Wait a year before gluing pressure-treated?!

[Tomtebo]

I will glue and screw 2"4 ntr-ab to deck posts. I was thinking of using PU glue Cascol Polyurethane. But in Casco's glue brochure (at the bottom of page 4) it says "Before gluing pressure-treated wood, the wood must be stored for a year so that the chemicals used in the impregnation have dried out." And this applies not only to PU glue but all their products. http://media.schoenox.net/casco/brochures/120111-trlimsbroschyr-klar.pdf :x

Do they literally mean that the chemicals weaken the glue effect or do they mean that the wood's shrinkage affects durability?

Are there other adhesives that work better on freshly impregnated wood?

 

[Helioz]

Since you are also going to screw, you can disregard that recommendation. PU glue cures better if it's a bit damp, so just go ahead and glue.

 

[Alfredo]

I glued new pressure-treated for a few weeks ago. Lucky I didn't read up beforehand... I thought it might as well be good if it's a little raw with PU glue. I'll see if it holds. Fortunately, it wasn't anything important.

 

[Nimajneb]

I have poles that are several years old and glued with PU glue, no problems.

 

[Tomtebo]

Since you will also be screwing, you can disregard that recommendation. PU glue cures better if it's a bit moist, so just go ahead and glue.

In what way do you mean it makes a difference when screwing, if the glue joint is weakened when gluing newly impregnated wood the result would still be worse regardless of screws?

PU glue cures better if it's a bit moist, so just go ahead and glue.

Sure, but a moist surface is one thing and wood that is so soaked it shrinks is another, if that's indeed the reason why Casco writes what they do. Or if it's the impregnation fluid that chemically weakens the glue.

I have posts that are several years old and glued with PU glue, no problems.

And that was newly impregnated wood?

I assume you not only glued but also screwed, how do you know that it's not mostly the screws holding your posts together? Or that the glue joint is quite weak.

I have a hard time going against a manufacturer's recommendations unless I have really strong evidence.

 

[Helioz]

The screws account for such a large part of the joint that any weakening of the glue does not significantly affect it. The PU and white glue for outdoor use available to regular consumers are not construction-rated and can't be relied upon to last 20-25 years outdoors; for that, you'd need to use epoxy or resorcinol glue. Therefore, they function more as a complement to mechanical joining.

Your 2x4s are likely cupped and poorly planed, so the glue won't adhere properly unless you hand plane or run them through a planer before gluing. There is also a misconception that PU glue should be used as a gap-filling glue due to its foaming, but this is not true.

 

[Nimajneb]

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And was it new impregnated wood then?

I assume you didn't just glue but also screwed them, how do you know it's not mostly the screws holding your posts together? Or that the glue joint is quite weak.

I find it hard to go against a manufacturer's recommendations unless I have really good evidence.

I have all variants, only screwed, only glued, and glued and screwed. On the ones that are only screwed, you can see on some posts a tendency to twist, which is not present on any of the glued variants. Coincidence or not, I don't know. Everything has been directly from the lumberyard.

 

[Tomtebo]

The screws play such a large part in the joint that any weakening of the glue doesn’t significantly affect it. The PU and white glue for outdoor use available to regular consumers are not construction-rated and cannot be relied upon to last 20-25 years outdoors; for that, one would need to use epoxy or resorcinol glue. Therefore, they function more as a supplement to mechanical fastening.

Your 2"4 beams are probably warped and so poorly planed that the glue won’t adhere properly unless you hand plane or run them through a thickness planer before gluing. There is also a misconception that PU glue can be used as a gap-filling glue due to its foaming, but that is not true.

Should it be interpreted as you mean that the glue provides so little reinforcement that one might as well skip gluing and just screw?

 

[Tomtebo]

I have all variants, only screwed, only glued, and glued and screwed.
The ones that are only screwed show a tendency to twist on some posts, which is not present on any of the glued variants. Coincidence or not, I don't know.
Everything was directly from the lumberyard.

OK, so you do believe that glue provides some reinforcement. What exactly did you use for glue, was it for "regular consumers" or "construction-rated"?

 

[Nimajneb]

OK, so you think that the glue still provides some reinforcement. What exactly did you use for glue, was it for "ordinary consumers" or "construction-grade"?

Ordinary PU glue from Biltema.
One has to consider that the sole purpose of the glue is to hold the post together, as there is no stress on the joint itself except for twisting forces.

 

[Chippe]

I glued new pressure-treated wood a few weeks ago. Lucky I didn't read up beforehand... I thought it would be fine if it was a bit damp with PU glue. I'll see if it holds. Wasn't important fortunately.

Great thread revival and perfect that a lot of time has passed. How did it go with gluing new pressure-treated wood? It can apparently twist quite a bit and maybe separate a little when it dries.

 

[Alfredo]

How did it go with gluing new pressure-treated wood?

And you think I should remember what I glued almost ten years later? But I actually have a vague idea, and if it's correct, the gluing held without any problems. However, as mentioned, it wasn't anything important or critical, so I find it hard to comment on how good the joint actually turned out.

 

[Chippe]

@Alfredo Haha, ok I thought it belonged to something still up. I've found info from carpenters who say it holds up just fine and the wood will not twist according to him.