Oak Strip Veneer on trap doors

[Bob Deroeck]

Hi,

I'm installing a traditional red oak strip (2.25" x 3/4") floor in my kitchen. There is a 30" x 60" trap door in the middle that is the only access to the basement. Not a great situation, but one I have to live with. To reduce the weight of the trap door, I plan to install two doors, each about 30" x 30" with piano hinges. Each door will have a light, but stiff frame with a 1/2" plywood top. I'm planning on gluing 1/4" thick pieces of red oak, each 2.25" wide to the top to match the flooring strips around the trap door opening, using Titebond I glue.

My question is "should I be concerned about differential expansion of the oak vs the plywood top, and, if "yes", what should I do to minimize the problem"? I was planning to glue each strip tight to the adjacent strip to "seal" the joint from the air and also thoroughly coat the ends and top of each strip with polyurethane to try to seal out moisture. Or, should I consider using a more flexible glue to allow the oak to "grow" more than the plywood without cracking the oak or stressing the oak-to-plywood glue joint. If so, what "flexible glue would you recommend?

All suggestions appreciated.

Thanks.

Bob

[Jamie Buxton]

If you butt the strips together, and thoroughly glue the strips to the plywood, your trap door will almost assuredly cup. The lumber is going to grow and shrink. The plywood will not. That gives you cupping.

Install the strips with a little gap between them so that they can breathe. Don't glue each strip down across the full width of the strip. Glue it only along one edge. Or nail it only along one edge, just like you're doing to the rest of the floor. Then each strip can do its thing without cupping the plywood.

[johnny means]

I would screw the planks on from the bottom, no glue. Prevents any expansion issues and leaves me options if there is any issue later on. If you don't have to commit to forever, don't.

[Jamie Buxton]

I would screw the planks on from the bottom, no glue. Prevents any expansion issues and leaves me options if there is any issue later on. If you don't have to commit to forever, don't.

Yeah, better.

[Rod Sheridan]

I'd be more concerned about the load bearing capacity of the 1/2" plywood panel that large..........Rod.

[Bob Deroeck]

The frame should be stiff enough to support the 1/2" ply. The frame around each edge will be 3/4" x 3 1/2" solid wood, probably pine. Then there are "joists" 2" in from each end that are 3/8" x 5 1/2" plywood. Other joists will be installed likewise with maximum joist spacing of 7 1/2" OC. The plywood top will be glued to the joists, embedded in epoxy stiffened with colloidal silica. And the plywood top will glued and screwed to the outer frame.

The oak strips are only going to be 1/4" thick, essentially a thick veneer, so screwing them in from the bottom is not possible. Let me ask the question another way. How thin does veneer need to be before it can be glued to plywood without concern for the veneer cupping?

[Mark Bolton]

Why don't you just either make the door solid material or veneer both sides.

[Bob Deroeck]

I'm trying to keep the weight down so a solid oak door is not a good option. I could put a thin coat of veneer on the bottom side of the door, between the joists to try to balance the surface exposure to the air on both sides of the plywood.

Bob

[johnny means]

I would build a heavier, better door then use a mechanical lift or strut to deal with the weight.

[Jamie Buxton]

The frame should be stiff enough to support the 1/2" ply. The frame around each edge will be 3/4" x 3 1/2" solid wood, probably pine. Then there are "joists" 2" in from each end that are 3/8" x 5 1/2" plywood. Other joists will be installed likewise with maximum joist spacing of 7 1/2" OC. The plywood top will be glued to the joists, embedded in epoxy stiffened with colloidal silica. And the plywood top will glued and screwed to the outer frame.

The oak strips are only going to be 1/4" thick, essentially a thick veneer, so screwing them in from the bottom is not possible. Let me ask the question another way. How thin does veneer need to be before it can be glued to plywood without concern for the veneer cupping?

Well, if you make the substrate sufficiently stiff, the veneer/lumber can't make it cup. So what's "sufficiently stiff"? A simple sheet of 1/2" plywood isn't. A well-built torsion box that's 8" thick is going to keep a 1/4" lumber skin from warping the torsion box. Maybe a 4"-thick torsion box will do the trick too, but I'm not sure. Part of the difficulty in predicting is that the bottom face of the trap door is likely going to be seeing a very different humidity than the top face. But you should study up on torsion boxes. They offer high stiffness with low weight -- exactly what you say you need.