Wood posts used in damp crawlspace construction?

[Kevinweg]

I'm not a structural engineer, so I figured this was a good place to get advice.

My father has a home on a lake in Northern Minnesota. The ground on which the 2 story house was built is sand/dirt mix. it is now about 10 years old. I went into the crawl space below the house to work on an air exchanger and noticed that the builder used 4x4 posts instead of what I'm use to seeing (which are metal posts). The posts are buried in this sand/dirt mixture (which is very moist) and they sit on top of concrete footings about 8 - 12 inches below the surface. The posts are space about every 6 feet. The base of the posts that are buried are damp, but there doesn't appear to be any appreciable rot that I detected in the ones I dug up.

It seems to me that this is not a very good design. The base of the posts are damp and over time, I expect these posts will rot. I have not inspected each post (there must be 40 - 50 of them).

Is this standard building practice? I would not have been concerned had I seen the posts resting on top of concrete that was above grade.

Maybe I'm worrying over nothing.

Thanks for the advice.

 

[Kevinweg]

I am including a really blurry picture of the base of one of these posts at the dirt line.

 

[msquared48]

Three things in the code:

1. Except in thecase of wood foundations for dry locations like a desert, wood is never to be in contact with soil.

2. There should be a vertical distance of 6" between the top of a concrete footing/stem wall and the finish grade/soil matrix below, so that there is a 6" gap between the wood post/studwall and the soil.

3. The foundation space should have adequate ventilation.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

Treated wood posts are commonly used where there is some contact as in pole footings and fence posts, or where the wood is in contact with concrete without any intermediate measures. However, I do try to limit even that myself.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Kevinweg]

Thank you very much. This is what I anticipated. I don't know how this would have passed inspection, but that's a different issue. The builder is long gone and moved/bankrupt.

At this time, I suspect I should get a structural engineer down there for inspection before anything bad happens. Like I said, I don't see significant rot, but the wood is wet. It does not seem that there would be an "easy fix" for this. If the code says 6" between the top of the concrete footing system and the soil, is it possible to cut each post out, dig down to the footing, drill in rebar, pour on top of the existing footing and then attach a new post to this concrete above grade. I can easily dig down to these footings by hand. It's the adhesion to that concrete that would be a bit suspect. Trying to dig new footings in this seems rather impossible. Not even enough room for a hand auger.

I realize this is a job for a licensed structural engineer to recommend, but being an engineer myself, I can't help but think of fixes.

Thank you again for the perspective!

 

[a2mfk]

I have quite a bit of crawlspace inspection experience in the opposite part of the country, Florida, but we have a lot of damp soil conditions as you could imagine.

MS has given you some good feedback. IN any crawlspace you want to have good ventilation, and most building codes have a formula for the amount of venting per the square footage of the crawlspace. In my experience, even a humid to damp crawlspace may not cause actual rot to occur. Of course, this means you need to excavate around the posts so no soil is in contact. You can then seal with a variety of materials, including bitumen (roof tar/cement may work), or wrap in building paper or sheet metal to form a moisture barrier, in case soil falls back into contact with the wood.

Also, if you can borrow or buy a moisture meter and probe the wood, you will know what you are dealing with exactly. Different species have different tolerances, many use a moisture content over 19% as getting into the danger zone.

Best of luck,
Andrew

 

[woodman88]

I have my doubts that the system used in the OP was designed correctly, but it can be done per the IBC/IRC's (see chapter 35 under the AF & PA) you can buy a copy of "ANSI / AF&PA PWF-2007 - Permanent Wood Foundation Design Specification" at

http://www.awc.org/standards/pwf.html.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Kevinweg]

Interesting, I had thought about excavating around the posts, but not sealing them. I have to presume that will not bring the current construction up to code, but will certainly improve the longevity of the existing posts.

The crawlspace has a power ventilator in it connected to a humidistat. It's far dryer down there now than when it was installed, however I have not really paid much attention to it up to this point.

I have tried to searching building codes for Wadena County MN, and I've come across a lot of information, but I've seen nothing regarding the posts supporting the structure. I have seen quite a bit on footing/pier depth. Do any of you know a good resource for something like this. Seems it would be simple to find, but I speak the wrong language obviously.

 

[dicksewerrat]

I would get 'plinth' blocks. They are made of concrete. Add solid concrete blocks to the existing until the blocks are at soil level.
Place the plinth block. cut off the existing 4x4 and reset. Of course you can only do a few at a time. Most lumber yards in Mn have the plinth blocks. Where up here in god's country are you?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Kevinweg]

woodman88 - went to the website and signed up. I will purchase the reference. Thank you!

 

[woodman88]

Here is a link to the Minnesota Administrative Rules CHAPTER 1309, International Residential Code

https://www.revisor.leg.state.mn.us/rules/?id=1309

per 1309.0010 "Subpart 1. Generally. The 2006 edition of the International Residential Code (IRC) as promulgated by the International Code Council (ICC), Falls Church, Virginia, is incorporated by reference and made part of the Minnesota State Building Code except as qualified by the applicable provisions in Minnesota Rules, chapter 1300, and as amended in this chapter. The IRC is not subject to frequent change and a copy of the IRC, with amendments for use in Minnesota, is available in the office of the commissioner of labor and industry. Portions of this chapter reproduce text and tables from the IRC. The IRC is copyright 2006 by the ICC. All rights reserved."
So (for the Residential Code) you need to go look at it, Or buy a copy of it. Or buy a copy of the 2006 IRC and view the Minnesota changes at the above link.
The ANSI / AF&PA PWF-2007 should be referenced in it. But as I only have the 2003 IRC I can not state for sure.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Kevinweg]

@dicksewerrat - Closest "big city" is Alexandria or Detroit Lakes. They would clearly have these blocks. My first reaction was to do something like you suggest, but I have to assume the block would need to be anchored to the pier (maybe I'm wrong). The other challenge (although it's not a big one, is to jack up the posts to the right height so-as to prevent sagging. I had that problem in my suggestion as well. That's why I like the idea of excavating around them and sealing them if it's a proven technique. Heck, I have to admit, I don't have a clue how those existing posts are anchored to the piers (if at all).

 

[msquared48]

Sometimes, there is only a 1/2" diameter, 6" long metal pin between the bottom of the post and the footing. Sometimes nothing too. It you have a rebar detector, you might be able to determine if one is present.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dicksewerrat]

My guess would be that the posts were only placed on the concrete blocks. Are they an 8x16x4 inch block? Use a nice little hydraulic jack, placed on a one of those concrete blocks and a 4x4 block on the top of the jack, spanning two floor joists. Remove the existing post. Place the new blocks, etc and put in the new treated 4x4 post. the excavating each post will take the most time. Dig out about 6 at a time. Do the remove and replace. Then crawl out and rest your back. Repeat till done. Don't wait till frost. Or wait till spring.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Kevinweg]

6" pin would be nice to prevent post movement on the pier. Will check for that. Probably, if I find them, remedy will need to be sooner rather than later as those posts are wicking and a hole in the base of the post where the pin sits is probably just another wet surface for wicking - cept on the interior of the post.

 

[Kevinweg]

dicksewerat - This house is probably around 10 years old and I have to believe that the piers go below the frost line, but what do I know. I did not dig around all the piers so I don't know what's below. All I know is I took a hand trowel and dug around two of them and hit concrete in both cases 6 - 8 inches below the surface dirt. I don't know if it's a block or poured concrete pier going 5 ft down. Now, this area is on a lake and has a high water table and maybe blocks are how they do it because digging too deep will just result in hitting lots of water. The part I was reacting to was the wicking of the damp soil into the posts, but it looks like I now need to do more investigation. The wet posts are clearly a problem, but I want to make sure I understand the local code for this all the way from the footing requirements to the post spacing, structure and attachment. My next trip up there is in a few months, so this will take some time.

When I go up next, I plan to do the following:
1) dig around the posts to determine what they are sitting on
2) Determine if there is a pin in the center or if they are just sitting on the concrete
3) Determine the moisture in the wood and condition of the wood in a larger sampling
4) Check against code (may even get an engineer in there).

Once that is done, I'll have a better idea of possible remedies. You guys are a great help!

 

[BAretired]

The picture is a bit blurry, but it appears that you have a black vapor barrier over the crawl space. The vapor barrier seems to be covered with some sort of granular material, maybe two or three inches thick.

The post has a faint black mark on the right hand face. Is this a grade stamp? Do you know whether or not the posts were PWF material?

PWF foundations entail wood against granular material but it should be well drained. Treated wood piles have been used to support buildings for many years in wet soils. If there appears to be no evidence of rot so far, maybe annual inspections would be an option.


BA

 

[Kevinweg]

BA -
I strongly believe the wood is treated but I didn't notice the mark on the wood until you pointed it out. Sorry for the blurry photo, my eyes aren't so good anymore and I was swaying I'm sure. The vapor barrier is covered with pea gravel about 2 inches thick. It would be great if this were PWF but as you pointed out, it should be against well drained granual material (and as you can see if you squint at the picture, wicking is present so there is moisture). The wood did not seem soft except at the very surface. I plan on reading the pwf information provided by woodman88 and can get a better idea of whether it's used or not. If it is, maybe this is a red herring of sorts and following the recommendation by a2mfk is enough to ease my concerns. If it's just untreated lumber, or if the moisture content is high (regardless of treated/non-treated), I may be looking at what sewerrat suggested. Wish I was still up there now, could easily figure some of this out.

Thanks!

 

[shobroco]

If the structure is performing as intended, why don't you just buy a pile of borate rods, drill a hole in each post & insert one. The posts will outlast us if you get it done before they start to rot. You can find suppliers by googling borate preservative rods.

 

[a2mfk]

If we are talking about posts supporting gravity loads of a first floor residential structure (not holding up much), I would only do something if there was a real problem, like wood rot. I have seen WAY worse than what you describe down here in Florida on structures that are 50+ years old and they are still performing. Hate to say it, "IF it ain't broke, don't fix it."

"The crawlspace has a power ventilator in it connected to a humidistat."
You have a vapor barrier and this, you are way ahead of the game.

I would invest in a moisture meter and probe the posts, if you are less than @ 19% or so then you are probably OK. Keep checking on them on an annual basis.

All of the other suggestions seem on point if you have actual wood rot.