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Community and Q&A

Details on monolithic slab (FPSF) for unheated garage?

Keith79 | Posted in General Questions on

Hi all,

I like the Frost-Protected Shallow Foundations I’ve read about in Martin’s article,

and in the recent FineHomebuilding article. I don’t understand if people slope their monolithic slabs for garages by making the forms taper upward towards the back of the garage?

The nice thing about the stem wall is that it can be perfectly level for the walls, but the slab can be sloped as needed. I think it’s required to be 1/8″ per ft.

Thanks for any help you can give,


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  1. GBA Editor
    Martin Holladay | | #1

    Different concrete contractors take different approaches. Some slabs are level, with beveled sills at the overhead doors. Some slabs slope to the doors. Some slabs slope to a center drain.

    There are several ways that concrete contractors create these slopes, so step one is to talk to your contractor.

  2. Keith79 | | #2


  3. Expert Member

    For ease of framing, sheathing, siding,drywall and trim you don't want a sloped perimeter on the slab. If you aren't using stem walls I'd slope everything to a drain, rather than the doors. If you do go with stem walls I'd reduce the width above the height of the slab. You have to do this each side of the doors for the tracks anyway, and if you carry it right around the perimeter you don't end up with an awkward ledge.

  4. Keith79 | | #4

    Thanks. That's a good detail about the stem wall. About the center drain: I thought the mandated slope for garage slabs was because of heavier-than-air, fumes and gases-to keep them from accumulating. I wonder if the center drain, which I hadn't thought about, is only for liquids? I guess it goes to a dry well? I read the Mass code today, a small part, and it talks about the slope and/or drain, but not what it is for. cheers.

  5. Expert Member

    The 2006 IRC calls for:

    "The area of floor used for parking of automobiles or other vehicles shall be sloped to facilitate the movement of liquids to a drain or toward the main vehicle entry doorway."

    Not a problem here, but I understand sloping the floor to the doors in cold climates can cause the door to freeze to the slab.

    It depends on your local inspector whether the floor drain is required to be connected to your storm water or sanitary drains.

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