Wood Movement Question

[Glenn Samuels]

I am finishing a wall display cabinet and the back will consist of 6 3/4" x 36" soft maple boards running horizontally. I plan on tongue and grooving them but I am not sure how to mount them to the cabinet. I know not to use glue because there could be considerable movement. Any suggestions would be appreciated.

[Bob Michaels]

Finish (stain) the tongues and grooves. Use elongated horizontal screw slots and tighten screws just enough so boards can move. Bob

[Bob Michaels]

Sorry, missed boards running horizontally...in that case slots will be vertical. Bob.

[Glenn Samuels]

Bob,
I have a vertical drill press but not sure if that is the best way to make the slot.. What do you think is the best method?

[Lee Schierer]

I would calculate the expansion/contraction of each piece. The make a ship lap joint that has enough overlap to accommodate the movement. Attach each strip in the center of its width with a single screw on each end. Align the pieces so the overlap is equal on each side of each piece. (Note: If you can check the moisture level you can adjust the overlap to more or less depending upon the moisture content) Make sure each piece is finished equally on all surfaces to minimize seasonal movement. The ship lap will allow the pieces to move as they need to without creating an opening in the cabinet back.

[Jamie Buxton]

Tongue and grooving is used for backs because it helps deal with hygroscopic expansion. When the individual boards are installed in the cabinet, they're not installed tightly together. Instead, there's a little gap between the boards. The gap means that each board can grow or shrink without colliding with its neighbors. The tongue bridges the gap, so you can't see through the back. And the tongue&groove align the boards, so they form a flat sheet, rather than each one flopping around on its own.

The individual boards can be installed with nails or screws, or can be glued. The cross-grain glued joint is only a few inches, and is not likely to be too stressed by the wood's expansion. Or if you're really concerned about a 6" glued joint, you can glue just the middle couple of inches.

[Danny Hamsley]

You could use 18 guage 2" brads. There is enough give in them to accommodate the seasonal movement, and the T&G will keep all the boards nicely aligned.

[Cody Colston]

I agree with Danny that 18 ga. brads will accommodate the seasonal movement of the individual boards. Also note that if the boards are quarter-sawn, the movement will be approximately half that of flat-sawn boards.

For construction at this time of year, shrinkage is probably a bigger concern than is expansion.

[John TenEyck]

When I built a cabinet with 6" wide boards on the back, I used T&G joints. I calculated the expected wood movement based on the actual MC of the wood at the time and the seasonal humidity change. I attached the boards closest to the sides with screws along the edge that was rabbeted into the side, and a single screw at the center, top and bottom. This helped make the cabinet more rigid. The others were separated with spacers I made based on the calculations I did, and attached with a single screw in the center, top and bottom. Three years later, all is well. They never quite touch in the Summer and air gaps never show in the Winter.

John