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How much rock under foam/slab in below grade basement?

silasjames | Posted in Green Building Techniques on

We’re building a new home in Central Iowa. The basement is below grade with 9′ foundation walls. The concrete slab will be approx 8′ below grade. Under the slab in this order is poly sheeting vapor retarder, 2″ Foamular 250, 1″ crushed rock then soil. There is 4″ drain tule spec’d for next to the footing on the interior below the slab. That tule will tie in to the tile on the outside and it will all drain to a sump pit. How much 1″ rock do I need under the foam and also under / over the tile on the exterior of the foundation wall? They have specd 4″ under the foam and not sure about the exterior side. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    The specification that calls for 4 inches of crushed stone under your rigid foam is correct; that is standard.

    When it comes to exterior footing drains, most specifications call for 3 inches of crushed stone, followed by a 4-inch perforated pipe (installed either dead level or with a slight slope, depending on your preference, and then connected to a solid pipe that leads at a significant slope to daylight).

    For more information on these issues, see these three articles:

    How to Install a Foundation Drain

    GBA Encyclopedia: Foundation Drains

    Polyethylene Under Concrete Slabs

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