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Replacing lumber with equiv strength steel/aluminum. Sizing?

  1. Nov 20, 2015 #1
    Hello, I'm in the design phase of a greenhouse building and am tossing around the idea of not using lumber. I have put up many stick built structures but am afraid the humidity will rapidly decay a conventional pine/fir. And I want to avoid pressure treated if possible

    Based on a roofing rafter span table the 8-9' span of the glazed south wall is easily supported by 2x6 lumber on a 24" center. 2x8 would do it with plenty of room to spare, Can anyone point me to a comparable strength steel tube or aluminum tube? Locally I can get 2x2 galvanized steel tube with 1/8" wall thickness and 2x4" aluminum tube with 1/8" wall.

    Also as a ridge beam I could easily use doubled up 2x12s. Locally I can get 4"x4" galv sq tube with 1/4" walls. Posts would normally be 6x6s. I would think the 4x4 galv steel should replace that with plenty to spare.

    So structural members I want to replace...

    Rafters 2x6 - 8' long span
    Posts 6x6 - 8' heigh (3, one per 8' of width)
    Ridge beam 2, 2x12s - 24' wide

    As much as I have tried to find any equivalent chart comparing lumber to steel and aluminum tube I have fallen short

    Thank you
  2. jcsd
  3. Nov 20, 2015 #2


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    Staff: Mentor

    So structures in the US need to be signed off by your local code enforcement folks (city or county). Have you asked them these questions? They are the ones who have to approve it.
  4. Nov 21, 2015 #3
    Not all structures do. This type in my area falls under agricultural use and has exemption. I did ask the county and the response was consult an engineer.

    I wanted to design something with a decent degree of certainty that it would work before paying an engineers hourly rate to review


    Last edited by a moderator: Nov 21, 2015
  5. Nov 21, 2015 #4


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    Staff: Mentor

    We will give you the same advice. Thread is closed.
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