floor medallion construction

**al ladd** · 09-19-2023, 7:41 PM

I'm building a 44" diameter 7 sided floor medallion for a very unusual house. The medallion will replace one in very poor condition, (and not well-made to begin with). I want mine to better endure the test of time. The medallion serves as a hatch for a crawl space used in theater performances, so it needs to be an integral single piece that's removable on rare occasions.
The vendor that specializes in this (Czar Floors) seems like they do 5/16" thick CNC cut jigsaw puzzles essentially, glued to a substrate for total thickness of 3/4" to integrate easily with wood flooring.

I'm wondering about glue and movement issues, and so ideal thickness of the hardwood, and glue type. Mine will be constructed of 7 pie shaped pieces, with the pockets CNC cut into each pie slice, and the male members cnc cut to match, the whole thing assembled and vaccuum pressed onto a Garnica ply backer. My CNC isn't big enough in either table size, or travel, to do the whole thing at once. The background wood is hard maple, and the pie slices are about 19" wide at the periphery, so there's a lot of potential force on them here in New England.

Seems like there's a difficult choice on wood thickness--thicker and more chance of bad movement problems, --thinner and short life for future sanding.....Polyurethane glue?
Rough preview attached--though the maple grain will run from periphery to center in each pie slice.preview2.jpg

**Kevin Jenness** · 09-19-2023, 8:19 PM

Interesting project. I would be wary of thick solid wood overlays. The shop I used to work at did a 48" diameter circular countertop with random 1/4" maple pieces glued to a ply substrate which developed some significant problems. If the finish is kept up thinner face material will work out better over the long run imo. I would suggest 1/8" with the same thickness backer laid up on a 1/2" substrate.

Are you planning to do v-inlays or conventional style?

**Clint Baxter** · 09-19-2023, 8:19 PM

I put together some six sided pieces similar to your design last winter and each of them had separations in the center during the more humid summer months. I'm located in the upper plains, not that far from the Canadian border and the humidity is quite dry during the winter. My maple pieces were slightly more than an inch thick, had the grain run from the perimeter to the center, and they were not glued to any type of backer. Fastening to a backer will definitely help, but I'd advise you to keep your maple thickness relatively thin. I don't know how thick you're able to get away with, but the thinner it is, the better your chances that the joints do not separate. Kevin's suggestion above sounds about what I'd try if doing it again.

Good luck on the project and I hope to see your end result posted here in the future.

Clint

**Jim Morgan** · 09-19-2023, 11:36 PM

In segmented turning, the grain runs tangent to the arc rather than perpendicular to it; you might be better off like that here. Pie-shaped pieces with grain running radially will move more at the periphery where they are wider than at the center where they are narrowest. This will make the assembly try to assume a conical shape, but if it's glued down it can't do that and instead will crack.

I agree with keeping the top layer no more than about ⅛" - that's enough to provide plenty of wear.

**Bradley Gray** · 09-20-2023, 9:27 AM

You mentioned that the hatch was removable. Is there a sub structure?

**Jim Becker** · 09-20-2023, 9:59 AM

I think that 5/16" or 1/4" thick inlays will likely not present a wood movement issue for this application. I'd probably do the 5/16" in this case if the assembly will be put in place and then the whole floor leveled by sanding prior to finishing. If you will be pre-finishing to match the existing floor height, I'd be comfortable with 1/4". Hopefully, there will not be a huge amount of sanding required over time, honestly.

**Edward Weber** · 09-20-2023, 2:07 PM

Originally Posted by Jim Becker

I think that 5/16" or 1/4" thick inlays will likely not present a wood movement issue for this application. I'd probably do the 5/16" in this case if the assembly will be put in place and then the whole floor leveled by sanding prior to finishing. If you will be pre-finishing to match the existing floor height, I'd be comfortable with 1/4". Hopefully, there will not be a huge amount of sanding required over time, honestly.

+1
Typically, 1/4" inch is about the max where you don't need to worry, anything over 1/4" is when I have to consider movement. That being said, I don't think 5/16" is a deal breaker. Laminating to a good solid substrate is the key. I would also align the grain to flow around the medallion.

**Tom M King** · 09-20-2023, 4:30 PM

If this is not the typical 30 foot rule theater construction:

https://www.youtube.com/watch?v=nTaQC7lZHTI

**Richard Coers** · 09-20-2023, 5:06 PM

**al ladd** · 09-21-2023, 11:03 AM

Thanks for these replies. There doesn't seem to be a consensus. Respected opinions recommending 1/4-5/16", with most, including most opinions on the many posts Richard linked to suggesting 1/8" max is safest. I think given that the house is for sale, and so I'm more worried about splitting this winter than sand-through 20 years from now, Ill go thinner.

My plan is 5/32" to start, glued to 1/8" bb so I can pocket to the same depth. Then the pie pieces will glue to 3/4" Garnica with hdf faces, and I'm thinking of balancing with 1/4" bb on the bottom. There is no super structure supporting it anywhere but the edges, so either a web of 2x4's or 3/4" ply to stiffen it so it can be danced on.

I'm wondering if anyone has experience with Unbond800 vs Gorilla glue in the sort of application. I guess the key quality is protection against glue creep.

Here's the setting (worth looking at !)and the original I'm replacing, when it still looked decent. (My recommendation was to replicate the original, but owners kids decided they had a better idea...)
https://www.trulia.com/p/ma/colrain/...1#lil-mediaTab

**Kevin Jenness** · 09-21-2023, 11:43 AM

Seems like you could use a thicker substrate for stiffness rather than adding framing after the fact. Assuming the medallion's edges sit on the subfloor they would be rabbeted to get the face flush w/ the surrounding floor. Could be a torsion box to minimize weight. How will the piece be lifted for removal?

I would suggest using the same or similar material with a similar grain layout on the bottom for balance. Plywood on the bottom won't balance the pull of thick veneer in a piecut pattern.

I was interested to see the variation of opinions on maximum recommended face material thickness for your project. My own experience leads me toward the conservative position favoring thin veneers, but others must have had success with thicker, perhaps more stable species.

**John TenEyck** · 09-21-2023, 1:37 PM

I would use Unibond 800 over GG. Both are rigid and waterproof, but Unibond 800 has a very long open time, doesn't require high pressure, and is far easier to clean up.

As for thickness, I'm in the group that says no more than 1/8". If your inlays are full depth then there's no reason. If they are V-bit carved then bad things will happen the first time the floor gets sanded regardless of how thick it is. With 1/8" thick veneer you can run the grain of the maple pie segments as you want. If they are 1/4" thick I'd be a LOT more comfortable if they went parallel with the perimeter.

John

**Christopher Charles** · 09-21-2023, 3:18 PM

I'd also tend toward 1/8" as a max thickness for the veneer, 1/2" (or thicker) ply with a rabbet around the perimeter to fit flush, and would consider an inlayed element in the center to minimize issues getting all the points to come together perfectly and future-proof from separation in a visually obvious spot. Cool looking house/project!

Best,
Chris

**Edward Weber** · 09-21-2023, 7:08 PM

Just some food for thought
https://flooring-experts.com/hardwood-floor-thickness/

**Tom Bender** · 09-24-2023, 6:39 AM

44" diameter might be a bit springy for a dancer in a theater situation. Adding a couple of ribs to the bottom might be wise.

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