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To ledge, or not to ledge: that is the question

rmelcher | Posted in General Questions on

I am currently designing a home in Zone 5, an hour outside of Philadelphia.

Current foundation design is Slab;

Footer
8″ poured stem walls
2″ perimeter insulation
4″ horizontal insulation
15 mil vapor barrier
4″ concrete slab

The stem wall design does not have an interior ledge; the stem wall will be 8″ from the footer to the sill plate.

The exterior wall assembly is currently…

PolyISO; 2″
Zip Sheathing; 1/2″
2X6 Wall; 5 1/2″

8″ wall assembly can completely bear on the 8″ stem wall.  Nice and Neat!  Only the 1X4 furring strips (ventilation air gap) and Hardi siding would sit proud of the wall.

I made the decision to not include the interior ledge solely based on the responses to the same question posted on GBA.

Stem wall ledge for slab support

” The ledge is an incidental by-product of thinning the top of the foundation so as to not have the intersection of the slab and foundation exposed on the floor.  It is not intended as a bearing surface for the slab” -Malcom Taylor

Today I received the May 2019 issue of Fine Homebuilding.  I truly enjoyed reading “Minimizing concrete in a slab-on-grade home” by Michael Maines, however, that same article has me rethinking the inner ledge of the stem wall.  In the article, Mr. Maines details the foundation, which includes a built-in shelf, or ledge, to minimize potential settling of the floor system at the perimeter and included as insurance.

I would assume that providing a ledge for 6″ EPS would also warrant a ledge for a concrete slab, for good insurance?

To Ledge, OR Not To Ledge, That Is The Question.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Ron,

    The two situations are not analogous for several reasons:

    - As I said in the quote you posted, on builds with slabs, the ledge is included to cover the foam where the wall meets the foundation. If a ledge is provided to give bearing surface for the slab at the perimeter, it impedes the ability of the slab to settle uniformly, as the middle of the slab is supported by fill. This differential settlement introduces forces onto the slab which can cause cracking - a consideration that doesn't occur on a floor made entirely of foam, with is much more elastic in bending and compression than concrete.

    The benefit of the ledge on the all-foam floor Micheal designed is that the subfloor above runs right out onto the concrete stem wall, so it makes sense to tie the foam under the slab to the same wall to keep settlement uniform at the perimeter - although i'm not sure I would have included one.

    I guess the question remains: If you don't have a ledge, what covers the perimeter foam around your slab?

  2. rmelcher | | #2

    Malcom-

    Thank you so much for explaining the difference. I had assumed that the difference between the two materials, concrete/foam, was all the difference, but needed confirmation.

    with regards to what covers the perimeter foam around your slab, I have spent many hours researching that same question. From what I can see, I have three options...

    1. cut the perimeter insulation top at a 45 degree angle

    2. allow for the ledge, 2" depth for the interior perimeter insulation. the stem wall would be 8" at the base and 6" at the top. have the 2X6 exterior wall cantilever 2" over the interior perimeter insulation, with the remainder 3 1/2" bearing on the stem wall. this would allow for my 8" wall assemble to fit nicely on the stem wall, leaving only the 1X4 furring and Hardi siding to sit proud of the wall.

    3. follow the FHB 2017 playbook. ledge with 4" perimeter insulation to top of stem wall. lay PT sill plate. screed concrete to top of sill plate, encapsulating the perimeter insulation. as the builder notes "Its not perfect from a thermal standpoint"

    https://www.finehomebuilding.com/2017/06/26/slab-appeals-potential-buyers

    my preference would be option 2, however, I am uncertain if a structural engineer would recommend/pass the load bearing exterior 2X6 wall cantilevering over the 2" interior perimeter insulation???

    I am open for suggestions!

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Ron,

      Over the years I've taken three approaches: A ledge, a 6' foundation (which was allowed due to the height of the wall and what it supported), and using a pt 2"x8" as the sill plate and pouring the slab up the top of that plate (which precludes leaving the slab as the finished floor).

      On my current design I have specified a fourth alternative, which is to cant the outside of the foundation leaving a flat 4" surface, although the builder has come back to me suggesting we switch to a ledge.

      Any of these work. I've never had a problem with engineers as long as there was 3 1/2" bearing for the walls.

      Good luck with your build!

  3. rmelcher | | #4

    Malcom,

    thank you for the reassurance that I am on the right track.

    the following are the specification for my stem wall...

    8" X 16" footer
    8" X 36" frost wall

    with an 8" thick wall, what could be the maximum ledge depth be?

    I guess the real question concerning me is; what is the minimum thickness required to support the exterior framed assembly? in my case, a single-story home.

    I have viewed many slab/stem wall assemblies which include a 4" ledge for 4" of slab perimeter insulation. the only part of the assembly which is not included is the stem wall total thickness. in my example, a 4" ledge would leave a 4" bearing surface for the framed exterior wall.

    and lastly, does the horizontal insulation require a minimum PSI? the slab assembly currently provides for a 4" compacted stone base, horizontal insulation, 15 mil vapor barrier, and 4" slab.

    Thank you.

    Ron

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Ron,

      You should confirm this with your local BI, but here you need 3 1/2" bearing for your exterior walls. That means a 4" wide ledge would work - but not in your case because you have so much perimeter insulation. The most your bottom plate (with drywall and baseboard trim) will cover is 2" of foam and I'm not sure you need more.

      The depth of the ledge is ideally the depth of your slab, plus the under-slab foam, plus 2" of fill to allow for settlement. Again you should confirm what reinforcement is required to allow that much foundation to be narrowed to 4".

      Sub-slab insulation should be 12 to 15 psi.

  4. AaronBeckworth | | #6

    Malcolm,

    Is what you are recommending similar to this detail?

    https://www.greenbuildingadvisor.com/cad/detail/slab-on-grade-with-frost-wall-w-1-in-rigid-foam

    It seems like this is similar to Ron’s option 2 in post 2 of this thread. Or can you post a detail to better represent what is being discussed?

    Thanks,
    Aaron

    1. Expert Member
      MALCOLM TAYLOR | | #7

      Aaron,

      Yes very similar. I would prefer the ledge was a bit deeper so that the under-slab foam didn't sit directly on it, but that's minor concern not shared by everyone.

      The problem with that detail is it limits how much foam can be covered by the cantilevered wall above. Ron wants 4" there. Probably the best solution would be to taper the foam and slab at the top.

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