Jump to content

Embedding Deck Post In Concrete


dtontarski
 Share

Recommended Posts

I've got a photographic memory....it's just not always in sharp focus....

I remember seeing an excellent explanation of why pressure treated deck support post should not be embedded in concrete. It spoke of this leading to the failure of the concrete. I understand the jist of this, it just tweaks me that I can't find the post. I could not find this discussion with a search. Does anyone remember this post? Or, could who ever posted this great explanation re-post it? I see this method of construction all of the time, and I want to assure that I'm explaining the issues as clearly as possible.

Thanks!

Dave Tontarski

Advance Home Awareness Home Inspections

Link to comment
Share on other sites

There is moisture in the soil. Concrete absorbs moisture. Wood expands when it gets wet. The expanding wood can break the concrete.

Water can also get in the joint between the wood and concrete. Water expands when it freezes. In a freeze-thaw climate, the pressure of the ice can break the concrete.

It's also not a good idea because the concrete is alkaline. This in combination with moisture will increase the rate of decay of the wood post. Preservative treatments on the wood will slow this down some but they will not stop it.

Link to comment
Share on other sites

There is moisture in the soil. Concrete absorbs moisture. Wood expands when it gets wet. The expanding wood can break the concrete.

Water can also get in the joint between the wood and concrete. Water expands when it freezes. In a freeze-thaw climate, the pressure of the ice can break the concrete.

I'll play devil's advocate here and ask: So what?

If the post is bearing on a footer then who cares if the concrete around the post breaks? It isn't any weaker than dirt back filled around the post. In fact I'd bet that even broken, it's still stronger because lateral loads would be imposed on undisturbed soils.

If the post is encased in concrete and is depending on the friction of the concrete against the hole, well, that's just wrong no matter what( I know there are friction pilings but we're talking about decks here, not bridges or beach houses)

Which brings me to the reason I don't like seeing posts encased in concrete. It gives frost or expansive soils purchase to lift the post even if the base of the post is below the problematic area. It doesn't bother me at all if the the concrete was placed in a smooth wall form that is has vertical walls or walls that taper to a wide base.

As far as the alkalinity destroyinge wood I have an analogy:

Stone walls built of sedimentary stone don't last as long as those built of igneous stone.

BTW Brandon, I've never seen anyone else go from being new to being a respected member of the HI community as fast and as thoroughly as you. I'd have you inspect a house for me in a minute.

T

Link to comment
Share on other sites

Originally posted by chrisprickett

The concrete forms a pocket around the wood, which traps moisture, which certainly can cause the wood to deteriorate.

Warning: Thread Drift!!!

I saw some guys on "This Old House" years ago who said the same thing. Their company had been building fences in New England for something like 75 years, and they never used concrete.

First they charred the last few feet of the post in a fire, on-site. They set them by putting 4-5 inches of packed gravel in the bottom of the hole, then added successive 2" layers of gravel and dirt, solidly packing each layer down as they went (they had flat-ended sticks just for that purpose). They claimed this method allowed for excellent drainage and faster drying out.

Frankly, I doubted any pole or post set that way would be very stiff. Dirt and gravel...nothing else? Well, I tried it with the service pole of my old mobile home, and I couldn't budge it when I was done. That pole was broken off during a hellacious storm in 2001, and I cussed a blue streak digging the damn stump out..inch by inch, pick and dig. It works.

Brian G.

Yankees Do Know a Few Things [:-graduat[;)]

Link to comment
Share on other sites

Originally posted by Brian G

Originally posted by chrisprickett

The concrete forms a pocket around the wood, which traps moisture, which certainly can cause the wood to deteriorate.

Warning: Thread Drift!!!

I saw some guys on "This Old House" years ago who said the same thing. Their company had been building fences in New England for something like 75 years, and they never used concrete.

First they charred the last few feet of the post in a fire, on-site. They set them by putting 4-5 inches of packed gravel in the bottom of the hole, then added successive 2" layers of gravel and dirt, solidly packing each layer down as they went (they had flat-ended sticks just for that purpose). They claimed this method allowed for excellent drainage and faster drying out.

Frankly, I doubted any pole or post set that way would be very stiff. Dirt and gravel...nothing else? Well, I tried it with the service pole of my old mobile home, and I couldn't budge it when I was done. That pole was broken off during a hellacious storm in 2001, and I cussed a blue streak digging the damn stump out..inch by inch, pick and dig. It works.

Brian G.

Yankees Do Know a Few Things [:-graduat[;)]

I remember setting them that way when I was a kid -- except for the charring part. I've never heard of that before. We dipped them in creosote.

- Jim Katen, Oregon

Link to comment
Share on other sites

Originally posted by Chad Fabry

When I lived in western Australia

Aaagh!,

When I was 19, I sent away for an immigration application packet to the Australian consulate in New York. I've been trying to get down there my whole life.

That's it, I officially hate you, Fabry. Better look under your bed at night for an IED. [:-gnasher

OT - OF!!!

M.

Link to comment
Share on other sites

Originally posted by Jim Katen

We dipped them in creosote.

Environmental terrorist!!! [;)]

I can't remember now whether it had to do the species of wood or what, but they definitely charred 'em.

Originally posted by Hausdok

That's it, I officially hate you, Fabry. Better look under your bed at night for an IED.

Yeah, what he said. I'd love to visit Aussie land. I even got to do an inspection for a young Aussie couple who were having a "walkabout" through the southern U.S. (thought about a piece of rental property in town). They were great. We had dinner together three times before they went home.

Brian G.

Down Under; Sharks, Crocs, and Golf [:-thumbu]

Link to comment
Share on other sites

Originally posted by Chad Fabry

There is moisture in the soil. Concrete absorbs moisture. Wood expands when it gets wet. The expanding wood can break the concrete.

Water can also get in the joint between the wood and concrete. Water expands when it freezes. In a freeze-thaw climate, the pressure of the ice can break the concrete.

I'll play devil's advocate here and ask: So what?

If the post is bearing on a footer then who cares if the concrete around the post breaks? It isn't any weaker than dirt back filled around the post. In fact I'd bet that even broken, it's still stronger because lateral loads would be imposed on undisturbed soils.

If the post is encased in concrete and is depending on the friction of the concrete against the hole, well, that's just wrong no matter what( I know there are friction pilings but we're talking about decks here, not bridges or beach houses)

Which brings me to the reason I don't like seeing posts encased in concrete. It gives frost or expansive soils purchase to lift the post even if the base of the post is below the problematic area. It doesn't bother me at all if the the concrete was placed in a smooth wall form that is has vertical walls or walls that taper to a wide base.

As far as the alkalinity destroying wood I have an analogy:

Stone walls built of sedimentary stone don't last as long as those built of igneous stone.

BTW Brandon, I've never seen anyone else go from being new to being a respected member of the HI community as fast and as thoroughly as you. I'd have you inspect a house for me in a minute.

T

If you pour a footer, set a post on it, and then have a second pour around the post to hold it in place, then as you stated, if the concrete around the post cracks, it isn't likely to have an effect on the post bearing on the footing. But what professional builds a deck that way? The pro pours a footing and a pier that extends to or above grade and then attaches the post to the pier with an appropriate connector. When I think of a deck post encased in concrete, I think of the amateur who digs a hole, sticks a post into it, dumps concrete into the hole, and then moves the post to plumb. With this type of an install, when the concrete in the hole starts breaking up, bearing could be more of a factor.

Bearing is one issue but there's more to it than that. Movement of a portion of the deck structure that is not supposed to move, and an accelerated rate of decay of the wood post are other issues. Once the joint between the post and the concrete opens up, what's to restrain the deck against uplift? And water getting in there not only promotes wood decay but increases the problems you noted with frost heave and hastens the destruction of the concrete (which destruction could, over time, affect bearing).

I'm not sure what to make of your analogy, but a wood beam resting on a wall made of sedimentary stone will not last as long as one resting on a wall made of igneous stone.

Thank you for the compliment. Coming from you, that means a lot to me. I know you are not fishing for praise, but I frequently find myself thinking "what would Chad do or say in this situation". Reading and actively posting on TIJ and other respected HI message boards has been a tremendous help and has shortened my learning curve, so kudos to all those who ask & answer the questions.

BTW, I'm not the author of the post that Dave was looking for. If someone tracks that down I'd love to see what it says.

Brandon

Link to comment
Share on other sites

  • 4 years later...

Hi, reading this site and thread with interest. Wish I had have read it a few weeks ago.

I'm building a cubby/tree house for my kids - one corner is supported by a horizontal beam attached to a tree, the other 3 corners are on 4x4 treated pine posts.

I'm in Western Australia. The 'soil' in our backyard is pure sand and so drains very well, but doesn't give a very solid base / sides of holes. I am following a book on tree/cubby house building and in line with that have set the posts into concrete by putting the post in and pouring pre-mixed concrete into the hole then plumming up - exactly as this thread recommends not to do!

I did this around a few weeks ago and have been up and down on the structure, hammering away, putting in a decking etc. The concrete around each post has cracked in a diagonal from the corner of the post, in a couple of them there are 2 cracks, on around 2mm wide, the other more hairline.

The structure feels solid, but it will be heavy - has a floor area of around 5m2 and sits about 8ft off the ground. At that height I didn't want to take any chances with the kids and so have braced the floor to the post at 45 degrees at each corner.

Anyway, seeing the cracks and reading this I'm worried whether the structure is going to be safe. I don't mind if the posts rot out over several years - the kids will be largely done with it by then anyway. What I don't want is a catastrophic failure within that time.

Any feel for how quickly the concerns raised here could create a rot-out / failure?

Also, wondering if there is something I could or should perhaps do now like (1) trying to put some form of concrete sealant into the cracks (bondcrete?) and or running beams around the lower section of the posts to further brace it - is this necessary in the short-term?

Any advice greatly appreciated. Have attached a picture.

Download Attachment: icon_photo.gif Treehouse.jpg

566.06 KB

Best Regards,

Link to comment
Share on other sites

Thanks alot Jim for the reply,

I do plan to put a cubby house on top by the way so this will add further weight to what you see here. Re-checked my books last night - one by Black & Decker and they do indeed set the posts straight into concrete (probably without the cracks though!).

Cheers,

Damian

Link to comment
Share on other sites

  • 10 months later...
  • 2 years later...

HELP!!!! my contractor in Pennsylvania pour a footer, placed a 6*6 on top to hold the roof of the porch, back fill with dirt, add modified, and is now planing to pour the floor of concrete on it... I'm a female and even I know theres something wrong here! I want to have an informative option, can anyone help????

Link to comment
Share on other sites

HELP!!!! my contractor in Pennsylvania pour a footer, placed a 6*6 on top to hold the roof of the porch, back fill with dirt, add modified, and is now planing to pour the floor of concrete on it... I'm a female and even I know theres something wrong here! I want to have an informative option, can anyone help????

Did you get a permit? Are there plans to follow? Call the local construction department.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...