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I've got a "crawl space" (really an unfinished space) under the main floor in a 2-story-with-basement house. I'd like to build essentially a deck frame and cover it in 3/4" plywood so that we can more easily use it as a storage area.

Picture of the space: Posts on the left, foundation not visible on right

I've been planning on attaching a band joist to the concrete foundation on the right side, and additional band joists attached to the support posts on the other side (left side of the picture). I'd then run floor joists between them (approx. 7.5' in length) supported by joist hangers.

Is attaching to the support posts a bad idea? Not being a structural engineer, I worry about the additional load, and the potential for weakening those posts by attaching the band joists with lag bolts. Should I be worried, or is there a better way to support that side of the floor that doesn't involve digging down and pouring concrete footings?

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Looks like you have concrete pillars with 2x's coming out of them. Perhaps you can frame a deck floor space, fastening treated lumber to them and maybe anchoring a joist to the concrete wall. If you anchored to the center posts at concrete pillar, there would be no support problems. Use 2x6, 2x8 should work. Good luck

  • Those are actually 4x's out of the concrete with the occasional 4x6. So I shouldn't need to worry about weakening the posts by attaching a (treated) band joist with lag bolts, or the added load on those posts? Note that the joists cannot rest on the concrete - the level of the floor will be about 1' above the concrete. – user2446 Jul 20 '16 at 21:29
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Because the new floor doesn't necessarily need to attache to the structure, and because I'd have some of the same concerns regarding additional load on the posts, I'd probably do a free-floating floor system on concrete post bases. Treat the situation just as you would an exterior deck.

I'd bring in a bit of washed rock to provide support for and allow leveling of the post bases. I'd space them as appropriate for a rim joist around the outside of the space, and for a doubled flush beam in the center. Frame the floor with treated 2x8 or 2x10 lumber, depending on spans.

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Thought I'd follow up with how I ended up solving this problem. I really wanted to avoid digging new holes and adding new support columns. That just seemed like too much work.

What I ended up doing was resting new 4x4 supports on the existing concrete footings, and strapping them to the existing supports. This way the load is directly on the concrete footings, but the straps provide what little horizontal support is needed. I also strapped the band joists to the posts, though that was probably overkill (though it did keep them in place while framing everything out). I attached the opposite band joists directly to the concrete foundation, and used joist hangers to support the joists. The floor is now built and is very solid. And the added storage space allowed us to get rid of three storage lockers!

eventual solution

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