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Community and Q&A

Dampproofing a Frost Wall

Talan | Posted in General Questions on

I’m doing a slab on grade foundation that sits on concrete 4′ frost wall below grade, 8′ out.

Do I need a drain or dampproofing for the 4’ frost below grade ? Climate zone 5a. Grade is above water table so no issues other than rain/snow. Excavator says it’s unnecessary.


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  1. Expert Member


    I think both American and Canadian codes allow you to skip perimeter drains and damp-proofing when it is unnecessary. Confirming with your building inspector that they interpret your site conditions as showing that would be prudent.

    You need to include details that stop any moisture from moving up to the bottom of the slab, and from the stem-walls into the framing above.

    - For the first that means a 4" to 6" layer of clear-crushed gravel under the slab as a capillary break. From your photos that will mean removing some sand.

    - To keep the moisture off the framing you will want to extend the under-slab poly up the inside of the stem-walls, and lap it with whatever sill-seal you are using under the plates.

    1. Talan | | #2

      Hi Malcom,
      Thanks for the info.
      There is no building inspector in my town so it is more for me than anything. If I dont dampproof or add drain - will that cause any issues in the future for the walls if soil contracts ?

      Also if I put clear-crushed gravel - wont that damage the vapor barrier ? Can I just keep sand ?

  2. Expert Member


    Unless you have expansive soils you don't need to worry about what happens around the foundation. If you do, neither damp-proofing or drains will remediate that problem.

    Are you using foam under the slab? If so that would protect the poly from the rock below. In any case it isn't something to worry about. Unlike air-barriers, holes in vapour-barriers aren't a problem.

    You can leave the sand and rely on the vapour-barrier to keep any sub-slab moisture away, but especially with no perimeter drains, the risk of a damp slab is a lot greater than if you had the layer of rock. A lot depends on how wet your site is.

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