I’ve voluntered to build a pergola in the flower garden of our church. The design is a 12′ x 12′ x 10′ high structure out of RSC. Six 8 x 8 posts in the ground are the foundation. The structure is to set against, but not ancored to, the church and it forms an entryway. I will be setting the posts 32″-36″ below grade in concrete. What should I do, or how should I treat the posts below grade? We want this to last a long time. I once treated a 4×4 RSC mailbox post with tar below grade, but never checked to see how it is doing.
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Use 6x6 PT pine for the in-ground posts, and wrap them with 1x cedar above ground to get the look you want.
Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
hey trimman, if you don't want PT posts w/ wrap another possibility is to use metal dowel set in concrete below grade and up into the center of the posts above-grade (it sounds like you've ruled out post base anchors?)
best, GO
Set posts ON concrete with sompson hardware but never set posts IN concrete because that will hold water to the wood and accelerate rot.
Posts I have romoved and replaced in gravel have the rot all confined to a point about 6-8" below ground surface at the worst and sometimes lasts for forty years. When in crete, the rot is all the way from top to bottom within ten years.
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Piffin is right. Water will puddle on top of the concrete (usually set at or just below grade) and rot the post right there. Had it happen too many times on fences I didn't build.
Growing up on a farm, we used hedge (A.K.A. osage orange) for fence posts. We never set any of them in concrete. Just put them in the hole (with a little gravel in the bottom), add some dirt, tamp it in, add more, tamp, dirt, tamp... An old farmer friend showed us a hedge fence post that was over 80 years old and still solid in the ground and in excellent shape. I know you won't use hedge, but the installation technique will work.
It also helps if you buy all your post holes from the same vendor, too. I got a truck full of them all of the time.Pete Duffy, Handyman