I have a rental house that was built up on 8x8 wood stilts in 1986. I think the stilts are just put into the dirt like a utility pole. I dug around the base of a few of them where they start to go into the ground, and they look to be in good condition. I have no idea how they were originally treated.

Being well over 30 years old, they still make me worry. I'm in NC, it's very wet and humid and things like termites are very very common. (We had a termite inspection - no sign of them) There are a few other things that were done wrong on this house so I don't really trust whatever the original builder did.

Is there something I can do to help preserve these posts and deter insects after they are already in the ground? Doing some research on this I found a bunch of things aimed at utility poles - either a liquid or solid borate rod that you insert into a hole drilled at the base of the post. It looks neat but I don't really want to drill holes. Is it worth using a product like this?
Example: https://www.amazon.com/dp/B0098WFHTU...v_ov_lig_dp_it

Also, is there anything I could pour in the soil around the base of each post?

 

Are these pressure treated posts? if not, do you know what species of wood?
If it was me, I'd devise a plan to replace the wood posts with footers and block piers.

 

I have no idea about the wood besides it's a bit grayed from the weather and does not have the tell tale puncture marks of pressure treated lumber. They've lasted fine for 34 years, I see no need to replace them with masonry. With that said they cannot be expected to last forever, but I want them to last as long as possible. When I dig below the dirt line they are a bit discolored, but not rotting. I might paint them, but that does nothing to protect below ground.

If I really had to replace a rotten post it wouldn't be that hard, it is really simple construction, and a small house. Imagine your average wood deck, 24'x30' 8 feet in the air, with a small house on top. I see no need for post replacement now or anytime soon...just want to make sure they last. If this was 30 years ago I'd probably dump used motor oil around the posts, but that doesn't really fly today.

They have to have been treated with something, no termites after all this time says a lot. They really love this environment.

 

I didn't realize the house was that high off of the ground. Is it near the coast or a flood prone area?
Pressure treated lumber doesn't always have the indentations in it. I think that is more modern than yrs ago.

 

It's built like you might find in a common flooding area, but I don't know why. It simply can't flood here. It does make for a nice carport below with one area closed off to make a room for mechanicals and storage. I like it this way, it's like a basement but without the issues that come with basements! Very easy to get to all the plumbing and duct work.

 

Jim Walters used to build one model that was always on stilts. I used to see a lot of them around here but I think one by one they've had the bottom enclosed. I assume it was cheaper to build them that way.

 

I would leave them alone.

They have lasted 34 years and you see no evidence of rot.
You do not know the type of wood or how/if they were treated so anything you do may just cause a problem.

 

Sounds like they could be White-Oak or Live-Oak. Southern live oak is a wood that is dense, tough and rot resistant. So dense, in fact, that the wood SINKS in fresh water. If so, they'll last another 50 years.

Live Oak was basically "Colonial Kevlar" In 1797 the USS Constitution was built with a triple ply composite of white & live oak, held with nails of black locust. During the War of 1812, the USS Constitution faced the British frigate Guerriere, and, famously, most British cannon balls simply BOUNCED OFF the White Oak-Live Oak hull, which is why USS Constitution is known as "Old Ironsides". https://historyofmassachusetts.org/u...-construction/

Tangent - A little-appreciated reason for the growth of United States industry during the 1800s is that North America has a HUGE diversity of native trees, with wildly varying material properties.("naturally growing scaled composites"). Some 1800s flour mills would select from 10-20 species of wood for the mill machinery based on hardness, durability, and flexibility in the same way modern manufacturers select plastics based on hardness, durability, and flexibility.

 

That's kind of like saying "I have an old truck that lasted 30 years, why bother changing the oil now?"

I don't know what type of wood it is, how does that matter? I've never seen a wood preservative specify a species. I've also never heard of the application of wood preservative of any type causing rot to become worse.

What "problem" would this cause? What problem could it possibly cause?

The truth is they have lasted a very long time, and reality says wood preservatives do not last forever. Whatever chemical they might have used originally, has most certainly deteriorated in the last 34 years. Even telephone poles soaked in creosote have to be replaced now and then. Utility companies also regularly re-apply different preservation methods to their posts.

 

I'm not great at identifying wood but this does not look like oak. Oak tends to have a very specific grain structure. If I had to guess, they're probably treated pine or fir like you would buy from Lowe's today. It was never a very expensive house, the reason it is on posts to begin with is that's probably cheaper than a concrete foundation. Everything it is built with was the cheapest stuff at the local supplier, or possibly a cheap kit as marksr suggested.

This was originally built as a vacation home (next to a lake) so there were several things they sorta skimped on that you might not on a full time home. They could be the strongest most rot resistant poles ever made, but I don't know that, and, all signs point to probably not.

 

IMO the best thing to do is thoroughly inspect them about twice a year. I would think damage would be apparent long before they got to a dangerous state.

 

Putting a coating on them will seal out some moisture but will also prevent any moisture that gets in from getting out. Woods like cedar do better when they are allowed to breath moisture.

Since rot is most likely occur at ground level you will have to dig down a foot or two to treat.
Now you are messing with the foundation and have no idea what effect that will have.

If you are set on doing it I would use something like end cut preservative.
Solvent based and not water based as the water based stuff is crap IMO.
Also check the local laws as many products are against environmental laws when buried.

"They could be the strongest most rot resistant poles ever made, but I don't know that, and, all signs point to probably not" Not sure how you come to this conclusion when they have lasted 34 years with no signs of rot.