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I'm planning to build an outdoor balance beam in a 3-part right-angle Z- shape ( ‾‾|__ ) and have access to some fresh-cut white oak (6"x6") which I think will do well outside, but I'm not sure what type of drying time I'll need, or if I need any at all. There are no glue-ups, just half-lap joints with lag bolts at the intersections. I've never worked with fresh/green wood before, so looking for tips or recommendations. The long sections are 12' and the short connector is 6'

I did also plan to use the wood for the posts into the ground (cemented) but might consider something different for the posts (like 4x4 treated SYP)

Basing it on this design: https://imgur.com/a/AWkoVlE

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  • People obviously work with green wood (and very much with green oak) building major structures, but the methods used are inherently tied to them knowing the wood is going to shrink. So no (few?) glued joints for example. So if you want to dry the rough rule of thumb for drying is, wait for it, a year per inch of thickness. Sorry! White oak is an excellent choice for exterior durability and it would do quite well with direct soil contact, so I think you'd be more than fine using it for the posts if they're going into cement footers. That is once dried of course, again, sorry!
    – Graphus
    Mar 17 at 17:56
  • Now regarding the pieces of wood you'll be getting, you do need to know a bit more about them before an assessment (read: guesswork) about how well they'll fare can be given. If one or more of the pieces are 'boxed heart' they'll act quite differently to if they are chunks taken from beyond the core wood near the pith line. But regardless, you can expect numerous and possible very severe cracks to form, starting from the cut ends if they aren't immediately coated with something to slow drying. If there's no access to Anchorseal best thing to use would be melted wax if you can arrange it.
    – Graphus
    Mar 17 at 18:01
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    Unfortunately I don't have 6 years to wait for the wood to dry, otherwise son won't go to this school anymore! If I really must use dried wood for this, then I'll have to find a different lumber source. My guess is that using green wood and allowing for shrinkage/checks will be fine, since it's mostly for kids to walk on. I may just periodically check for splinters and tighten the lag bolts as shrinking occurs.
    – xdumaine
    Mar 17 at 18:54
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    I knew I should have stressed the immediately. It's best to seal the end grain within an hour of milling; you wouldn't believe how fast cracks can develop in just a couple of days of exposure to exterior air and it's fairly dry this time of year. But regardless, for various reasons which I'll flesh out into an Answer I don't think you should go with green oak for this. Too many unknowns.
    – Graphus
    Mar 18 at 4:42
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    @Graphus I'm still trying to gather information while the beams are being sourced, but I suppose there's probably enough information to accept an answer.
    – xdumaine
    Apr 6 at 20:28

2 Answers 2

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A lot depends on the nature of the wood you have. Everything @Graphus says about checking is true, but ... if the wood you have does not have boxed heart, and is reasonably close to quarter sawn in its composition, and it's really White Oak, then you'd be fine going with green wood. Quarter saw White Oak is unlikely to check more than minimally, and will remain structurally sound if it does. I've got White Oak fence boards that were put up green 45 years ago, and those that are pure long grain (quarter sawn) are still strong, and while quite weathered, not seriously checked or split. Those with knots, flat or short grain are long since caput.

So short answer: I'd go ahead if and only if you have quarter sawn lumber.

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    Your fence boards are not similar to 6x6 green beams, cannot make parallels.
    – Volfram K
    Mar 22 at 10:07
  • I disagree. A 6X6 cut in quarter sawn conformation out of, say, a 30" white oak log would be very straight-grained, and would dry/weather very much like a quarter sawn 2" fence board. Straight-grain white oak is incredibly stable and resilient wood.
    – Walnut Close
    Mar 22 at 13:26
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    Oak is a 'refractory hardwood', difficult to dry like beech! Only stable after careful drying.
    – Volfram K
    Mar 23 at 6:29
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Do I need to dry white oak before it can be used for an outdoor balance beam?

I think so yes. Green wood, and very much green oak, is used for building things but I don't think this project is a good fit for using it for multiple reasons, but perhaps most importantly because of various safety concerns.

One face of each beam is intended to be a walking surface, and there's no way to ensure that surface remains flat. And as the wood shrinks from green to air-dry over the next few years it will inevitably crack — and the number, severity and position of the cracks can't be predicted — so it's just a throw of the dice whether you'll get one forming on the top face of one or more of the members.

And be under no illusion of just how large the cracks can get. If any of the pieces have 'boxed heart' you can almost guarantee you'll get a radial crack (AKA heart check) and on 6x6s these can certainly be wide enough to trap a small fingertip or toe.

Here are some photos to show how bad the cracks in thick oak members can be, and this is even with management (in a mill's yard) or under cover/indoors, not uncovered in the open air and left to their own devices.

Cracks in air-dried green oak

From the Comments:

If I really must use dried wood for this, then I'll have to find a different lumber source.

Obviously building this from dried oak would be a different story, but I doubt it's going to be economically viable in the current lumber market. Although there could be significant local variation the latest I heard (about two weeks ago) typical lumber prices have risen again to roughly double pre-Covid levels; and oak of these dimensions would already have been pricey before!

So for cost, availability and reliability reasons I'd suggest just going with with pressure-treated SYP as you were considering using for the posts. This sort of wood is what is most commonly employed in the construction of playground equipment anyway.

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  • Respectfully disagree on two counts. Splinters from pressure treated wood are nastier than untreated. (I know: the wood industry assures us that the current PT is perfectly safe; just as they assured us the now-disgraced-old-formulation was perfectly safe.) Second, checking will happen, but can be managed by occasional visits to sand/cut/smooth any terrible edges. If, in the most dire example, there was a huge crack, it could be filled with an epoxy. At the end of the day, I view this as playground equipment, not furniture.
    – Aloysius Defenestrate
    Mar 20 at 14:10
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    @AloysiusDefenestrate, I get where you're coming from and just to go into the PT side of things briefly, the now-disgraced-old-formulation was probably perfectly safe. Did you know that the surface arsenic levels were lower than those naturally found in some soils? Yes, lower. It's little nuggets like that that begin to give an insight into why that process was — not completely — phased out or banned, and it had little to do with consumer safety.
    – Graphus
    Mar 21 at 1:49
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    @AloysiusDefenestrate, now about the green oak, like I said right at the start in the Comments we're just guessing how this will turn out. The OP doesn't even know yet how clear the wood will be (relevant to splitting and warping), whether there'll be any checks before he even gets it into his hands. There are in addition other issues I deliberately didn't include mention of to avoid TL;DR, including how the wood and fixings will interact (in the UK stainless is virtually universally recommended) and LBNL how much sapwood will be present. How it will fare once installed is anyone's guess.
    – Graphus
    Mar 21 at 2:00
  • It seems like treated SYP isn't actually much cheaper than oak these days, at least in my neck of the woods.
    – xdumaine
    Mar 24 at 15:50
  • @xdumaine, yeah there was always that risk :-) One thing though, it's important not to extrapolate from board prices — there's always a premium on length, and anyway 6x6s in kiln-dried oak are possibly unobtainium. Lumber prices do innately vary widely place to place, especially so in the US because it's so large and there being wide variations on what's available from local sources due to the differences in climate.
    – Graphus
    Mar 24 at 17:04

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