Troubleshooting Warping in a Mirrored Door

Attaching a mirror to one face of a large melamine door leads to warping in the course of a year. December 1, 2013

So I did a job last year and one small part of it was a mirrored door for a closet. It was a large door, maybe 40" wide x 80" high or so, and I told the client it was a bad idea for several reasons, mostly related to inadequate structural support. He was the type who made design decisions on the fly and just threw money at the problems. So we made the door, barely squeezing it into the space designated, and I walked away knowing I'd get a call down the road… but also having saved all the various e-mails and warnings about it!

So I got the call this week and although part of it was a small easily fixed issue, there is another larger issue beyond any repair. The door has warped slightly by maybe 1/4" over the long edge. It can't be fixed so is likely just going to stay as is, but it leaves me wondering… Why is it when you take a flat surface, in this case 1/4" mirrored glass, and adhere it to a flat melamine panel, it moves over time? The door is secured by 7 SOSS hinges on one side so the panel has no pressure in either direction. Neither material should want to move, as they are both flat and stable to begin with. Yet there is a measurable warp nonetheless?

Forum Responses
(Architectural Woodworking Forum)
From contributor P:
An unbalanced panel and the difference in moisture absorption.

From the original questioner:
So how, or why, does a fully sealed melamine panel transfer moisture? Seems to me with the plastic coating and PVC edgebanding on all edges, it would be pretty much sealed, at least in terms of normal indoor air?

From contributor P:
Melamine is not sealed. If it was sealed it would not have warped.

From contributor D:
The moisture change happened slowly, over the past year. Finishes, including melamine, don't prevent moisture gain and loss, but slow it down. The melamine really slowed it down, but didn't stop it.

If this is a thin panel - 3/4" or so, then the weight of the mirror will throw it off balance. The hinges are likely in the center of the panel, and the mirror - extra weight on one side - will throw the weight center off of the panel centerline.

Which of the above (or a third cause) is anyone's guess, but I suspect a combination of the two, plus gremlins as yet unidentified. Was the panel hung during a full moon?

From the original questioner:
No but maybe by a full Loon;>)

For whatever reason I figured the plastic coating on melamine products would be much more resistant to moisture exchange than, say, plywood with a finish on it! Guess that's what happens when you assume!

The hinges are centered on the 3/4" panel, so the weight is off balance, but they also keep that side of the door straight, so…?

The funny thing is the client originally wanted a balanced panel! He wanted mirror on both sides of the door. Being that there wasn't really adequate structure to support the single sided mirror, nor enough room for the extra thickness, I didn't really give him that option. But maybe if I did the door would still be flat. Probably would have ripped the hinges right out of the side panel, but still stayed flat!

So for future reference… Is there a way to accomplish attaching a mirror to a 3/4" panel without warping? It would still have to be a panel that can be routed to receive hinges and drilled for screws, yet not be prone to balance issues.

From contributor W:
Another consideration is the melamine panel has a small mc from the mill in all likelihood. Plywood veneer cores are run through a dryer before they get pressed into sheets but not down to 0%. The pb/cb/various panel cores may also have a small mc. With that in mind the mc was no longer centered.

From contributor R:
Mirror expands at one rate, panel at different. Not sure I would expect a 40 x 80 door to stay flat.

From contributor K:
So the swing edge has T mold. I would be thinking about pulling it, and maybe replacing it with a metal edge of something stiff enough to counter that bend. Maybe the T-mold can be bonded on outside of that, to keep the look.

From contributor P:
No offense, but that won't work. This may be heresy, but is a 1/4" of warp in 80" really a show stopper?

From contributor K:
To answer the second question: no. There is no wood-like panel that you can attach a mirror to that will not do the same thing as yours, unless it is balanced. On the other hand, you can do frame and (mirror) panel all day; a mirror of adequate self-supporting thickness would also work with different hinges.

From contributor B:
Good advice. In my experience, MCP doesn't always stay flat. There is stress in the panel caused by the manufacturing process, how it is stored, and how old the panel is. MDF is more stable than MCP with a decent finish. Your panel was definitely unbalanced. The mirror and MCP do more at different rates.

Just maybe if the mirror wasn't glued to the MCP, attached with some kind of trim (which isn't always possible), the 2 panels could have moved independent of each other. Sometimes you can't do something the way you want to!

The suggestion of the 'L' channel handle free edge might have helped keep that edge straight, with a prayer. That was a big door to only have 3/4 for a substrate.

Hafele has a threaded rod system that you route diagonally into the back of the panel to correct warp-twist. It is in their closet hardware area of the catalogue. Which is a lot of work-time-cost for a hope and prayer that it will work.

Over the years I have bought instead of built large doors. I was surprised to see that the larger manufacturers state that up to 3/8'' warp is an accepted industry standard for large doors. That covers their butts for many reasons.

Next time, design a thicker door, but you may have the same problem if you (1) can't balance the panel or (2) build a core box substrate that is stronger than a 3/4 panel.