6×6 post foundation question
I am planning to build a small workshop in central Virginia and I am considering pole-barn style construction using PT 6x6s. A local lumberyard sells a product called “Green Post” which has the bottom 48″ of the 6×6 encased in a layer of bitumen and polyethylene to keep the preservatives from leaching out and prevent it from rotting.
I would like to use as little concrete as possible, but I am wary of burying wood in the ground. Does anyone have any experience with this sort of product and know if it is significantly better than an unwrapped PT 6×6?
Product website is: https://www.planetsaverind.com/pdfs/greenpost_buildings_2pg_Generic_818.pdf
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Replies
Tim,
The primary (and important) distinction between types of PT lumber is whether it is rated for ground contact or not. If the Green Post uses lumber with that designation, the additional features are simply a bonus.
Pole structures are great for simple uninsulated sheds and garages. If you ever decide to finish and condition the space they add so much complexity you are better of using standard framing.
Adding on to what Malcolm is saying, remember that pole buildings are meant to be a cheap and short-lived way of providing a lot of space that is protected from rain and not much else.
For the most part they aren't structurally sound enough to comply with the IRC so they are limited to places where there is no code enforcement.
I’m not familiar with that product, but you want one that is classified as UC4B treated. You should be able to order it from most lumber yards. If you look up that specification, you can see what treatment retention level is required for each type of preservative. I prefer the old 0.6 CCA treatment, which can last 40+ years when backfilled with gravel. Laminated posts are also nice as they don’t tend to warp.
Thanks for the feedback on this! Good to know what preservative treatment to look for if I decide to bury the posts.
I know utility poles which are essentially treated wood hold up, still not a fan of it for a structure.
You can use screw piles and build on top. Simpler install and you won't have any of the decay risks.
Utility poles are exempt from some of the enviornmental regulations pertaining to treated wood preservatives. They can still use the CCA treatment, which, AFAIK, all the "green" poles still use. They also have creosote treated poles, which usually have an orange-ish color with black streaks. There is a gray/blue treatment too, but I'm not sure what that one is as I've never personally worked with it.
I'm not sure you'd be able to purchase poles like that for a pole barn though. Some of that stuff is restricted, and the suppliers will only sell to people with commercial accounts that they know. I've only ever bought this stuff at work, for commercial/utility projects though, so I'm not sure how they would handle sales for private use.
Bill
Bill,
Our utility also periodically injects more preservative into the poles though a capped hole near grade.
You are making a pay less now and pay more every month for as long as you own this shop choice.
Yes, the pole barn will cost the least to construct but it will always be a drafty poorly insulated building that is expensive to heat and cool.
If you can afford the monthly heating bills for a pole building you can afford to build a tight well insulated conventional building. The hard part is convincing the wife to write the bigger check for the building that cost less to own over the next 20 years.
Walta
I've ready many posts in this and other discussions of this topic claiming that a pole building is incompatible with energy efficiency. Maybe I'm missing something here, but why couldn't one do the following:
- frame with 6x6s @ 8' oc with a supporting continuous beam above
- frame with 2x6s @ 24" oc between the posts, using a bottom plate sealed to the slab
- sheath the outside with OSB and foam, including whatever typical air sealing is done to sheathing. OSB would run over and encapsulate the 6x6s on the outside face.
- insulate between the studs with R-21, as usual.
-install drywall on the interior, as usual. Drywall would run over and encapsulate the 6x6s on the inside face
Sure, there is more money in the posts and the beam on top than you would pay for a continuously bearing 2x6 wall, but you use a lot less concrete and have a lot less excavation than pouring a continuous footer, since you only need to dig a hole every 8' and pour a 24" x 24" x 12" footer (or something about that size). Even a shallow, frost-protected, thickened slab footer would use somewhat more concrete than a pole framed structure, wouldn't it?
The approach I laid out above doesn't seem that complicated to me. I don't see why it would not work to air seal the above wall assembly to a standard that is comparable to a normal stick-framed wall.
Look at my post #24 here for a simpler low concrete option:
https://www.greenbuildingadvisor.com/question/poking-holes-in-a-double-wall-wood-on-gravel-frost-protected-shallow-foundation-assembly-in-4a
No beams and pretty standard construction details. Those 8x8 beams are also a pretty big thermal bridge without exterior rigid insulations.
Where I am anything below grade has to be concrete. So it's apples-to-oranges to compare code-compliant construction with non-compliant. A fair comparison would be concrete piers vs a stem wall. Either way the bearing surface is going to be the same, it's going to be dictated by the loads of the building. Piers might end up using slightly less concrete because from the footer to the surface you don't need as much concrete.
If I were doing piers I still wouldn't do poles, I'd run doubled 2x beams between the piers and platform frame from there. That gives a nice floor with no concrete. And it's conventional construction the rest of the way up and everything goes smoothly.
Tim,
A few years ago I built a couple of shops as post and beam structures, off a slab with pad footings I wouldn't do it again. Everything about it was more complicated than conventional framing - and they weren't even insulated.
“- frame with 6x6s @ 8' oc with a supporting continuous beam above
- frame with 2x6s @ 24" oc between the posts,”
When you build 2X walls that fill the spaces between the posts exactly what are the post now doing since the 2x is more than strong enough to carry the load?
Absolutely everything that makes pole building cost less to erect becomes an expensive problem when you decide to make it tight and well insulated.
Walta