In a coastal climate with rain can I use untreated beams for an exterior deck if the deck has a torch on membrane?

The deck sits on a retaining wall to provide a 4' crawl space. The crawl space may just be a gravel floor with a vapor barrier.

Considering a 5.25"x11.78 PSL @ ~18'

I did some research and their is a PSL plus which is treated and carries a 30 year warranty for exterior use. The lumber yard doesn't typically carry it and the manufacturer has all products to this yard on a 6 week backlog or something.

The lumber yard is telling me that they can send out a standard PSL or any structural member for pressure treating and for the beam as specified it would ~$100. The standard PSL subjected to pressure treating isn't going to carry the 30 year warranty and I don't have a good way to determine how much better the Weyerhaeuser PSL plus is going to be over a Weyerhaeuser PSL treated in a chamber even if they use the same stuff - probably I'll give their tech line a call and see what they think. $100 is nominal so if I can't get the PSL plus I'll go for the manually treated PSL as it seems like as long as I have adequate venting even a standard PSL would meet code.

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    In a rainy, coastal climate, I'd be inclined to get as much protection for the wood as possible. I'd go with PT & a membrane is you're adding a membrane. Don't forget, moisture will move up from the bottom of beams and in from the end. Unless you completely wrap all 6 sides of each beam, but then you've got penetrations from nails/screws to seal.
    – FreeMan
    Nov 3, 2020 at 18:19
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    If you have more than 12" clear to the beam from the soil that you pass code. The best answer will come from your local jurisdiction. Just call them, they would be happy to answer such question.
    – Ack
    Nov 3, 2020 at 21:51
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    Pressure treating a psl beam would be pointless since the treatment would not penetrate the surface due to the adhesive saturation involved in manufacture. The strands of wood glued together to make these beams would have to be pressure treated and dried beforehand
    – Kris
    Nov 4, 2020 at 22:20
  • @Kris I am not sure I agree. I have installed PSL beams and they have gotten wet and the beams have grown in height. I suspect this is more than just the water penetrating the surface. I suppose you could test this theory by taking a piece of PSL, weighing it and submerging it for a few days in water, dry it off and then again weight it. How much more it weights would give a good indication of how much treatment could be absorbed into the PSL. I like the line of thinking though I wonder if weyerhaeuser treats the wood before the beam is assembled or if they treat it in the same fashion. Nov 5, 2020 at 2:22

1 Answer 1


Would it be legal, on the west coast it would be. I would reconsider and at least get a paint on wood preservative and put a coat on it prior to building the deck. Moisture is not the only issue with wood outside.

I live in Oregon and have lived in California and Washington state. I have built many decks and replaced quite a few. Some of the issues with decking is the dampness. your torch sealed coating will do a good job for the top deck wood if it has enough slope to drain. That should last a long time. the framework not being protected from rot or wood loving fungi and wood chewing insects may end up shortening the life of the deck. At a minimum coat the top of the joists and the bottom foot or more of the posts then you can keep an eye on the rest. The top between the boards is usually where I find rot and the bottom few inches of the posts above the ground level on top of the pier blocks or if you pour pads. Concrete helps but the chemicals keeps the critters from attacking the wood.

The stuff with arsenic was the best but I don’t think it’s legal anywhere, a good preservative will help extend the life with a minimal amount and check it every now and then to see if it needs more.

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