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What are the pros and cons of pouring concrete in an existing crawl space, and how thick should it be?

Insular | Posted in Green Building Techniques on

I admit I am a bit of neat freak, but the idea of have a nice concrete floor in the crawl space appeals to me.

This is a spacious crawl space: 20×24 feet and nearly 6 feet tall. It houses the propane furnace and the electric water heater.

The walls I believe are concrete black. The floor is currently stone and dirt above a white plastic sheet which shows through in a few places. The crawl space opens to a smaller basement below it.

I believe the crawl space s currently vented to the outside (a couple of small, probably ineffective vents), and I am also considering sealing it.

If I do pour concrete (by “I” I mean a contractor not me personally), are there any considerations or drawbacks I should be aware of? Are 2″ sufficient for this application? Should I request a specific kind of concrete?

I am going for cleanliness, ability to use space for storage, making access to mechanicals easier, and possibly lowering Radon (which is already below the EPA action level).

There is no water or humidity poblem currently, either because of the plastic sheet or because the land gently slopes away from the crawl space.

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  1. GBA Editor
    Martin Holladay | | #1

    Your questions are best directed at a contractor.

    Most basement slabs are between 3 and 4 inches thick. It's possible to install a so-called "rat slab" in a crawl space that is as thin as 2 inches, but such a thin slab will be prone to cracking. You probably won't save very much by installing a 2 inch slab instead of a 3 inch slab.

  2. Expert Member
    Michael Maines | | #2

    An inch of concrete for that square footage is about 1.5 cubic yards. A yard of concrete is roughly $100. Like Martin said, you might as well pour 3". You should read various blogs here about insulating the floor and crawlspace walls before you pour, if you live in a cold climate of any sort.

  3. davidmeiland | | #3

    Whenever possible, when we pour a foundation we also pour a rat slab on the crawl space floor. We grade it relatively flat, lay down a sheet of 6 mil black poly, then pour 2-3" of concrete over it. We use a water reducer so that the concrete flows out easily (it still takes some raking) then bull-float it flat... and leave it.

    I like doing this because it makes the rest of the work a lot more comfortable, it makes the vapor barrier permanent, and it keeps rats and whoever else from digging.

    If I were doing this under an existing house, I would set up a fan to draw air out of the crawl for a few days. A slab loses a lot of water as it cures, and a lot of the will evaporate up into the house if you don't vent it.

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