I hate sinking posts in the ground but I’ve got a contract to build a free standing pergola. The architect wants the posts (8×8 red cedar) to be buried in concrete. This is to allow there to be no brackets or cross bracing on the structure which is 16’x16′. I still want to set those posts on concrete piers slighlty proud of grade. Anyway to do this and keep the structure from being wobbly. I’m tired of thinking for myself.
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Replies
I drill a hole in the bottom of the post (I think 2"x @ 18") and insert a 3' chain link fence pole. Fill pole with concrete, set pole in concrete with post "floating" a couple of inches above grade.
Corner/gate poles are stronger than regular poles used for the rest of the run, if you need even greater strength. But this might be what you mean anyway, as I have no idea what the actual dimensions of the poles is.And this is a great idea by the way. I may steal it for some of my fence stuff.How do you drill the holes in the wood posts? What bit do you use (I'm thinking self feed bit with extensions ?) and how do you keep it straight?
I use self feed bit with extensions. I started to write a description of jig but it's too much for a one finger typist. I will say you don't have to get the pole real straight since only 2 or 3 inches shows between post and grade.
The jig is post secured to table with a sliding block drilled to pass extension through. Hole in sliding block is drilled to keep extension centered to hole in post. I hope that makes sense.
a friend has two cedar trellises with gates that are about 4'x4' with 4"x4" cedar posts set in concrete sonotubes. i'll bet they are about 10 years old and now the posts are beginning to rot and need replacement.
i brought this up on this forum looking for a solution because of my concern about lateral gate forces moving the posts around if i just set them on top of the concrete piers.
ultimately what i decided to do was rip out the posts and concrete piers an install new posts in oversize holes backfilled with 3/4" clean crushed stone tamped hard.
i think this may work for you, but i definitely wouldn't recommend setting your posts on top of piers without any form of lateral bracing..
carpenter in transition
I'd go with burried. Just be sure that you have ~6" of pea gravel in the bottom of the hole, and another 4" or so up the sides of the posts. Then when water gets in (not if, when) it will have somewhere to go other than just rotting out the posts.
You don't really need the concrete - tamped dirt works great if it's not too sandy or loamy - that's how phone & power poles are set. One of the fences in my folks yard was installed that way - in 1955. Still solid. (The fences with concrete were replaced recently).
Seems to me that this would better be solved above ground rather than below. Can the architect use some hidden steel L and T plates somewhere?
Otherwise, the steel post suggestion seems best.
The non-concrete burrying sounds doable as well (using compacted gravel instead). My understanding is that works great provided you have sandy soil (so water filters through the gravel into the soil) but bad if you have clay (as it then just becomes a hole full of water).
With all the engineered lumber these days, you'd think someone would have come up with a structural version of the lumber that could be used for posts. Could they not encase steel bars or something into these engineered woods?