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Community and Q&A

Treated or untreated top plate?

jackofalltrades777 | Posted in Building Code Questions on

Which provides the best attachment for a top plate for a concrete core (ICF) wall?

A foam sill gasket will be used between the raw concrete and the wood top plate to provide a break.

Hearing two sides. Some claim a treated top plate provides better rot protection, especially as the concrete cures in the wall. The other side is that a treated 4×6 piece of lumber will not provide as much screw bite and remain true & straight as an untreated 4×6 Doug Fir would.

Out here the treated lumber is the brown cote lumber. It does have a tendency to split as the chemicals leach out.

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  1. user-2310254 | | #1

    Peter. Do you mean the sill plate attached to the top of the ICF wall. Peter Yost has an article here:

    Code typically requires pressure treated lumber when it is in contact with concrete. Of course, not every area enforces code. If you are using a foam sill gasket, it is likely acting as a capillary break between the concrete and the sill plate.

  2. GBA Editor
    Martin Holladay | | #2

    If the plate is fully supported by concrete, I'm not sure why anyone would specify a 4x6 in this location instead of a 2x6.

  3. charlie_sullivan | | #3

    Leaving code aside, I would think a factor would be how high it is above grade. Are we talking an ICF foundation or an ICF above-grade wall? PT is more important on top of a foundation than at the top of an above-grade wall.

    Climate might be a consideration as well, and I didn't see what climate you are in.

  4. Expert Member
    Dana Dorsett | | #4

    An EPDM sill gasket would be a better solution than pressure treated lumber. They air-seal remarkably well without goop (assuming there was at least some attempt to trowel the concrete smooth), and are an extremely good capillary break.

  5. user-6184358 | | #5

    My guess is that you are in a high seismic area and are following the 3x lumber requirement for anchorage of concrete walls to a wood roof diaphragm. Here in California you can easily get Borate treated 3x6 lumber. It is commonly used for heavily loaded shearwalls. The 3x6 is most always Douglas fir and of pretty good quality #1 and better and not hem fir. hem fir is not as dense as Douglas fir so different design values. It will also keep termites away. The Borate treated lumber won't corrode fasteners like the other type will.
    You should be able to special order it. I would also want to have an EPDM gasket or peel and stick barrier. After all this anchors your entire house to the foundation or roof to the walls.

  6. jackofalltrades777 | | #6

    It's an ICF wall and then the 4x6 wood top plate is there to mount and secure a SIP roof.

  7. charlie_sullivan | | #7

    I wouldn't use PT in that case. But the EPDM gasket is still a good idea for air sealing.

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