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Outdoor Sculpture Joinery

1052 Views 28 Replies 10 Participants Last post by  Unknowncraftsman
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I've designed an outdoor sculpture with a colleague of mine that is going to be installed later this summer, and we aren't able to agree on best practices for the joinery of the pieces. The project is a series of 24' high cattails made of cedar and steel. The stems of the cattails will be large steel poles (3" diameter) and the pods of the cattails will be a series of 1.5" thick discs of cedar. The diameter of each disc is about 10.5" with a 3.25" hole in the middle where it will slip over the pole. The overall height of each pod will be 5' - so each pod will consist of about 40 discs. We'll be using 2×12 cedar boards to achieve this. The bottom disc will rest on a welded plate on the pole (think giant washer welded in place). This sculpture will be outside in Minnesota year-round.

The question is this: what, if any, fasteners or adhesives should be used between each disc? My belief is that other than the top few discs, we shouldn't use anything between them, so as to allow each disc to move on its own. I think something on the top few would be good so that any cupping doesn't create an unsightly gap. I believe that a) any adhesives between the discs will be temporary in any effectiveness, b) screws holding the discs to one another will prevent each disc from moving at its own will, therefore potentially causing cracks, and c) that there is virtually no chance of an individual disc cracking so much that it would be able to fall out of its position on the pole and to the ground.

My colleague worries that without screws and glue that pieces will crack and fall off and be a liability issue. I worry that the liability issue increases if we do use screws. Glue, I could get behind, but again, I feel it's only a temporary hold with the weather changes.

Anyone have advice that might steer us in the right direction?

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Yeah, that's kind of my thought. A bit of epoxy or construction adhesive between the discs, because if the wood NEEDS to move, it'll snap that connection instead of potentially cracking under the pressure of the screw.

All of my woodworking experience is telling me to run far, far away from using metal fasteners between the discs.
Yup

"This sculpture will be outside in Minnesota year-round."

I have zero experience with Minnesota weather so hopefully our betters can give the benefit of their knowledge
and experiences.

Best
Don't use any kind of mechanical fasteners or rigid glue. Glue the disks together using a good quality clear elastomeric caulk. This will have the best potential for allowing seasonal movement of the wood and sealing moisture out.
I say no on epoxy. It's dries hard too ridged for cedar. You don't say what species of cedar western red or Tennessee aromatic. There's also a northern white. Another point I would like to make is all the woods I mentioned above are really in the cypress family.
Cypress is a great out wood.
I would like to suggest either gorilla glue or silicone. Test test test. You will be surprised how well caulking bonds to wood.
If your boards are flat sawn they will expand mostly in their width. Very little up and down.
I would also paint them leaving the wood exposed to Mother Nature isn't going to last as long.
Good Luck
Maybe a composite material instead of cedar?
Maybe a composite material instead of cedar?

- 1thumb
I've thought about that, but unfortunately it's too late to make that change. The client and structural engineer have signed off on cedar.

I say no on epoxy. It's dries hard too ridged for cedar. You don't say what species of cedar western red or Tennessee aromatic. There's also a northern white. Another point I would like to make is all the woods I mentioned above are really in the cypress family.
Cypress is a great out wood.
I would like to suggest either gorilla glue or silicone. Test test test. You will be surprised how well caulking bonds to wood.
If your boards are flat sawn they will expand mostly in their width. Very little up and down.
I would also paint them leaving the wood exposed to Mother Nature isn't going to last as long.
Good Luck

- Aj2
It will be western red.

Gorilla glue/silicone and really where I'm leaning as well.

We're going to likely stain the boards with a high quality wood stain (read: not big box store) and then let them gray out when that starts to wear off.

So far it seems like folks are in agreement with not using screws. I'm glad to hear that, and it's been my instinct as well.
Maybe a composite material instead of cedar?

- 1thumb

I ve thought about that, but unfortunately it s too late to make that change. The client and structural engineer have signed off on cedar.

- JustinHWW
That's a good thing if the cedar happens to fail. I'm far from an expert and many here are, but I don't think I'd use any adhesive. Hard to test for one. Leave up all 4 seasons and see what happens? I've seen what happens to beautiful finished products that were glued in places when they shouldn't have been. Suicide watch. Take their pistols, shoe laces and belts. Lot of money was not exchanged.

I'd stack 'em, maybe counter bore the first and devise a hidden stop kind of thing to hold the stack in place. *Sorry. You listed that in post.
Worth noting that the structural engineer, who was hired by me, not the client, suggested using screws and silicone. Not for structural soundness, but to minimize the rain that gets in between the boards. Silicone I can see, and while I'm not expect at all, I just can't fathom why screws would be a good idea. Rather, I believe they're a very bad idea.

I wish I had the time to test for 4 seasons, but sadly I don't.
How do you plan to hide the joints between the discs?
I don't.

I'm planing and then CNCing the discs so they'll be exactly the same in dimension, and my theory/plan is that the overall weight will hold them pretty tightly together. The seasonal movement and slight gaps just has to be part of it, as I don't know any method to keep them tight to each other without sacrificing their ability to move, which in turn, will weaken the wood.

Plus, they're pretty high up in the air - at least 12', and viewers will mostly look directly up at it, so I think the gaps between the discs won't be terribly noticeable.

Maybe I'm way off here on this. I'm open to accepting that! :)
Those are way bigger than the Costco hot dogs.

I think personally I would drill the center hole just slightly bigger than the skewer, say 3 1/4", then use 100% silicone around the hole. Then layer each piece with alternating grain direction, allowing some silicone, say about 1" from the hole on each piece to adhere to the next piece. This way they are siliconed together and the alternating grain will keep one big chunk of hot dog from falling off when it splits.
"Then layer each piece with alternating grain direction, allowing some silicone, say about 1" from the hole on each piece to adhere to the next piece"

Just like plywood ^ Good idea
You are going to weld a giant "washer" in place on the bottom to hold them in place, is there a way to do the same on the top to hold them down from the top, to sort of compress them? The discs aren't that big, I would soak them in stain once you cut them. Here in Texas they submerge fence pickets in large tanks of stain when you by pretreated fence pickets from a fence supplier. They last a long time. The interior would be protected for 10 - 15 years I would guess as long as the end grain and edges are periodically re-treated, every 2-3 years.

If your going to cut them on a CNC how about routing interlocking groove a few inches from the outside edge? When stacked they would lock together. If a piece were to break and fall off it would be limited to the outside of that ring.

That is awesome by the way!!!
I would use the silicon just like glue.
Spread it quickly notch it with a trowel. Bring each disk together with good even pressure I would look to get a bead of squeeze out on the outside edge. Don't not disturb it let it dry and cut it off later. You don't want to smear it on the end grain.
Hopefully you can get a clean cut on the edges of the disks. Cedar isn't that easy to get a clean cut on the end grain.
Your going to have a lot of end grain. That end grain need to be perfectly sealed that's where the wood will start cracking.
WRC is great outdoor wood. If you have a chance or choice look for the dark heart wood boards.
Good Luck
The problem with silicone is that you have to get it so thin to avoid visible gaps, it might not be that effective and any squeeze out will be very visible. And if you do not get squeeze out, you will have gaps for water to infiltrate. If you are able to get a good thin bond, even if the silicone matches the wood tone now, it will age differently than the wood and become ugly unless you plan to paint it periodically and it may not accept wood stain.

You want an adhesive that joins the pieces into one with no gaps to seal out water while not creating a contrasting line between them. Without gaps almost any glue will work as long as it can withstand some moisture. Epoxy, Titebond 3 and Gorilla Glue would all work, IMO because they can withstand some moisture. I really do not see an issue with screws as long as you orient the grain the same way in all pieces (you should do that regardless) and you predrill and coutersink the heads. With the grain all oriented the same, wood movement on 12" pieces will not be an issue. Plus without screws, it will be difficult to get a good tight joint because you may not have any other way to get adequate clamping pressure between individual pieces to get a good bond and squeeze out excess glue to close gaps. It might not be a bad idea to route some shallow channels (not visible from the outside of course) to give the excess glue closer to the center a place to go while clamping.

All theoretical of course but you could do a little joinery experimentation with a few piece just to see how well you can assemble it and if you can achieve tight joints. Just do not use clamps because they are probably not possible unless you can come up with something special for this.

BTW, have you calculated how oversized the center hole needs to be to accommodate the slight curve in the stem? I would consider using silicone to seal any gaps between the discs and the pole, especially near the top to prevent water from seeping down the pole. Any water that gets in that way may never dry out.

EDIT to add: I would not weld a compression cap on the top. Because of the grain direction, the wood needs to be able to expand vertically and preventing that could cause a problem
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"Then layer each piece with alternating grain direction, allowing some silicone, say about 1" from the hole on each piece to adhere to the next piece"

Just like plywood ^ Good idea

- waho6o9
That works on plywood because the plies are very thin. That is not a good idea when gluing 2×12s together. If they do expand across the grain, alternating the grain may cause all kinds of problems because wood expands significantly across the grain but almost none along the length. There are numerous forum topics where people have done that on various projects with disastrous results.
Thanks for the comments Lazyman - lots of good info there.

Titebond 3 and Gorilla Glue would all work, IMO because they can withstand some moisture.
I've always worked under the assumption that glue on cedar is temporary at best - because of the movement of outdoor pieces, and the oil on the cedar. Am I wrong in that assumption?

BTW, have you calculated how oversized the center hole needs to be to accommodate the slight curve in the stem?
The section where the discs are going is a straight piece of pipe. The rendering may not make that clear.

EDIT to add: I would not weld a compression cap on the top. Because of the grain direction, the wood needs to be able to expand vertically and preventing that could cause a problem
Agreed. I think it's best to let it move vertically if it needs to.
Even though the pipe is straight where the wood will be, it looks like each disk still will have to slide over a curved section of pipe to get into place. For thinner boards, that might not be an issue but you might want to at least try it before you drill the dozens of center holes and find you need them to be a little bigger.

I have glued WRC with no problems though not for weather exposure. By orienting the grain the same top to bottom, the wood will move together and not stress the glue joint that much (theoretically of course). I do not really consider it an oily wood but I suppose that compared to some other woods, it may be more so. If that is a concern then epoxy is generally considered the best option for oily woods and another reason I would add screws to help get tight joints. If you do not use glue, I think that the water will definitely infiltrate and may never dry out inside causing it to rot more quickly. Tight joints and a good stain on the outside will give you the best durability. My thinking is that even if the glue joints fail at some point, it will have helped prevent water infiltration until then and the screws will help with clamping as well as if the glue fails. Note that if you do use glue, I would not stain the surfaces between the discs because it could interfere with the bond. If you do not glue them, then it cannot hurt. I would only use a stain and not a clear film finish or varnish. Film finishes inevitably crack and peel and once cracks form, they actually trap water under the surface leading to mildew and other staining.

BTW, when picking out the lumber, try to avoid any boards that have the pith or growth rings that are small radius curves. These will be the most likely to cup or move in a nonuniform way. You want nice relatively flat rings that are fairly similar from disc to disc. This can be a little bit of a challenge with 12" wide boards so you may have to cull through the stacks to find boards without the center growth rings running through them. Save the worst boards for the smaller discs where you can cut the disc off center to avoid the pith. If you orient the growth rings with the curve all in the same direction, that will help prevent 2 adjacent boards cupping in opposite directions should that occur and by facing the tree center down, any cupping that may occur will not create a place for water to collect should there be some infiltration. Mostly just thinking out loud about how I would do this.

This is a cool idea. Make sure that you post some pictures when it is done.

EDIT: One other thought. You want to run these boards through a planer to get a nice glue surface. If you leave them rough like most WRC comes, the joints will not be as tight and more likely to be "temporary".
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