Black locust vs. treated wood -- for barn poles

Comparing black locust to treated lumber for use in barn construction. June 21, 2000

I have a lot of standing black locust at least 10 inches in diameter. If I cut them into 12 to 14-foot sections and have a bandsaw mill square them up at 5 or 6 inches, could they be used as poles in a pole barn?

If I used them green, would they warp or twist excessively later, after the barn is built? Or would I be better off using 5-inch treated pine poles? All things said and done, the cost might be the same. Would black locust with the sapwood cut off last longer than treated pine?

Forum Responses
Treated pine will last for a century or more. Locust might last for 20 years in a wet location; longer if dry. Treated pine may also be graded, so that you know the design strength. Certainly for ground contact, use treated pine.

Gene Wengert, fourm moderator

In a study I conducted of old barns in the mountains of western North Carolina, I examined one that was built in the late 1920s and made extensive use of 6 to 8-inch diameter black locust logs for its frame (stripped of bark, crudely notched, nailed) and was then clad in rough sawn boards of various species.

I didn't check the roof, but none of the wall poles/timbers showed any obvious deterioration. It was earth-fast construction, so there was some deterioration where the black locust was in direct contact with the soil. It must have been really dry soil over a lot of years, but I was still amazed that the wood held up so well.

In Indiana, one of the preferred fence posts is black locust. They last many decades without deterioration. A locust grove we are logging has lots of black locust blowdown from 15 years ago. Even in direct contact with the wet ground, they are in great shape. About 1/4 to 1/2 inch of the sapwood is soft, but the rest is fine.

From the original questioner:

Gene, about your comment on the lifetime of treated pine: I live near Houston and have a lot of treated pine around in the form of flower bed borders, fencing, etc. The stuff that went down 12 years ago when I moved here is about shot. I wouldn't give it another 2 years. I can jab a screwdriver about halfway into the wood.

Some of this is landscaping timbers (not the best quality) and the rest is fence posts and rails. Even the treated pine on my trailer is not going to last 15 years. Do we get different treated stuff down here? Or is Houston just Hell on everything?

My black locust is not here, but on my place in Oklahoma. Treated pine lasts a lot longer in OK. I once used untreated pine on another small trailer down here and in two years it was totally rotten. You could crumble it with your hands.

You have wood that is "treated to refusal" rather than being certified.

They take green wood and try to force in some preservative, but being green it refuses to take very much at all. So it appears "green colored" or "treated" on the outside, but it really is not much better than no treatment at all.

Always look for stamps or labels on the wood with the AWPB or AWPI symbol.

Gene Wengert

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
I have some black locust fence posts set in a swamp by my grandfather before 1930. The barbed wire has long since rusted away, but the posts are still quite sound. They are about 8-10 inches in diameter.

Comment from contributor B:
I'd say always go for the natural alternative to toxic preservatives. CCA is being discovered to be pretty nasty stuff when in contact with exposed skin. I really dislike the treatments. They work because they are so darn toxic. Sure, cedar is toxic too, but at least I won't get skin cancer by walking barefoot on it (like a couple of folks did with CCA treated decks).

In New Hampshire they say locust posts last about 1 year shy of granite ones. There are standing fences near rivers using BL posts that are better than 50 years old and not even punky. Black locust is the Northern hemisphere's wonder wood. Is is strong, incredibly decay resistant and bears up to wear. Great for any structural application. Also a substitute for white oak in any application.

Comment from contributor J:
We recently pulled old locust posts at our farm in eastern WVa and are reusing them. They were put in the ground by my grandfather over 80 years ago and seem to be good for another 80. Soil is shaly and well drained. Around here, mountain-grown black locust is prized for its longevity in contact with the ground. Black locust taken from low areas (along the SB of the Potomac), however, does not last nearly as long.

Comment from contributor C:
This new style treated lumber has not been around for a hundred years. Power poles that were treated with creosote, do not last more than 50 years. I have seen decks not in contact with the soil deteriorate in 10 years. The black locust posts used in N. GA. are superior to any other wood product for longevity in the ground and are natural. Termites will eat an oak post up in 2 or 3 years. The only down side to black locust is after they age, the wood becomes too hard to drive a fastener into it.