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Why would there be an 8in gap under slab?

thegiz | Posted in General Questions on

I’m guessing this may be bad news or there is a reason. There was a hole in basement slab by furnace. I noticed couldn’t see or feel solid ground with my finger. So I fit a tape measure in and there is 8in gap from top of slab to what feels like solid ground. Why would that be? If so would I technically have less work to pour a new slab. I mean that is plenty of room to put in insulation poly and new 4 inch pad. I have repaired a different part of floor years back when there was a damaged piece of concrete and it was dirt right under thin layer of slab. If pouring a new slab is necessary this would force me to do so. Is this gap because of water lines running under floor?

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  1. Expert Member


    Most probably just uneven subsidence of the substrate due to poor initial compaction. It's a regular enough occurrence that a whole industry exists to fix it called Mud-jacking.

  2. user-2310254 | | #1


    Are you in a part of the country where sinkholes are an issue?

  3. Andrew_C | | #2

    @ Joe: more info may be helpful. Obviously the slab isn't suspended in mid-air, so one question is how wide is the void beneath the slab? How far does it extend under the slab from the edges of the hole? How large is the hole? How thick is the current slab? Answers to these questions would help clarify how big an issue this might be, and also let the experienced builders guess at the cause.

    The basements that I'm familiar with always have a floor drain in the near vicinity of the furnace/air handling unit for a condensate drain.

  4. thegiz | | #3

    I don’t know if sink holes are common around downstate NY. , I have never heard of such a thing. Not sure how wide void is without taking apart floor. I was able to get a tape measure 10in to side before it hit something. Slab seems to be about 3inches thick. There was a very old oil burner there that I replaced about 5 years ago. Maybe there was some type of support there or as mentioned a drain.

  5. user-2310254 | | #4


    I suggest hiring a plumber with a video system to see what is going on or making the hole bigger so you can do a visual inspection. I asked about location because natural sinkholes are pretty common in Florida.

    1. charlie_sullivan | | #6

      You can buy such a system for cheaper than you can hire someone.

      1. johngfc | | #8

        Check Amazon for "Wifi endoscope". I think I paid ~ $30 years ago for one that uses my phone for a screen. It's great for the 2 or 3 times a year I use it.

  6. user-723121 | | #7

    I know of a house in Minneapolis where the entire basement slab had about 6" of air space underneath. It was supported by the center footing and the attachment to the block foundation walls. It was discovered because the plumbing would no longer drain. The plumber put a camera in the main waste line going to the street and saw it had a big sag. The neighbor across the street was there when the house was built and claimed it was backfilled in the middle of winter. The fill under the basement slab must have been frozen and in time settled about 6" or so. The plumber cut a couple of holes in the basement floor and reset the waste line to drain to the street. It had been that way for decades , the 6" gap remains today.

    1. Expert Member
      BILL WICHERS | | #10

      Basements built in areas with expansive clay soils are sometimes built with a sort of suspended slab like this. The idea is to prevent the slab from breaking if the ground heaves. You end up with a sort of crawl space under the slab. If this is the case, there will be an open area under most of the slab with the slab supported by the perimeter footing and some piers and not the rest of the ground underneath.


  7. thegiz | | #9

    I have to check out the camera as an option. Plumbing doesn’t seem to be in an issue. I might have a mason check out the floor further, make a bigger hole basically. Hopefully it isn’t something serious.

  8. Expert Member
    Michael Maines | | #11

    Do you ever see water in the basement? I've seen gaps under basement slabs that seem to be caused by underground water channels carrying soil away, or somehow causing subsidence.

  9. thegiz | | #12

    Apparently before I moved in it would flood, until a company came in and solved the problem. I think they installed a perimeter drain. Maybe soil did move from years of water flowing through?

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