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How to deal with concrete poured against a rim joist (for porch or garage slab)?

Dennis Miller | Posted in General Questions on

The 1st floor of a house is often built 12″ or so above the foundation. If you want to build a porch with a concrete floor that has maybe a 4″ step down from the house, then you need to pour concrete to a level more than half way up the rim joist. Clearly the rim joist and concrete should not make contact. What sort of barrier do you use in this place? My builder friends say that around here (PA climate zone 5) they use a strip of Ice and Water Shield. They stick it to the rim joist a couple inches above the intended concrete level and run it down the foundation wall with the intention that it will stick down there as well. Whatever is used, it seems to me that it needs to be watertight AND should also be a vapor barrier so that no moisture can migrate from the concrete to the wood. Thus it also seems to me that the other side of the rim joist inside the basement needs to have a vapor-open treatment — no closed-cell foam or styrofoam or anything else that resists drying to the interior. What say ye??

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Dennis,

    Let's just deal with porches. Garages have other considerations.

    There are best, and alright solutions.

    The best is to step up the concrete foundation wall for the length of the porch or patio. You can either make this step half the width of the foundation, or set a PT ledger into the form on the inside and use joist-hangers.

    Alright is the solution your builder friends propose, but the membrane should be protected by a metal flashing compatible with exposure to concrete, and both should run right up to the subfloor. You can either pour up to the flashing, or leave a gap for air which will accumulate debris and need periodic cleaning.

    Either way, think through what happens to the grade on the outside of the porch where it meets the foundation. It may require a bit of a retaining wall.

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