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How can I minimize concrete usage when lowering a basement floor?

manbeardo | Posted in Green Building Techniques on

I’m considering underpinning my 1912 poured concrete foundation in order to get more ceiling height. At the slab’s highest point, it’s 6’10” from slab to joist. I’d like to get 8′ between finished floor and finished ceiling. Our neighbor’s house was built at the same time by the same builder and doesn’t have a footing, so ours probably doesn’t either.

I’d like to add as little new concrete as possible in order to avoid the embodied carbon. I can think of a number of ways to go about doing that, but I need a gut check on whether they’re feasible.

Option #1: Underpin with concrete and build a non-concrete “slab”

This seems like the most conventional option. It looks like the easiest way to do a non-concrete slab is by building a floating floor. Do underpinned foundations need some kind of structure to resist horizontal forces? If so, could that structure be provided by lumber at the walls or concrete beams poured between the footings?

Option #2: Lift the house and replace the concrete foundation with a permanent wood foundation

This seems like it might be cost-prohibitive. Is it possible to build a permanent wood foundation in place without digging a trench around the exterior? Since we’d be digging down after lifting the house, we’d have to put the jack posts in at or below the desired depth. Would we have to pour footings for the posts?

Option #3: Underpin with a permanent wood foundation

This seems the least possible. We’d need to install PWF in phases without an exterior trench and we’d be using lumber to support the weight of concrete. Is this even remotely feasible?

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Replies

  1. Expert Member
    Zephyr7 | | #1

    I would absolutely not trust a wood foundation. I think this is one place where you should really just stick with concrete. Remember that if you are trying to build green, one of the goals needs to be not having to RE-build every so often, since the extra materials use and waste from each rebuilding cycle will rapidly offset any intitial issue you might have had using concrete.

    Concrete has two main advantages here: it doesn't care about water, so no issues with mositure degredation, and it lasts. 100 year old concrete foundations that are still fine are not unusual. If your foundation fails, everything else follows and this is EXPENSIVE work to have to do down the road.

    In terms of using lumber to support concrete, yes, it's possible -- that's how concrete forms are built. If you mean to use the lumber as a concrete support in the long term, I suppose that would be possible but I've never seen it done that way -- concrete structures usually use steel for support.

    If you want to try to minimize your concrete use (and I really don't think that is a good idea here), I'd use the concrete for walls and footings, then crushed stone to possible allow for a thinner slab floor. You would probably have some issues with building codes here regarding minimum slab thicknesses, and you'd have other tradeoffs too (more prone to cracking, etc.).

    There is a lot of downside to what you're considering here for a minimal benefit which, in my opionion, will be more more than offset by future structural needs.

    Bill

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