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Recently we replaced some of the rotten and cracked true size 4 in x 6 in. posts under the house in crawlspace. The old posts were resting on concrete pads 24 in. x 24 in. and supporting a true size beam 4 in x 8 in.

The lumber we used is 4 in. x 6 in. x 8 ft. Pressure-Treated Landscape Timber , since we couldn't find any true size to match the old one. This lumber is rated as #3 grade but does not have any knots or cracks and labeled as not being used for structural purposes.

Should I be concerned about its strengths? I presume that is Douglas Fir.

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    Hmmm...let’s see...it’s, “labeled as not being used for structural purposes,” and you chose to use it for structural purposes....hmmm. Well let’s see, if it’s pressure treated lumber, I doubt if it’s Douglas Fir...probably spruce. Spruce 1) has a significant lower bending stress values than Douglas Fir, 2) #3 grade is significantly less bending stress value than standard framing lumber (no. 2 and better), 3) stress values are less because it’s been injected with water treatment. Having said all that, residential construction is grossly over designed. If we knew the span of the beams, – Lee Sam Aug 6 at 19:44
  • If we knew the span of the beams, contributing load on the beam, etc. we could make an educated guess as to its strength, durability, etc. – Lee Sam Aug 6 at 19:46
  • Dimension ( eg. 2 X 4 ) and treated wood in the US is mostly southern pine ( loblolly , short leaf or long leaf) . It is relatively strong. Critical engineering wood construction is commonly "stress grade spruce" ( in the US). – blacksmith37 Aug 6 at 19:54
  • @Lee the distance between beams is 5ft, and distance between posts is 8.3 ft. I just had an answered question from the Home Depot about the type of the lumber I purchased, it is "either Hem-Fir or Doug Fir". – cadobe Aug 6 at 20:13
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Not using graded lumber can get you in trouble. Or what I mean is using a lower grade than allowed for a residence. If you ever have an earthquake this could be an area that the insurance company says they won’t cover your loss. And you want to use material that is below grade 2 and specifically listed as not for structural. I would not have this prior to my loss, don’t give them any reason for denying your claim.

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Why wouldn't you just use 6x6 treated #2 SYP posts? You must have a reason why you felt that getting a 4x6 was more important.

Otherwise 4x6 treated is available as curb blocking at lumber yards. Typically it's #2 if I remember correctly.

  • The 4x6 it is not a real size. It is 3.5x5.5 which is lower than the size I have. – cadobe Aug 6 at 19:13
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I would be heeding the warning. Did you try a real lumber yard or just your home store for the replacement lumber? At least replace the wood with some rated for structural use.

  • Did you intend to post this as an answer? Seems more like a comment. Home stores often sell better grades than lumber yards because their customers consider appearance a more important factor than builders do. I have lots of experience with both. Let's not big-box-bash without cause. – isherwood Aug 6 at 19:00
  • I'm not bashing big box... have a lot of stock in them. It was mentioned he couldn't find the true size he needed so I was just asking the question. I have found true sizes in lumber yards that the home stores didn't carry. – JACK Aug 6 at 19:11
  • no lumber yard has a 4x6 real size – cadobe Aug 6 at 19:14
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    Probably not, but they could cut it for you and they used to do that. That was the one great reason for going to the lumber yard. – JACK Aug 6 at 20:01

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