Deck Support Posts buried 24 inches ????
Hey guys,
One of my employees is getting ready to build a deck on the back of his house. It will be about 40″ off the ground, approx 12′ x 20′ When he applied for his permit, they gave him some drawings of how the support columns and footings were to be constructed. They show a 18″ dia hole in the ground, 32 inches deep. At the bottom of this hole they show a poured footing about 18″ tall with the support post imbedded in the concrete about 4 inches. Once the footing is poured and the post is lowered into the hole and into the footing, the rest of the hole is filled with dirt, leaving the bottom 20″ or so of the support post buried in the ground ! This simply goes against everything I have ever seen. I have built two decks and both of them have the support posts attached to the top of the footings with Simpson ties. The top of the footings have been out of the ground, not 20″ below ! I am trying to talk him out of doing it this way, but he is trying to make sure he keeps the inspectors happy.
Any thoughts on this method of construction ? This is in Prince Georges County Maryland, just outside of Washington DC.
Bill Koustenis
Advanced Automotive Machine
Waldorf Md
Replies
Your are correct. Buried posts are a bad idea...unless the posts are PWF (permanent wood foundation) grade. Most deck posts are not treated enough to be exposed to moist earth all the time without rotting on the inside (where the treatment is inadequate.
I just removed an 800 sq.ft. treated deck last month that was built in 1994 with buried posts that were already hollow inside. It was falling apart.
never, ever, ever embed a wood post partially into concrete.
ever.
it forms a cup which fills with water and prematurely rots the post.
carpenter in transition
bill.. we pour our footings below grade.. then set the posts on top of the footing and backfill with granular matl
we do not embed them
and we don't use a bracket
i'd say this is the way we've done it for 25 years
Mike,
Not to knock 25 years of success, or question your obviously excellent building practices. but what about lateral and uplift forces on the posts??? How close to grade do you finish the top of your footings??
H
the bottoms of our footings are 40 " below finished grade... i'd guess the tops are about 30" ... the 30" gives a lot of lateral support ( sway brace )
most decks do not have an uplift design..
BUT if we are building a roofed over deck like a screened porch
then we have to design for uplift, at that point i pour conventional sonotubes with rebar and Simpson hold-down brackets and use the PT posts to tie the roof structure to the sonotubesMike Smith Rhode Island : Design / Build / Repair / Restore
Buried posts are OK, but putting posts in concrete is stupid.
If you MUST burry the posts, 6" pea gravel, insert post, 6" pea gravel around the post, then tamped earth (best) or concrete. At least when the water gets in it has someplace to go and doesn't rot the post. Still, like you said, this is NOT the first choice!
I live in an area with pour drainage, a high silt content in the soil, and a frost that penetrates the ground to 3+ feet at times. Putting a post in a hole with gravel is no different than pouring a post in concrete. Just a bigger cup. I put post on piers. Haven't had any problems yet.
Pea gravel (river rock) provides little lateral stability. Crushed rock is better.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
well,there you go... and now you know
Mike Smith Rhode Island : Design / Build / Repair / Restore
I'm with Mike Smith. Exposed piers are ugly, and if not laid out perfectly, look even worse with the post off-center.
Here (upstate NY) we have to go 48" deep. Bag and a half of concrete mix in each hole, level it off, set posts the next day. Again, about 25 years of doing it this way, no problems, no complaints from inspectors.
Around here the soil has a high clay content. I've always thought using gravel for backfill would hold more moisture than using native soil. We mound it up a little so it slopes away. The latteral forces are minimal in either case.
Agree totally about not surrounding post with concrete.
Off-topic, but I've always wondered why loose stone is placed under flat concrete work, when the whole thing is sitting in a clay "tub". Seems like that just gives water a place to go, and later freeze.
We do lots of paver work, and use crusher-run as base. No voids for water to go into.
Bear
Well,
I guess I have learned something. I wonder if the PT he is getting from HD or Lowes is rated for the ground contact ? I think he was looking to just backfill everything with dirt, I will suggest the crushed stone instead. Bill Koustenis
Advanced Automotive Machine
Waldorf Md
Mr. Bill (from another Mr. Bill), it's all about money & code minimum for structural support.
It costs a good bit more to pour sono-tube piers, lined up precisely so they look good, plus making sure to account for rack forces (since the posts are no longer stabilized by being buried. And you have to get the inspectors to sign off, which probably means stamped plans. But there is no question in my mind that it lasts longer if you build it that way. OTOH, do it the way the County shows, & make sure you get the right PT wood, and it'll last plenty long enough. Just make sure you get the right wood.
2 things come to my mind.
Telephone poles. I never have seen one in any part of the states I have been in, set IN or with concrete...I wonder why.?
Piles. Driven into the ground with NO drilling, often underwater.
I can understand that there is no real uplift on a power pole, and very little load...so they are drilled and backfilled with the same tailings that were the now hole.
Piles are away from oxygen so no real threat to bugs or rot..
Points to ponder.
Spheramid Enterprises Architectural Woodworks
There is no cure for stupid. R. White.
Around here, when they're setting several poles, I generally see them set with crushed rock. They sometimes use a concrete mixer truck, loaded with the dry rock.With individual poles I would imagine that it's a matter of convenience -- use what's at hand.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
Just to show that there are exceptions to every rule, I just tore down an elevated deck that was built 24 years ago (not by me). The 4x4 PT posts were buried about 18", the bottom ~12" just backfilled with soil and the top ~6" being a concrete "donut" around the posts level with the surrounding ground. The sections of post below ground level were in better shape than the parts above ground, with no evidence of rot or softness at all. This is in middle Tennessee where we have plenty of rain (except in July and August) and humididty, not in a desert somewhere.
I'm struggling with whether to pretty much replicate this method with a new replacement deck or to do what is "right" and attach the posts to footers above ground. I think I would rather bury the posts to provide additional lateral support and racking resistance since I plan on making this a free standing deck rather than attaching a ledger to the house, and it is about 5' off of the ground.
-Chuck
The most important thing is to mound the backfill up a bit around the post, and go back from time to time and add more as it subsides. The most active point of rot is right at ground level, where water will stand against the post if allowed.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison