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Best rot prevention tips for Doug Fir deck?

GreyWolf92 | Posted in General Questions on

Hi everyone, I posted last week about building a deck outside my tiny house (which will have a awning roof over it).

After further research, I decided to go with Doug Fir. The only other option is composite (which is much more expsensive and not as thick) or Cedar (which is new growth and isn’t really much better in terms of rot resistance. 

With that being said, I have researched some excellent tips to help prevent rot such as, taping deck joists, sloping deck, 3/16″ space in between boards, allow for deck overhand, no seams directly over joist etc.

I have a few questions:
1) Is there a recomended tape for taping the joist beams or do they all perfom similar?
2) What are some eco friendly/ low oder sealant options? This deck is right by my window and I don’t want any strong smells (which is why I’m not using PT)
3) If you have any other framing/construction tips on preventing rot, please share!
4) Is #2 Doug fur sufficient or should I go with #1?

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  1. Expert Member

    Grey Wolf,

    Sounds like you are on the right track. I like Resisto deck tape, but I've also just used strips of their peel & stick membrane. My experience is that one of the most common places for rot to start is on the laminated beams made of dimensional lumber. If you don't use pt wood anywhere else I'd suggest using it there.

    Like any other wood, how long D. Fir will last depends on the age of the tree and which part of the log it was cut from. I just finished a deck made entirely of MPS treated wood. It has virtually no smell, and takes finish quite well.

    My own deck was made of ACQ treated lumber. Its over 20 years old and still looks alright. I used Penofin on it - which I wouldn't' recommend if you are sensitive to odours, they take several weeks to dissipate. You might try Siko deck finishes. I've had good luck with them too - although I can't say I've been particularly enamoured by any products I've tried. They simply don't last the way they say they will.

    1. GreyWolf92 | | #2

      Thanks for the tips Malcolm. Looks like a great deck!

      I will check out the finish you mentioned.

    2. GreyWolf92 | | #3

      What is your reccomendation for deck seams?

      I like the look of sandwhiching a 2x6 inbetween seams but is there a way to do that, that will allow for adequate drainage and avoid rotting?

        1. GreyWolf92 | | #7

          Thanks Tyler this is the exact explanation I was looking for!

    3. maine_tyler | | #5

      Malcolm, what is MPS treated wood? Are you referring to MicroPro Sienna products (MCA treating)?

      1. Expert Member
        MALCOLM TAYLOR | | #6


        Yes. Most lumberyards here have switched over to it from ACQ. The two big advantages are that when you buy it it isn't soaking wet so can be finished as soon as the deck is done, and it doesn't eat fasteners the way ACQ does. Neither last anywhere near to what CCA pt wood did, but it's only available by special order now.

  2. GreyWolf92 | | #8

    One other question guys...

    What percentange moisture content should I aim for before sealing the doug fir? I have 10 boards that are ~18% right now.. How many days in the sun would those take to get to below 15%?

    Other options are to get kiln dried, or frame the deck, and wait a few weeks to seal .

  3. markgh1 | | #9

    I agree with Malcolm, I have also used scrap EDPM rubber roofing on top of the joist and beam (free usually) on a deck I built in 1982. We just replaced the deck boards this summer and the joist and beam looked new underneath the EDPM.

    Also, Malcolm is dead on regarding sealing. Penofin deep penetrating oil is really the best product I have ever used and you also have to like the fact that maintenance reapplications are fast and effective as the deck ages. Also, I would think a penofin clear penetrating oil would look exceptional on fir. Another thing I started doing a long time ago was to treat the bottom of the boards before I attached them to the joists. It is overkill but if you are in a damp, rain soaked environment it will help a raw wood product like Fir.

  4. gusfhb | | #10

    I would treat the underside of the areas where it touches the joists with a preservative. I removed a deck on my house that was solid 4x6 Doug Fir, and the 8 foot spans were fine, but wherever it touched the beams, rotten to the core.
    I had a can of some foul green stuff that worked pretty well but it ran out and I don't think they make it anymore, the replacement stuff [copper napthenate?]doesn't smell as bad. Can't possibly work if it doesn't smell bad, right?
    If you are using standard 1x4 and have the option of selecting your wood, pick the tightest most vertical grain boards you can.
    My previous house had a 1x4 fir deck that I had built in 2001. It was still in good shape when I sold it in 2009, but it was obvious that where the heaviest water hit it the light colored wood on the widest grain was literally wearing away. the wider grained pieces were more worn than the tighter grained pieces.
    Fir can last a long time, but it will rot where water can sit. Give extra clearance around balusters or any place that two piece can come together to give moisture a foothold. If building it yourself you can stain the end grain to help a bit there. The uncovered back stairs on that same house were over 15 years old and solid except where the balusters were tight against the treads and IIRC the top step against the house

  5. BirchwoodBill | | #11

    I have used both Penofin and TWP, I found TWP 1500 last longer and is more mildew resistant.

      1. Expert Member
        MALCOLM TAYLOR | | #13

        Grey Wolf,

        You want something that also protects against UV damage.

          1. Expert Member
            MALCOLM TAYLOR | | #16

            Grey Wolf,

            It's worth considering the distinction between products which emit high VOCs when being applied, and those that continue to do so over time. Some really nasty stuff, like Xylene based concrete sealers, quickly become inert once they gas off. This is especially true of things that will be outside and not in confined areas.

  6. gusfhb | | #15

    I have never found any of those water sealer products of any use. UV and water kill the wood. Classic stains will protect it. If you want lifespan you kinda hafta take your choice of chemicals.
    Pay me now[PT wood, plastic] pay me later[stain, stain, stain]
    Fir at least takes and holds stain pretty well

    1. GreyWolf92 | | #17

      Thanks Keith. Yea alot of reviews say great things about this product and the company claims it's a new technology that mimics oil based stains.

      I think I will take my chance because it will have a roof over it. And worse case I will recoat next year.

      What moisture content do you suggest waiting to seal? I thought it should be under 15% but I called my local lumber yard and they said their kiln dried is 20% which makes no sense to me.

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