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Thread: Moisture egress considerations for steel tube on trailer frame

  1. #1
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    Aug 2015
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    Moisture egress considerations for steel tube on trailer frame

    I just purchased a really nice 20ft trailer frame from a wrecked enclosed car hauler. For $500, I could not pass it up.

    It is a bare frame (no wood, sides, nothing) w/ dual 6500 pound drop axle and good brakes on all 4 wheels.

    I am going to use it as a base for a really nice flatbed/car hauler.

    The original floor was likely some plywood, but I will be installing 2x6 pressure treated planks and therein lies the issue...

    I plan to install 82" wide runners every 2' on-center in order to provide the wood deck with the needed support - and surface to fasten/secure all the planks.

    I would LIKE to use 1 or 1.25 inch square tubing. From there, I am planning on welding up a 4"x.25" flat bar down each side of the deck - essentially closing in all the 82" tube runners. The real issue comes in: I plan to secure the planks down to the runners w/ actual self-tapping deck screws. But what about the inevitable moisture that will work it's way inside the tubes? Should I drill a couple holes on the bottom of each tube?

    Is there a more practical way I should do this?

    I considered - as an alternative - using 1.15" angle iron for the runners instead of the square tube - welding it on-end to the frame. But I feel like the square tube provides much more rigidity.

  2. #2
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    Re: Moisture egress considerations for steel tube on trailer frame

    May be worth noting that the existing frame looks like 2"x5" box tube (not sure the proper name for it), but it is NOT c-channel or anything like that. Does not seem the factory placed any such egress holes in the original frame, but then again - I have no idea it the frame was dipped in a preventive coating at the factory.

  3. #3
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    Re: Moisture egress considerations for steel tube on trailer frame

    FWIW, I did a similar conversion last spring. Started with a commercial backup power generator trailer and added cross beams to support a wood deck. I used 3 inch 3/16 channel, 16 OC. Welded inside the existing side frames of the trailer. No moisture entrapment possible, plenty strong.
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  4. #4
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    Re: Moisture egress considerations for steel tube on trailer frame

    I wonder if 1 1/2" x 3/16" angle iron would work better to attach the wood to. You could even tie them together down the middle with pieces of the same angle to prevent them from twisting.
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  5. #5
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    Re: Moisture egress considerations for steel tube on trailer frame

    I can see the concern about trapped moisture when using tubing. But, how does that square with using treated lumber in direct contact with steel? Albeit the steel is painted, for now................ I'm assuming that deck is treated, being as it's sorta greenish.

  6. #6
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    Moisture egress considerations for steel tube on trailer frame

    I alternated 2x3x0.120 tube and 1.5in angle for the cross members (maybe 16in OC?) and then screwed the decking down to the angle. I painted all four sides of the deck boards before install and itís holding up surprisingly well. Iím not a big fan of treated lumber - it seems like they use lower quality wood (hemlock vs fir) and Iím not convinced it holds up any better than painted. At least around here.

    Rebuilding the trailer was still probably more work and expense than it was worth, but at least itís been useful.


    (The tires got some air before it went back on the road.)


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    Last edited by ccogg; 04-15-2021 at 12:57 AM.

  7. #7
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    Re: Moisture egress considerations for steel tube on trailer frame

    There will be no rust,, if there is no large path for oxygen to get in the tube,,

    If I was REALLY worried about rust, weld in the tubes, then attach a piece of 1/4 X 1" (or whatever) to receive the self tapping screws.

    If there is no oxygen, there is no rust,, (the oxygen in the sealed tube will rust a tiny amount, then stop)

    I think this is why most trailers use channel rather than tube,, the channel is easily painted, and protected.

    I am pretty sure the cross pieces under my 20+ year old, 10K LB capacity, dump trailer are channel.


  8. #8
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    Re: Moisture egress considerations for steel tube on trailer frame

    The carpenter/home builder part of me suggests using a self sealing membrane under the deck as that does away with any water issues and isolates the pressure treated wood from the steel frame and eliminates any associated corrosion potential. Use either hot dipped galvanized or stainless steel screws and you will be fine. My experience has been that ceramic coated screws tend to loose their coating when driven through structural thickness steel.

    https://www.lowes.com/pd/TITE-SEAL-S...ashing/1007825

    Cut down the middle, each roll would give 66' of coverage....

    Best regards,


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