Keeping large exterior doors flat?

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Forum topic by William Shelley posted 09-07-2017 02:19 AM 663 views 0 times favorited 6 replies Add to Favorites Watch
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William Shelley

609 posts in 2241 days

09-07-2017 02:19 AM

Wasn’t sure which forum to put this in, so apologies if this belongs elsewhere.

I’m trying to figure out the correct or best way to design a pair of large hinged doors so that they will remain flat. The opening size is about 94” x 94”, so each door will be about 47” wide and 94” tall, and the doors will have two or three (as needed) large strap hinges each.

My first version used a sheet of 1/2” CDX plywood as an exterior skin with a frame of straight 2×3 lumber, and a “K” shaped bracing from 2×3 material. Between spring when I built the doors and summer now, the doors turned into potato chips and are almost 2” out of plane at the corners.

These are exterior doors for my workshop. I don’t want to go crazy in effort or material cost, but I’d like to improve on the garbage that I built earlier.

-- Woodworking from an engineer's perspective

6 replies so far

View TungOil's profile


1382 posts in 1267 days

#1 posted 09-07-2017 02:31 AM

I’d suggest you start with dry wood (not construction grade) and skin the inside the same as the outside to balance the stresses. Be sure to paint it as well to prevent it from absorbing too much moisture.

-- The optimist says "the glass is half full". The pessimist says "the glass is half empty". The engineer says "the glass is twice as big as it needs to be"

View 000's profile


2859 posts in 1671 days

#2 posted 09-07-2017 02:48 AM

I’m not sure how thick you can go,
but I would start with a square tube frame welded up, then clad it.

View Woodknack's profile


13384 posts in 3152 days

#3 posted 09-07-2017 03:54 AM

The door on my shop is not quite as big as you want to go but is a big door. It’s made of 2×6’s with a 2×6 frame and bracing with diagonal brace and 3 strap hinges. Tougher than a brick outhouse. But when using construction lumber it helps to let it sit and air dry for awhile because there is still a lot of moisture in it. I’ve built shed doors with cdx and they also potato chipped. I’ve had better luck skinning with hardie panels but you need strong bracing.

-- Rick M,

View Loren's profile


10477 posts in 4420 days

#4 posted 09-07-2017 04:21 AM

I concur with Jbay. A steel frame is cheap
materials and will stay flat. Welded is
best but I think finding a way to bolt it
together wouldn’t be that difficult if welding
is not convenient.

View Rich's profile


5609 posts in 1361 days

#5 posted 09-07-2017 04:45 AM

If you insist on a solid wood door, watch your grain patterns. Try to get quarter-sawn or riff-sawn for the stiles. That said, for doors that large, jbay probably has the best advice.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View William Shelley's profile

William Shelley

609 posts in 2241 days

#6 posted 09-07-2017 05:22 AM

I have a welder and welding skills to go with it, but I already have a bunch of wood that needs to be used. I’m sure everyone has been in this position.

At this point, I’m thinking that i should build a low-density torsion box and call it good. I also have several new surplus pieces of ribbed metal roofing leftover from putting a new roof on my shop this spring. I was considering using the surplus roofing panels, with the ribs oriented vertically, as the exterior skin surface of the doors, and a piece of 1/4” hardwood “underlayment” plywood on the interior.

I believe the torsion box design should guarantee flatness (assuming it was built correctly)?

-- Woodworking from an engineer's perspective

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