Troubleshooting a Bowed Slab Door

Cedar slats on one face, Oak slats on the other face, bowing about a quarter inch ... what went wrong? November 5, 2013

We are having a problem with a flat slab entry door. The door is about 2 1/4" x 36 x 96. There's a 1 3/4" ladder core frame made of FJ pine. The exterior face is made from horizontal slats of 1/4" western red cedar. Cedar is air-dried to about 10%. The interior face is 1/4" horizontal slats of white oak, kiln dried to about 7%. All the slats were edge glued with Titebond III and the door was vacuum pressed front and back. The door is now bowed toward the cedar side at about 1/4" overall. No finish has been applied, so the bow may have occurred right out of the press (not sure). I have ideas as to why this has happened but opinions would be helpful. A solution would be even better!

Forum Responses
(Architectural Woodworking Forum)
From contributor G:
Might be the vast difference between cedar and white oak. Could also be the FJ pine that doesn't have the strength to be a good core. As for a fix, if it's stabilized, might make a real nice 1 3/4" door.

From the original questioner:
We have built dozens of ladder core doors with FJ pine and there have been no problems. I take your point, however. I suspect the cedar might be drying enough to pull the door in that direction. I would have thought that 1/4" veneers would not have enough mass to bow a 2 1/4" door, but I may have been wrong.

From contributor D:
The solution is to not build a door that way. Your problem lies 100% in the fact that you edge glued all the slats together, making a single board face on each side. That is cause enough for spectacular failure, but by using different species and thicknesses, you assured it.

The purpose of the ladder core is to provide a solid base for panels that are allowed to move. "Allowed to move" being the operative statement in any discussion on plank doors, vertical or horizontal. By gluing all the panels/boards/slats together you overrode the whole purpose of the design.

The purpose of a v-joint (or u-joint) in solid plank work is not decorative; it is to provide a crush zone where the movement of the boards will occur and can be seen, but not self-destruct or be evident.

If you want a smooth, one-piece face, build a veneered face door. If you want different species from side to side, build a veneered face door. If you want v-joints like a plank style door, you can use solids, but leave a gap between the boards just like if you were running siding. Calculate this gap with the aid of the Shrinkulator and a calculator. Then each board can move in its own area, just as all solid woods will, and all will be fine. Pack the joints in tight and/or glue them all up, then movement is not allowed and the door is doomed.

From what little I read - and I assume "slab" actually means "flush" - you want a flush door with horizontal oak on one side and cedar on the other. Veneer it - this is why veneer exists.

From contributor L:
Seems like a classic case of unbalanced construction. That is a lot of width of solid material. Even small changes in moisture content would result in big stresses.

From the original questioner:
The door design calls for flat surfaces on both faces. Am I not actually using what many would refer to as a thick veneer? The solid wood on inside and outside faces is 1/4" thickness. I would never use a thin veneer on an exterior door. Also, is it possible to now thickness the door and remove the bow? I suspect you might compound the problem as the veneer would be of different thicknesses across the door.

From contributor D:
"The door design calls for flat surfaces both sides..." Simple enough.
"Am I not actually using what many would refer to as a thick veneer" No. You are using solid wood.

The door is scrap. You need to reconcile yourself to making a simple flush door - with veneers. Why do you think you should never use a thin (relative term....) veneer on an exterior door? There is no magic dimension where wood stops being solid and becomes veneer, and all the properties associated with solid wood disappear. The thinner the wood, the more it tends to behave like veneer and allow cross grain construction.