattered. I have done almost exactly what you are involved with now. The only difference is that where we excavated was under part of the existing home. Anyway, we went the route that Ryan and Robert mentioned. Think grade beam. Easy and satisfactory.
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Thanks for the suggestions. It is a slab on grade. I hadn't thought about not forming the back side, but that makes a lot of sense and will probably be much easier than forming it up and dealing with getting the forms out and the fill back in. I think I'll go with it!
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We've got a situation that has elicited a number of interesting and...other ideas. Maybe someone here has done this before? We have an existing concrete slab floor and are replacing the floor adjacent to it (which also used to be concrete) with a wood joist floor. The new floor will need to have 18" of crawl space beneath the joist, so we need to excavate approximately 30" down immediately adjacent to the existing slab floor. Obviously we're concerned with undermining the concrete slab floor or reducing the strength of its base by digging that deep right next to it. We also need to have some way of keeping the first joist (which will run parallel to the edge of the concrete slab) away from the soil at the edge of the concrete/joist floor edge. We're thinking of putting in a concrete stemwall that would be in the crawlspace of the new floor but adjacent to the edge of the concrete slab--then backfilling the small gap between the stemwall and the soil directly beneath the slab. The subfloor would then sit directly on top of the stemwall, and the first joist would actually be attached to the side of the stemwall. Has anyone done this before with success? Is there a better way to approach it?