+ Reply to Thread
Results 1 to 6 of 6
  1. #1
    Join Date
    Mar 2009
    Posts
    237

    Default Hollow under basement slab

    Recently purchased a 1920s 2-story rowhome and now have a horrible feeling. I've noticed that the slab had lots of cracks and i expected this in such an old home but today i saw a hole the size of a dime in it. Maybe i dropped something at some point to make this hole but i saw that it was hollow for a good 3 or 4 inches. I grabbed a hammer and made the hole larger and poked around with a stick to see how far it goes. The stick was a foot long and i could only find an edge in one direction. So i grabbed an 80 pound bag of mason mix i had and used half of the bag filling this hole.

    My questions:
    1. Should I be worried? Because i really don't have the money to pay for a whole new slab and im afraid one day im going to take a step and go right through!
    2. Is this common?
    3. What would cause this since its an older house I would think it wouldn't need to settle any more.
    4. What are my options here?
    My advice and opinions come from hands on knowledge...and This Old House Hidden Content

  2. #2
    Join Date
    Jul 2010
    Location
    SoCal
    Posts
    6,533

    Default Re: Hollow under basement slab

    I'd ask an engineer for his opinion. We don't know what type of soil the house is built on and whether or not there is a dangerous erosion under the slab.

    Do you have any pipes under the slab?
    Is the house on a flat land or on a hill?
    What's around the house? is there a slope or a hill?

  3. #3
    Join Date
    Aug 2007
    Location
    Maryland
    Posts
    1,587

    Default Re: Hollow under basement slab

    You should be able to figure out how big the void is by tapping on the slab with a hammer & listening to the sound. Hard to say what caused it. It could be a leaking pipe or possibly fill was used that wasn't compacted and has settled. I would have waited to fill until the extent of the void was determined and possibly a cause. A loose mix that can flow and fill the void should work. If it's extensive, it would be best to figure out why.

  4. #4
    Join Date
    Mar 2009
    Posts
    237

    Default Re: Hollow under basement slab

    The land is flat, no hills nearby, attached building on both sides. No underground pipes and the sewer line is to the opposite side of the basement. The entire slab slopes pretty drastically in my eyes but i only assume it was purposely done this way. The tapping method sounds the same all across so i'm not 100% sure but i tend to assume the worst.

    Who would be the best person to check this out? (structural eng or gc?)
    How much might I expect to pay to get this checked out?
    Can anyone recommend someone from the Philadelphia, PA area?
    My advice and opinions come from hands on knowledge...and This Old House Hidden Content

  5. #5
    Join Date
    Aug 2007
    Location
    Maryland
    Posts
    1,587

    Default Re: Hollow under basement slab

    I'm not sure an engineer is required, since you don't see any other foundation issues. Although if the footing is undermined you may have a more pressing problem that an engineer could address.
    A gc or even a concrete contractor may be able to figure out the problem & offer a solution. Be prepared to knock an observation hole(s) in the slab. Should the void be filled & the slab topped to level it if that's a problem or would mudjacking possibly fill & raise the slab without causing any problems? Have any neighbors had this problem? Slabs in basements are typically poured level, not sloped. I think figuring out the extent of the void is the first step. Knowing why it happened would be nice, but it may not be possible.

  6. #6
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: Hollow under basement slab

    Quote Originally Posted by dcalabro View Post
    Recently purchased a 1920s 2-story rowhome and now have a horrible feeling. I've noticed that the slab had lots of cracks and i expected this in such an old home but today i saw a hole the size of a dime in it. Maybe i dropped something at some point to make this hole but i saw that it was hollow for a good 3 or 4 inches. I grabbed a hammer and made the hole larger and poked around with a stick to see how far it goes. The stick was a foot long and i could only find an edge in one direction. So i grabbed an 80 pound bag of mason mix i had and used half of the bag filling this hole.

    My questions:
    1. Should I be worried? Because i really don't have the money to pay for a whole new slab and im afraid one day im going to take a step and go right through!
    Worried ? -- I doubt you have anything to worry about
    2. Is this common?
    It's not uncommon for old homes. Lots of old homes had brick or wood floors just placed over the dirt. Then sometime later a homeowner decided to pour concrete and usually it ends up being an inch or two thick of a poor mix.
    3. What would cause this since its an older house I would think it wouldn't need to settle any more.
    4. What are my options here?
    Depending on how thick and how good the existing concrete is -- mudjacking might be an option. Otherwise , it's tearing out the old floor and pouring a new slab. Chances are you may even have plumbing issues with the old drain lines -- in which case that would be a good time to replace all the old drain lines.
    "" an ounce of perception -- a pound of obscure "
    - Rush

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •