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I’m building a set of outdoor stairs down a 14 foot drop over 60 feet. I’m going to use 6x6 as the front edge treads and then backfill.

This means the wood will be in contact with the ground. I am aware this will limit its lifetime. My question is whether PT will last significantly longer in this situation than hemlock?

The local sawmill has rough 6x6 hemlock for 550/1000 vs about 20 for a PT 8’. Unless the PT lasts way longer the 40% difference seems worth it?

Any other suggestions on outdoor stairs?

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Pressure treated will last significantly longer. Hemlock performs about the same as Southern Pine and its above ground life is significantly less than treated pine’s in ground life. In ground, you can’t expect untreated wood to last more than 3-5 years.

In ground, I think you can reasonably expect pressure treated to last six times longer (even more if you treat with an additional product like copper naphthalate). Given that plus installation costs, pressure treated is clearly the choice.

As far as other options, I think concrete or blocks would be the only other cost effective options.

References

https://www.fs.fed.us/t-d/bridges/documents/tdbp/decayres.pdf https://bct.eco.umass.edu/publications/articles/wood-myths-facts-and-fictions-about-wood/

  • Is there any point in painting the PT with a waterproofing coat in this role? I've been told it helps with decking. – Maury Markowitz Aug 26 at 17:36
  • @MauryMarkowitz it will help extend the life even more than regular PT. There are plenty of articles that recommend soaking PT lumber overnight in copper naphthalate to extend the life. I haven’t done it and don’t know how much of a difference it makes – UnhandledExcepSean Aug 26 at 17:39
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I have not used hemlock but I have repaired decks that were made of it. I would definitely go with the PT. Just make sure you specify that the PT will have ground contact. From a lot of personal experience, I'd go with stainless steel screws for planking, galvanized bolts for framing and avoid sinking posts in concrete unless you wrap them in plastic. Good luck

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