Tongue and Groove over 6" wide - need advice on best way to keep them from cupping/moving

you could try putting a type of backer board or furring strip and attach it.

-- You can get alot further with a kind word and a hand gun, than just a kind word alone

I wonder if its cupping because its glued to tight? Wood needs to move, and your not going to stop it short of some sort of permanent seal. The reason for tongue and groove is to allow for the expansion and contraction. That’s why smaller pieces don’t cup, there is enough joints to allow for the movement.

Try incorporating some sort of break, like a seam. Use a tenon type piece maybe and let it slide.

-- There is nothing like the sound of a well tuned hand plane. - http://timetestedtools.wordpress.com (timetestedtools at hotmail dot c0m)

Using quarter sawn material will prevent cupping. Flat sawn will always want to cup to some degree. You really can’t stop it other than using a stable material like plywood.

-- Bondo Gaposis

I’m thinking along the same line as Don and Bondo. Too much pressure on the clamps will cause cupping and the other issue of wood movement, especially on a garage door but you probably know this already since your statement of “all wood is 10%”.

There is one adhesive you might want to try. I was introduced to it by my wood working guru who claims this stuff will not turn hard, yet it holds better then wood glue. It’s called “LOCTITE”

-- Bernie: It never gets hot or cold in New Hampshire, just seasonal!

We run into this problem with exterior siding. One side in sun and humidity, one in shade. Do the pieces have any kind of back cut?? (like flooring) this can relieve the pressure on the front side. Also, when we have milled ourselves, we always try to install with the natural cup crowned to the shade side. That way it will tend to flatten when warmed on one side only. +1 on leaving room for expansion. also +1 Bernie In glued applications like this, we use a urethane glue, like flooring, or PL. It will allow for movement.

-- Who is John Galt?

Wood moves. Especially in an exterior situation, where humidity and temperature swing a lot more. Try and glue or screw it to stop the movement and it will likely split.

-- Dwight - "Free legal advice available - contact Dewey, Cheetam & Howe""

Hey Joe – just went down to the workshop and the LOCTITE I mentioned is a urethane glue used in flooring and “PL” is on the tube. From what you are saying, my guru is right as usual. It’s been about 6 months since I first used it and no problem thus far… with your comment, I’m in the relax mode now… thanks!

-- Bernie: It never gets hot or cold in New Hampshire, just seasonal!

Maybe a picture or two would make it easier for us to help you.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush