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Community and Q&A

Interior Slab- Tear-out, or Pour Over?

user-6770312 | Posted in Energy Efficiency and Durability on

Hi, great community here…

I have a 400 sqft addition on a 1960’s house that was built on what appears to be an old patio slab, so it’s slab on grade, adjoining the rest of the house, which is over a basement.  

I have no idea if the slab has a vapor barrier or even stone underneath, nor do I know how old it is.  Doubtful it has insulation.

The soil around the perimeter of the slab tends to stay very moist (I’ve re-graded to help but will install a french drain as well in the spring).  

There is also an uninsulated concrete block stemwall that the wall framing sits on that is 16″ high.  It has caused me mold problems on the drywall covering it, so I tore that off and will do something different.

The slab is WAY out of level,  2″ over 12′ in one direction, 3″ over 20′ in another.  So tiling it, or even doing a “bathtub” of rigid foam on the interior of the existing slab and stemwall with an osb subfloor and plank flooring on top of the insulation, would require a lot of very expensive self-leveler.

So, I have two options as I see it.  The first is to pour a slab on top of the existing slab, and stain and seal it.  Done, pretty cheaply at that.

The second option is to jackhammer it out, lay down all the right stuff including vapor barrier, clean stone, and rigid insulation.  It will be at least $1500-$2000 more expensive to go this route. I’ll also acid stain and seal it if I go this route.

My question is, will the energy savings and moisture control afforded by the removal and replacement of the existing slab justify the cost?  Or should I just go the cheaper option and pour a slab over the existing one?  I’m grateful for any feedback.

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Replies

  1. Keith Gustafson | | #1

    What condition is the concrete in?

    If it is solid and not drifting or cracking the bathtub method seems workable.

    If the height penalty is not excessive, you could pour concrete over it rather than try to do floor leveler

    1. user-6770312 | | #7

      Thank you for your reply, Keith,

      The slab seems quite stable and has no cracking, so I assume it's not settling much, if at all.

      The problem with the bathtub method is that I'd have to use very expensive leveling methods, as the slab is very out of level (described above).

      I may do the pour over and call it a day, in the end. Thanks!

  2. Expert Member
    Peter Engle | | #2

    You should also look under the slab for foundations. I've seen plenty of patio slabs turned into additions, with no real foundations. Could be the cause of the settlement. Especially if you are in a freezing climate, the foundations should either go below frost line or be frost protected with lateral insulation. Lack of foundations might be enough incentive to replace the slab, and add foundations at the same time.

    1. user-6770312 | | #8

      Peter,

      Thank you kindly for your reply. The reasons why it is so out of level are a mystery to me. Since there is no cracking, and the control joint in the center of the room doesn't indicate that the two subdivided sections of the slab haven't settled out of alignment with each other much, if at all, then I don't think it settled, but was probably poured that way. Strange thing is, it slopes toward the rest of the house. Weird.

      In any case, it doesn't look like there are substantial footers under the short block stem wall. If I were to tear out and replace the slab, how would I go about adding footers without undermining the stem walls and that section of the house? I'm nervous even tearing out the slab and excavating near the walls for fear of unsettling the walls. What would go into that?

      I've added an image of the room and the block at the perimeter of the slab to my original post. Thanks again for your advice.

  3. Walter Ahlgrim | | #3

    If you pour on top it seems like the bottom of the walls will now be lower than the new floor. The bottom of the wall will still be wet and with less air movement is more likely to mold and rot away.

    It sounds to me like you have 2 choices one is to live with the problems and when you sell the claim ignorance and to be unaware of any problems the way your seller did for you. The other choice is to spend the money to rebuild it correctly starting 3 feet below ground and ending with a new roof.

    As to your question of recovering the cost with energy savings my guess is that is very very unlikely as the heat loss from a 68° room to the 55° ground is not a lot.

    Walta

    1. user-6770312 | | #9

      Thank you Walter,
      I've added a photo to my original post to better illustrate the block. The sill plate for the framing is 16" above grade, so adding to the slab will only affect the sill plate for the front door and for the sliding door, but I'll keep them clear to dry out to the outside, if needed.

      Thanks for the info on energy savings, I'll keep that in mind as I make my decision.

      Cheers!

  4. Chris Charron | | #4

    Aside from the question Peter brought up about the foundation.

    What about Vapor barrier over existing slab, and pour concrete over that? This would give you vapor barrier, and effectively treats the existing slab as the subgrade/gravel.

    They do this in the buildings I work at, pour a rough concrete to get it close, then pour the final slab to grade.

    1. user-6770312 | | #10

      Thanks Chris,
      The reason I wouldn't include a vapor barrier if I pour slab-over-slab, is that it would serve as a bond-breaker, and because I'm doing a modified mix and pouring as thin as 2" in some places, I need to bond it to the existing slab as closely as possible. This was recommended to me by an experienced concrete guy who has done dozens of thin slab pour-overs with success in the cold climate of Maine without any problems.
      Cheers

  5. Austin G | | #5

    How long do you plan on staying in the home? This is the biggest variable it seems like. Makes a big difference if it’s a lifetime house or something you’ll leave in 10 years.

    1. user-6770312 | | #11

      Thanks Austin,
      I'll hang on to this home probably forever, though I may not live in it longer than 10 years. I live in a good vacation rental market (mountains of Western NC) Either way, I'll be affected by the energy use of the house for some time. My biggest concern is total project cost and if it's safe to tear it out, as the block wall that sits on the slab doesn't seem to have good footings. Any further advice is welcome. Thanks!

  6. Brian Wiley | | #6

    I’m having a little trouble envisioning exactly how much moisture you’re dealing with, but Christine from Building Science Fight Club posted a detail to her Instagram account about using an epoxy floor coating to handle moisture that may be applicable if you don’t want to add any noticeable dimensional thickness.

    1. user-6770312 | | #12

      Brian I have considered Epoxy but the floor is so dang out of level, and the surface has worn down enough in places to expose some aggregate, so I'd need to at least do a resurfacing, and then the epoxy. I figured it would be easier to top it off, acid stain it, and then seal it with a clear sealer. Any comments on that? Do you think a poly sealer would do a similar moisture vapor retarding job?

      Thanks for your comments.

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