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Community and Q&A

Is it OK for a basement floor slab on slab edge on only 3 sides?

boathook | Posted in General Questions on

I am having a new house built and when I looked at the foundation there was a floor slab edge only on 3 sides and when I called the general contractor he told me the concrete contractor had already poured the floor and said it was all fine. the rest of the concrete work looks fine, but missing the slab edge on one side worries me and I don’t know what to do???

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  1. GBA Editor
    Martin Holladay | | #1

    I am not familiar with the term "slab edge" -- perhaps it is a local term. Are you talking about vertical rigid foam insulation, or something else?

  2. boathook | | #2

    the home style is a split level with a rear walkout to the back yard. what I'm calling the slab is the actual basement concrete floor, and in my plans it shows the footings to be wider on the inside edge so the basement floor rests on the footing, and it does on the front and both side walls, but the rear wall is 4' deeper because the rear has a walkout door. So when the concrete contractor formed the back wall he dug 4' down and laid a footing then a 6"x 4'wall on that for the 6" wall to be built on top of that, and there was supposed to be an 8" wall leaving a 2" ledge that the basement floor slab can be supported by, but he didn't create that edge, so the rear edge of the concrete rests on the gravel bed but it isn't supported by the concrete wall.

  3. GBA Editor
    Martin Holladay | | #3

    If the concrete contractor failed to follow the plans and specs, then the concrete contractor is responsible for the cost of correcting the error. You will have to contact an engineer to determine the best way to correct this error.

  4. GBA Editor
    Martin Holladay | | #4

    Once you have solved any (possible) structural problems, you can return to GBA (if you want) to let us know whether your basement slab and the foundation walls have adequate insulation details. Walk-out basements often have poor insulation details, especially under the door threshold.

  5. Expert Member

    Your concrete floor is supported by the compacted fill underneath it, not the footing around the edges. In fact having the slab poured directly on top of the footing can cause it to crack as there will be more settlement in the centre than the edges. From your description there is nothing wrong with what your contractor did.

  6. wjrobinson | | #6

    We do it the way you have it done. I'm with Malcolm. There's a lot of second guessing builders today via the Internet. Start with a good builder and enjoy the process. Be good to your crews, it comes back in spades. Always show up with compliments Coffee and sandwiches at lunchtime. We love customers like that and return the goodwill and tell others how wonderful it is to have such nice people to work for.

    It's a two way street, there are as many good and bad customers as there are contractors.

  7. rocket190 | | #7

    I see no problem either as long as the gravel fill in the trench was properly back filled. People get all warm and fuzzy about slab bearing ledges, but I think they were only implemented due to the fact that many builders cut corners by filling underneath slabs with unclassified and uncompacted fill. My area has silty clay soils, and it amazed me that so many builders elect to fill garages and stoops with this material. So while the bearing ledge might support the edge of the slab, the balance of the slab is supported by air within a few years.

    If it makes you feel better, commercial foundation plans never spec a bearing ledge. The integrity of the slab depends on granular fill and good compaction. If you have that, no worries!!

  8. rocket190 | | #8

    One point I would like to bring up is that you were shorted by receiving on a 6" thick wall. 6" walls are typically only buses for garages with light loading. I thought an 8" wall is the minimum for a house wall, but maybe that's only for basement walls with static pressure on one side only.

  9. davidmeiland | | #9

    6" stemwall for single story, 8" for two (and three?) stories... at least around here.

    We don't use slab-supporting ledges for floor slabs either. We pour the slab entirely within the stemwall, bearing entirely on compacted fill. In some cases there are piers or grade beams below the slab, to support interior loads, but the perimeter of the slab is not supported directly on the foundation.

    You may have issues around the slab perimeter where it's supported on a ledge, if the fill was not installed and compacted correctly. I have seen this in more than one house in the form of a crack running parallel to the exterior wall, with vertical displacement, a few inches inside the house.

  10. boathook | | #10

    Thanks for all the advice I feel a lot more comfortable with the input from people who know the business.

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