Pressure treated wood - drying/shrinking

[JHZR2]

I'm making some playground equipment for our backyard, and using galvanized conduit for ladder rungs, a fire pole, etc.

My approach is to drill into wood either through or to a set depth, using a forstener bit, and then after putting the conduit in, drill though the wood and tubing to hold it in place. It makes for some sturdy connections.

All I use is SPAX screws from HD.

I want a tight or even interference fit, but I also don't want to crack the wood with differing expansion and contraction of the wood vs the metal pipe.

So my question is this:

If I drill a 1 1/2 hole in new, wet wood, will the hole get larger as the wood dries and shrinks, or will the hole get smaller since the entire piece of wood is shrinking?

My understanding is that new pressure treated wood (this stuff is wet and sappy) shrinks quite a bit. I saw a recommendation that pressure treated wood decking should be installed with no gap, as the gap will form as the wood dries and shrinks.

If the whole thing will shrink, I'll make the 1 1/2 hole larger than it needs to be by 1/8 or so. The through-bolting will keep it secure in use.

Thanks!

 

[Donald]

Get the pressure treated wood from a quality lumber yard. You would be at HomeDepot all day picking through the pile for a few good pieces. Some people stack it and let it try for awhile.

For awhile it seemed the local school playgrounds were made with pressure treated wood. Now they are all bright colored plastic.

 

[dnewton3]

John -
It's too hard to tell you if the hole will grow larger or smaller. This is dependent upon the orientation of the grain in a radial sense. This is why some 4x4 treated posts "twist", some "bend" and some stay straight.
I understand what you want and why, but the answer is that it's too unique to each piece of wood to give you a "straight" answer (pardon the pun).

I've built many a deck, barn and playground over my years. Used PT wood nearly all the time. All you can do it put it together as best you can and hope for the best. There is no way to read the wood and tell if it's going to twist, bend, crack or whatever, that I've ever found to be reliable in a sense of predictable foresight.

For the steps/rungs, you might consider using galvanized pipe with termination ends like these:
https://www.menards.com/main/plumbi...0.htm?tid=7206356325533214321&ipos=2
I have used these to make the ladder rungs with 18" pipe sections. Screw them onto both ends firmly, then place in between the wooden side rails. You can then bolt the flanges solidly to the rails. You even have the ability to custom fit each one, as you can screw the flanges on/off a tiny bit on each side to make up the correct end-to-end dimension. Put together, it would be like this ....
wooden rail, pipe flange, pipe section, pipe flange, wooden rail
The added benefit of this approach is that the strength of the pipe/flanges helps hold the wooden rails in alignment and reduce twisting of the rails. Also reduces the intrusion of moisture into what would otherwise be a large untreated bored hole in your planned approach.

 

[PimTac]

Originally Posted by dnewton3
John -
It's too hard to tell you if the hole will grow larger or smaller. This is dependent upon the orientation of the grain in a radial sense. This is why some 4x4 treated posts "twist", some "bend" and some stay straight.
I understand what you want and why, but the answer is that it's too unique to each piece of wood to give you a "straight" answer (pardon the pun).

I've built many a deck, barn and playground over my years. Used PT wood nearly all the time. All you can do it put it together as best you can and hope for the best. There is no way to read the wood and tell if it's going to twist, bend, crack or whatever, that I've ever found to be reliable in a sense of predictable foresight.

For the steps/rungs, you might consider using galvanized pipe with termination ends like these:
https://www.menards.com/main/plumbi...0.htm?tid=7206356325533214321&ipos=2
I have used these to make the ladder rungs with 18" pipe sections. Screw them onto both ends firmly, then place in between the wooden side rails. You can then bolt the flanges solidly to the rails. You even have the ability to custom fit each one, as you can screw the flanges on/off a tiny bit on each side to make up the correct end-to-end dimension. Put together, it would be like this ....
wooden rail, pipe flange, pipe section, pipe flange, wooden rail
The added benefit of this approach is that the strength of the pipe/flanges helps hold the wooden rails in alignment and reduce twisting of the rails. Also reduces the intrusion of moisture into what would otherwise be a large untreated bored hole in your planned approach.





This is true. Hemlock for example will twist and bend even after kiln drying. It's a poor wood but plentiful in the west.

One tip is to get a small amount of wood treatment and brush into the holes you drill or over any cuts you make.

 

[ragtoplvr]

Poor quality pressure treated wood may have an untreated core. So drill it and you may have the middle rot.

As stated use good quality wood. The flanges are a good idea, be sure to use the special coated screws for the type pressure treated wood you use, also to conform to local building codes.

I age pressure treated wood in my attic for a few months.

Rod

 

[expat]

As suggested ALL the wood will shrink. The wood will shrink away from the hole as the wood dries. It will also expand again as it gets wet.

Don't over think it. But try to get PT wood that not is right out of the tank, but is semi dry, but not so dry it is cracked and warped.

 

[danez_yoda]

Do yourself a favor and spend the extra coin for stainless screws for the rest of the project. I used the pressure treated and hot dipped lag bolts and back in 2009 and the screws are now rusted through. Replaced the screws a time or two once I found them being loose. heads just snap right off. my daughter fell when one of the "galvanized" hooks rusted through and broke. Replaced everything with more galvonized since the kids will not be using it much longer (3-4 years) but if I i had a choice I would go stainless the first time through and save all the drama.

Its the CCA they use in the wood nowadays has copper that will rust and corrode regular steel more quickly than the old pressure treated pine.

Fir your center rot concerns where you drill the holes for the swing you can use boric acid totreat the wood against rot (TimBor, Borocare). Or do what I did and used PT 2x4 as a step with a 2x2 under it in a T shape as support. I (6' 270lbs) can climb in the castle with no problem.

 

[JHZR2]

I do have stainless Spax screws for parts that need them (e.g. where we are using cedar due to human contact).

The Spax outdoor/treated lumber rated screws, I have reasonable confidence in.

I like that Spax are all made in the US or Germany, and arent that $$$. And they DO work well.