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Community and Q&A

Using existing slab

mtaylor12345 | Posted in Green Building Techniques on

Hello again,

I’m putting a one-room addition (about 11×14′) on the back of our walkout basement on our 1960 ranch in northern Utah (dry and sometimes snowy winters; dry and hot summers).

The site is difficult to access and everything needs to be hand carried to the back and debris hand carried to the front, including hand digging the foundation trench and hand-mixing bags of concrete.

My contractor has the idea of leaving the existing concrete patio slab, which doesn’t have a crack in it after 60 years, in place and only sawing out 2-3′ wide strips of the patio where necessary to dig out and put in the foundation wall. The old slab would be joined to the new foundational wall in some way.

This would obviously save time and money but also the embedded energy and debris waste in retaining the existing patio slab and not using new materials.  For reference we used about 500 therms of natural gas in 2018 for heating 1450 ft2 above grade and 1200 ft2 walkout basement with our 80% efficient furnace – about $500/yr locally.

We wouldn’t be able to put rigid insulation or a vapor barrier under that part of the slab.  Our existing house obviously doesn’t have these either.  I assume it wouldn’t meet energy codes but if the local permitting authority would approve it…

Structurally is this a bad idea?
What are your thoughts on the energy and/or carbon balance of reusing the concrete slab vs. saving some energy by properly insulating a new slab?
Other thoughts?

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  1. Expert Member
    BILL WICHERS | | #1

    The usual way to tie old concrete to new concrete is to drill horizontal holes into the edge of the old concrete, then epoxy (use proper anchor setting epoxy) short pieces of rerod into those holes so that they’ll extend into the new concrete. When you pour the new concrete around those rerod pieces the be concrete and old concrete will be locked together. This is done all the time for concrete roadway repairs when a damaged section of concrete is cut out and replaced.

    You can insulate the outside of the new foundation wall. You can put insulation on top of the old slab and then put a plywood subfloor on top of that. Insulating on top of concrete and then putting a subfloor in is done all the time without problems.


  2. charlie_sullivan | | #2

    The usual challenge for insulation on top of an existing slab is that when you make it thick enough, you lose headroom in a basement that might not have had enough headroom to begin with. But if you are building new, you don't have that problem--you might have the problem of whether it is level with the floor in the existing structure, but you might be able to have a step if not, or maybe a good insulation thickness will actually make that match well.

    This probably doesn't apply here, but in general, helical piles are an option for foundations where concrete is problematic for one reason or another.

  3. mtaylor12345 | | #3

    Thanks for your replies! I am actually really limited on headroom. I'm actually taking down an existing wooden deck, putting in the addition and then putting a tile deck on top of the addition. The existing slab aligns with the basement floor now and the top of the new deck needs to align with the floor above it.

  4. GBA Editor
    Martin Holladay | | #4

    M. Taylor,
    Don't forget to install vertical insulation at the perimeter of your slab -- that's the most vulnerable area for heat loss.

  5. Expert Member
    Peter Yost | | #5

    Hi mtaylor -

    In terms of the embodied energy of the slab: it's the portland cement in the concrete where the lion's share of the environmental impact is. And it is a big deal, well worth your re-use efforts. See this blog by Martin and Bruce King's book on the topic: Bruce King is both a structural engineer in the Bay area of California as well as a leading thinker on the environmental impact of building materials and processes.


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