Minimize warp in long narrow door

Four 9" x 80" panels adds up to 36" by 80". Hardly a custom opening for a door. When I make doors, I allow 1/4", meaning that for a 36" opening, I build a door that's 35 3/4" wide. That's a common figure if you look around. In your case, you'll need to allow some relief between each door unit as well. If you build four 9" units for a 36" door opening, it's not going to work.

I fear you're getting well-intentioned advice, but some that's not based on experience and, in my opinion, flawed. For example, the thin strip reinforcement that's later milled off will serve no purpose other than to complicate the job.

I'm probably wasting my breath, but I'll say it again. I've never had a door warp, but that panel layout you have is going to give you headaches during assembly. I also think your proportions are off on the rails.

If you're going to build that door, I strongly recommend building it as one piece and ripping the four units apart after it's constructed. If you build them individually, you'll have problems getting it all to line up.

I constructed this door as a single unit and ripped it in half. It fits in a 24" wide linen closet opening.

Just for fun, I went looking for some numbers. As it turns out the data refutes the statement that "Laminated wood is MUCH stronger" than a single solid piece of wood.

I found this PDF from the USDA Forest Service web site. Here is a quick screen grab from it:



You can see that plywood has a lower MOE an MOR than any of the woods. Moreover, they aren't even listing woods with the highest values. For example bloodwood has a MOE of 20.78 GPa and a MOR of 174.4 MPa.