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Non-Pressure Treated Sills

David_the_Farmer | Posted in Construction Techniques on January 31, 2022 07:42am

Hi all, 

I have a 36×36 barn that I’m looking to convert into a living space. The sills of the barn don’t appear to be pressure treated and are laid directly on the concrete stem walls. From what I gather, the barn was constructed in the early 80s. I’ve checked the condition of the sills in quite a few locations and found only a bit of rot where the siding had failed. 

Should I try replacing the sills with PT before continuing with the build? Any recommended treatment that I can apply in place? Should I not worry about it and move on? 

Thanks in advance

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Replies

  1. eddo234 | Jan 31, 2022 11:26am | #1

    It depends. My house, built in 1840, has white oak sills sitting directly on a stone foundation. Two problems I've had are, poor roof drainage from an addition caused one 10' section of the addition's white oak sill to rot, and the back of the house sits on bedrock, and the sill is very close to the ground, causing some rot from splashing. The rest of the 182 year old sill is in excellent condition. Same with my barn, built around the same time, all its sills are in excellent shape. Here in Vermont before PT existed it was common to use hemlock (if no locust or white oak was available) for sills and spruce or pine for all other framing members, although I've also seen a lot of poplar joists in barns and old houses. Do you know what the sill material is? I'd probably think about replacing it if it's pine or spruce, but if it's white oak, locust, maybe hemlock, or other rot resistant wood, I'd leave it alone.

  2. User avater
    tfarwell | Jan 31, 2022 11:35pm | #2

    If it were me, based on minimal rot, I'd try to get a layer of asphalt shingle (3 tab or rolled roofing) between the existing sill and the foundation. It should stop any moisture wicking up over time. Maybe you could get foam sill sealer in there, which would be ideal, but I think the rigid asphalt is easier to insert.

  3. plumb_bob | Feb 01, 2022 12:37pm | #3

    The sill has been able to breathe for its entire life, this has allowed moisture to dissipate. If you build up a proper wall assembly with vapour barrier etc. the drying potential will be reduced, and could cause problems down the road. If possible replace any rotten sections, and install a capillary break between the concrete and wood.

  4. User avater
    ericpanderson | Feb 02, 2022 10:48pm | #4

    My 1978 tract home has untreated sills. When I resided, I sealed the sill to slab with Polywall blue and troweled the liquid flash over the top of the 2x. this stops air and water from getting under the sill and if there is any water from inside, it can dry to the inside. The sheathing was R-zip with the bottom of the sheathing zip liquid flashed on the bottom of the sheathing/insulation layer onto the slab which should do a pretty good job of keeping water out of the interior stud cavity. Again, the sill isn't sealed on the interior side (and the slab was not floated very well so is quite wavy) so any possible moisture can dry. Also, after many years there was minimal rot, a few places I epoxied with minor rot. Hopefully that is the end of any moisture intrusion from the exterior'

  5. User avater
    george_gordon | Feb 03, 2022 01:38pm | #5

    If the rot is not extensive, I believe you should keep the sills, try to get a barrier between them and the concrete as others have mentioned, and use good flashing details. You could treat the sills with Borate or ACQ liquid. I prefer the former; the latter is messy and somewhat toxic.

    Pressure treated lumber also rots (often the center rots out where the treatment didn't penetrate), and is generally a very poor quality building material. I would choose a good tight grain white oak sill (treated with Borate) over PT every time.

    No matter what material you use it has to be protected from water and moisture, or it will eventually fail.

    1. User avater
      mistered1957 | Feb 04, 2022 04:53pm | #7

      I agree that with George Gordon that you should treat the existing sills with Borate. I've done this with ordinary Douglas fir in making outdoor runs for chickens, and after 8-10 years the wood is in excellent shape. All you need is a Borate product, such as Penetreat, and a garden sprayer...

  6. David_the_Farmer | Feb 04, 2022 07:55am | #6

    Thanks everyone. It seems that the consensus would not to leave it as it and try to get some sort of capilary break between the wood and concrete to prevent wicking.

    Because I'm not sure that I'll be able to fit some kind of barrier between the sill and concrete easily, I'm going to try something similar to option three here: https://www.youtube.com/watch?v=Bosvy8bNb7M

    As for the replacement sills, I'll use PT lumber or treated white oak and focus on good water/moisture details. Impressive the amount of effort some builders put into attaching the sill plate as demonstrated in last months FHB 305 https://www.finehomebuilding.com/project-guides/insulation/concrete-free-slab. I'll aim for the best.

    Thanks for all the insight. I enjoyed reading your collective experiences with sills!

    David the Farmer

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