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Straightening Old Bowed Floor Joists
Overloaded wood deforms over time (it's called "creep"). There's no quick way to reverse the process. December 1, 2005
Question
I am disassembling an old potato barn for the joist timbers (doug fir) and I notice they are bowed with time. The beams are 20' in length and 3"x10" full dimension. Does anyone have ideas to take out the bow? I will try supporting them at each end with weight in the middle, but this will take some time. Maybe moisture application? They are probably 40+ years old.
Forum Responses
(Sawing and Drying Forum)
From contributor T:
If you load it as you propose, it will probably take another 40 years. The coopers used to heat the wood under load to some 200 deg F to bend it, but I doubt that you want to do that. I have been re-sawing a large oak wine cask and, although the wood was beautiful, there were old damages from the bending that would have been important for the strength characteristics of the material if used for structural purposes.
From Gene Wengert, forum technical advisor:
I agree with the above... it will take about 40 years to straighten them. Incidentally, this "bowing" is actually called creep. It results because wood is plastic, so it flows with time.
From the original questioner:
Thanks for the quick reply. There are about 100 of these timbers and unless I get some ideas, I will just have to live with the creep deformations… find some use where the bow is not a problem, or cut the lengths to 10 ft.
From contributor R:
Maybe you can consider turning them into vertical grain flooring. Short pieces are no problem then.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
If I had to guess what formula to use to predict the sag of those beams, I would choose a hyperbolic cosine (cosh). But comparing cosh (x) to a parabolic function with the same sag doesn't show any significant differences. The parabolic function has constant curvature, and hence all sections of your beams should be approximately equally deformed from straight.
Have you reviewed the related Knowledge Base areas below?
KnowledgeBase: Knowledge Base
KnowledgeBase: Lumber and Plywood
KnowledgeBase: Primary Processing: General
KnowledgeBase: Wood Engineering
KnowledgeBase: Wood Engineering: General
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