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'll have to look up borate rods - this is in line with an earlier suggestion regarding sealing the posts.

I'm just trying to get ahead of any rot... I would like to think that there isn't any but I believe a meter test and getting the soil away from those posts is the simplest most prudent things to do. If there is no rot, I will coat them or use borate rods to preserve them better. All of this is a minor amount of work in comparison to if they rot.

Thank you for all the excellent advice. This is an excellent forum.

 

[BAretired]

I am not sure that coating them would be effective. Coating with what? It is no longer possible to effectively seal them from the outside because the bottom will always be in contact with water and will permit wicking up through the post.

Borate rods sound like a good idea although I have never used them and cannot comment on how effective they are. This would be something you might want to read up on.

I tend to agree with the philosophy "If it ain't broke, don't fix it" and give it periodic inspections to confirm that rot has not become a problem.

BA

 

[a2mfk]

Good point BA, unless the end bearing condition is a Simpson post base or similar isolation connection, any moisture in the ground or the concrete would be wicked up in the end of the post, especially since the bottom of the post is open-grain fibers. If the end is bearing on ground or concrete with no moisture barrier or metal base, I then would consider not sealing, to allow the wood to "breathe". I think sealants may prevent moisture from leaving the wood if the air were kept dry in the crawlspace, therefore exacerbating the situation.

Perhaps a product that inhibits microbial growth but does not form a moisture barrier would be the better prescription, and do your best to keep the humidity down in the crawlspace which is being done. All of this effort is far more than I have ever seen in any other crawlspace, which is commendable!

 

[Kevinweg]

Regarding sealing, I was looking at the "roof tar" idea for sealing. I'd clean off the post and the pier completely... let it dry out and then seal the post and the top of the cement. However, I do like the borate idea. I did do a little reasearch and it certainly seems to be applicable in this situation (and relatively inexpensive).

Yeah, it's a lot of work to go through for a crawlspace, but we want this home to last well into our retirements (when we won't be able to afford any big sructural repair bills). Regardless, I plan on fishing for walleye with my spare time, not crawling around in that crawlspace trying to fix something I coulda/shoulda prevented years ago

 

[chicopee]

Since this is a private residence,without too much expense you can brush clean motor oil as a preservativeon the bottom of the posts since the bottoms appear dry.

 

[woodman88]

Kevinweq - As you are planning to stay with the house that long and fixing up what you can, and if you determine that the wood posts may be a problem down the road. I would suggest looking at the top of the wood post connection. It may be possible to add a new concrete pier with a adjustable post/beam support, both sides of the wood post, without having to jack up the building. This may (or may not) be doable, but is IMHO worth looking at and could be done on a “as money and time allows basis” at least until (or if) a problem occurs with existing posts.

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.

 

[ajhengineer]

@Woodman88,

I have not been able to tell if you have properly preservative treated posts... there is nothing wrong with embedded wood posts in soil provided they are properly preservatve treated. I specialize in Post Frame Engineering (see

http://www.postframeadvantage.com

for more information about Post Frame) where at least 75% of the commercial building projects I work on in Wisconsin - Minnesota - Iowa utilize embedded wood posts... completely code compliant. If your columns are not properly treated, it may be possible to shore up the building temporarily, replace the embedded portion of the existing posts with properly treated posts and make a structural connection to the existing 4x4 posts above grade.

I applaud you for taking this observation so seriously, but I wouldn't presume the posts are bad just because you're not familiar with the practice. We tend to distrust things we're not familiar with, right? If the 4x4 posts are not properly preservative treated, or if you're not sure, you may be able to hire a qualified post frame contractor to come in and give you a better opinion for less cost than hiring a structural engineer to come in. I think a qualified post frame builder would probably give you better results than a structural engineer not familiar with this type of construction. Consider checking out the Find a Designer or Find a Builder feature at the postframeadvantage website.

Note that the American Society of Agricultural & Biological Engineers (ASABE) has an engineering practice #EP486 called Shallow Post Foundation Design and this is referenced in the International Building Code 2006, adopted by the Minnesota Building Code. Although this is a dwelling, structural considerations are allowed to be analyzed under the IBC provisions to meet the requirements of the IRC.

 

[ajhengineer]

Crud! Just realized I addressed my last post to @Woodman88... it was intended for the author of the Thread, @Kevinweq. Sorry!

 

[Kevinweg]

Was gone for the weekend but back and I see there were a few more responses.

My first reaction to what I saw was "WTH?!" as I am not familiar with wood posts supporting a home like that. However it appears, based on the feedback, that this can be done to code and there are several options I can choose from if problems actually occur. I am headed back up in Oct and will get a much better view of what's going on there. I will take the advice from ajengineer and if I need a pro down there, will look up someone on postframeadvantage.

Clean motor oil eh? So simple.