The ends are secured, so how much load can a 6' span of SPF 2x4 support when turned on 3.5"?

  • 1
    It depends on the loading distribution. Point loading? Evenly distributed across the span?
    – Doresoom
    Apr 2, 2012 at 21:30
  • Also, the modulus of rupture varies widely both with wood species and moisture content.
    – Doresoom
    Apr 2, 2012 at 21:41

3 Answers 3


The Sagulator says it will support about 375 pounds (evenly distributed) before there is noticeable deflection. I recommend you go to the site, read the notes, and play around with the numbers yourself.

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  • 1
    Note that while this is a good resource, it won't tell you when the beam will actually break. Try replacing 375 lbs total with 90000 lbs/ft. The calculator gives a sag of nearly 29" per foot, which is obviously impossible.
    – Doresoom
    Apr 3, 2012 at 17:16
  • 2
    Good point. The "Sagulator" is really geared more toward shelving, whereas the span tables that shirlock referenced are geared more toward structural engineering. It's not entirely clear from the question what the OP's intent is here.
    – mwolfe02
    Apr 3, 2012 at 19:16
  • my only question about this answer is why center load was not selected in the screenshot. @doresoom and mwolfe02 a footnote in the tool reads "Target sag: 0.02 in per foot", it goes on to explain why that value was selected.
    – Jason K.
    Jul 22, 2022 at 14:29
  • @JasonK. Because I picked an arbitrary setting when OP didn't specify if it was a distributed or point load.
    – Doresoom
    Jul 23, 2022 at 15:53

Check out this site : http://www.msrlumber.org - it has all the answers to spans for most applications. There are several good sites that have tables, just google "lumber strengths".

  • 1
    The PDF proves to be a vital source of information. Very nice
    – Piotr Kula
    Apr 3, 2012 at 10:49
  • Many years ago, we won't say when..... I had to study strength of materials charts in architecture class. With all the new LAMS, beams and trusses, the charts are more important than ever. Apr 3, 2012 at 20:15

The American Wood Council offers just the tool you need at http://www.awc.org/calculators/span/calc/

Maximum Span Calculator for Wood Joists & Rafters/ You'll want 'live load', meaning people (and/or stray deer).


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