Thermo Treated Decking Lumber Question

GeneWengert-WoodDoc · January 29, 2013, 05:37:38 PM

The end of lumber will have a sticker indicating the level of treatment. Basically, most of us will have only two possible treatments--"above ground" and "ground contact" with "ground contact" having more chemical. Ground contact will often have more chemical in the core too.

The label will indicate a limited lifetime warranty, which provides for decay and insects, but not fire or bad construction. Bad construction would include cutting the end of a piece off but not treating the freshly exposed end, or drilling a hole and not treating the exposed wood. A treatment level is for the outer regions of the wood, so if you do something that exposes the core, whcih may or may not have adequate treatment, you need to apply a treatment chemical on site.

Axe Handle Hound · January 30, 2013, 09:15:45 AM

Dr. Gene,

Very informative post. I just got schooled on a whole lot of pressure treated information that I never knew before. In regards to the treatment you mention, what would you suggest? Is there a particular chemical/product that a builder should use for exposed cuts in PT lumber?

GeneWengert-WoodDoc · January 30, 2013, 01:47:52 PM

There are several including UltrawoodR, Wolman ExtraR, and WeatherShield.

One other item that is often overlooked is that the deck boards should be oriented so the bark side is down. Wood cups toward the bark so with bark side up you will have more cupping than with bark down. However, as the upward side in either orientation will be wetted and dried with rain (sprinklers, etc.) and sun, the exposed face will be moving more. As the exposed face will shrink more due to heating and drying from sunlight, no matter what face is up, you will see some cup. That is why I said that bark side down will give less cup, as the heart side has more quartersawn grain which shrinks and swells less.. The bark side will move more than the heart side. Having said this, there is certainly some information that suggest bark side up. The best is quartersawn.

GeneWengert-WoodDoc · January 30, 2013, 01:52:47 PM

Incidentally, if you do cut the end off of a post, put the original "factory treated" end in the ground. Then consider a cap on the cut end, or else use the above chemicals every year on the end.

beenthere · January 30, 2013, 02:22:34 PM

Better idea, is put the cut end in the ground, as it isn't going to decay on that end being away from the ground line. The part of the post at ground line and above will be prone to decay conditions.

Den Socling · January 30, 2013, 03:41:37 PM

BT I don't know why you would say to put the cut end in the ground unless you live in Arizona. Around here the ground is damp.

beenthere · January 30, 2013, 03:47:29 PM

Same here, but the end of the post is deep enough in the ground where it doesn't decay. It is the 6-12" at the ground-line that decays. Not the end of the post 3-4' down.

Den Socling · January 30, 2013, 03:59:42 PM

Come to think of it, fence post rot off at ground level and leave the end of the post in ground.

GeneWengert-WoodDoc · January 30, 2013, 04:29:31 PM

Put the cut end up in the air and the factory end into the ground. That way, no decay can enter the piece. Indeed, the ground level seems to be the most decay prone. When treating under pressure, a lot of chemical will go in the end, so the factory end will have a bit more preservative for two or three feet.

rooster 58 · January 30, 2013, 04:55:34 PM

I just saw some thermo treated wood the other day at a wood shop. The owner actually machines wooden parts for toys. I noticed the stack of thermo wood. He said he makes adirondak furniture from it. It almost looks and feels like an exotic wood, more dense and probably brittle; which would reduce the strength. I'm sure that if it would make good decking other than the strength issue, all one would need to do is put joists 12" o.c.

As far as p.t. lumber, the sun has a far greater effect on it than moisture. The wood dries out badly and is prone to checking and splitting, and can become sort of brittle in it's own right, which reduces strength and is a leading cause of replacement.

Doc is right about putting the "bark" down. Even taking time to inspect the growth rings at the ends pays off by putting the rings concave up to reduce cupping.

Posts are or were treated to mainly two different levels, .40 and .60 retention. This retention level was checked by boring holes into the side of a posts and inserting a meter to measure the amount of retention. If you ever saw a post that had a dowel plug in it, that was the result of a retention test. I encountered them many times while constructing many pole buildings

T Red · January 30, 2013, 05:00:58 PM

I have learned a lot in the thread.

But I don't see how any deck built with today's PT will last a century. First the penetration is not enough IMO. I've seen it rot to the point of being dangerous. Second it's really hard to get quality lumber or at least I've had a hard time. The decking will warp and shrink (from not being dry properly I'm sure) and crack to the point of being an unattractive. It's not that people need replace a rotten deck. They want to replace the eye sore that's attached to their house.

As far as the post in the ground it doesn't matter which end you put in the ground water will get in the top and in a few years ? 20, 30 you'll want to replace it anyways.

My experience, ymmv.

beenthere · January 30, 2013, 06:12:54 PM

I won't continue to discuss which end down, but that doesn't mean I think the Doc is right. Just the opposite.
Then, I don't have a PHD

tyb525 · January 30, 2013, 06:44:07 PM

Ground level rots the most, but then anything anywhere in the ground rots. I've pulled posts that were rotted completely at ground level, but also rotted on the end.

Cut end goes up in the air with a metal cap to prevent water in the end grain.

And there really is no quality decking anymore, no matter how much you sort through the pile. The better stuff is sold at Menards and is over 1/8" thicker than the stuff at lowes and home depot, but even then they all warp and split once exposed to sunlight.

I once saw a deck board, after just 1 month, bend upwards on the very end of the piece at the last joist over 3" in the air, it was screwed down good with 3-2 1/2" screws and pulled them right out.

I have also seen deck boards shrink up to 1" lengthwise. Now how in the world does that happen?

jueston · January 30, 2013, 09:48:33 PM

Quote from: tyb525 on January 30, 2013, 06:44:07 PM
I once saw a deck board, after just 1 month, bend upwards on the very end of the piece at the last joist over 3" in the air, it was screwed down good with 3-2 1/2" screws and pulled them right out.

i have seen this exact thing as well and it was on the menards pt decking...

GeneWengert-WoodDoc · January 30, 2013, 10:33:41 PM

The reason you see this excessive warp and lengthwise shrinkage is due to compression wood most likely, and if not cw, then juvenile wood. Both are not acceptable in 5/4 radius edged decking lumber. I suggest that someone look for the grade stamp indicating this grade on every piece of lumber. There are a lot of similar looking products, but without the quality.

tyb525 · January 31, 2013, 12:24:55 AM

I must not be getting the real stuff then, cause a lot of that lumber has the pith in it

beenthere · January 31, 2013, 01:20:50 AM

I buy Premium grade 5/4 x 6 radius edge (no knots) SYP for decking. Sort out all the pieces with heartwood and return it.

1woodguy · January 31, 2013, 10:08:43 AM

The pt they have at the Lowes in my area is flat garbage in my opinion
Spent a few days replacing a bunch that lasted 3 1/2 years

Bugs didn't used to bother it here.