I'm building a large outdoor living space in the pacific northwest (Vancouver, Canada) with rafters, purlins, beams, and posts all douglas fir. It will all be covered with transparent polycarbonate roofing (Suntuf). The roofing will block most UV light and all the rain except some occasional horizontal/windy rain.

I love the dark red color doug fir turns indoors when totally untreated. What will happen in 5-20 years if I leave my outdoor fir untreated (no paint/stain/oil/anything)? Is ambient humidity in a wet climate enough to rot the fir? Or make it go grey or black?

Thanks so much for your experience!


Since it will be covered it won’t rot away but it will turn gray. I also live in the Pacific Northwest south of you , condensation is the biggest issue I see and occasional condensation is not a problem. My riding arena 120’ with poly the entire length for light it is 30 + years old with no issues but is 22’ up so the color is far enough away we don’t notice the gray. I did paint the back porch because I thought the bare grey wood looked bad after a few years. So since it will be covered it won’t rot away but will turn gray.

  • I live in the northeast as well. Even treated fir will begin to gray after a few years if additional sealing isn’t done.
    – mreff555
    Aug 26 '19 at 0:57
  • That's too bad. I guess I'll have to oil it or something, thinking about using this stuff: sherwin-williams.com/homeowners/products/…
    – Noland
    Aug 27 '19 at 15:25

Yes, the Douglas Fir will decay in a few years if left untreated: 1) dryrot in end grain, 2) grain will raise, 3) major dis-colorization, 4) knotholes will become loose and fall out, 5) warping due to excessive moisture penetration.

You say the lumber will be covered, but where you live, wind blown rain and just a generally wet environment will affect the lumber constantly.

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