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Background

I am a very much a beginner, and I have been trying to make a long (3.6m) pelmet. (It is not a pelmet box - it goes from wall to wall, and hangs from the ceiling.)

Attempt #1 was with a single piece of pineboard that was delivered with a mild warp (mainly twist with a touch of cupping). I let it sit for a couple of weeks inside before got around to painting it, but I didn't take any active measures to flatten it first - I thought it wouldn't be noticeable once it was mounted up high and painted the same colour as the walls and ceiling. I was very wrong, and think I have to abandon it and start again.

For Attempt #2, I am thinking of trying MDF instead.

MDF Stability

I've seen suggestions here that MDF is less likely to be warped (as long as it is kept dry):

chipboard (aka particle board) and MDF are considered dimensionally stable and will not warp with changes in humidity

but I've also seen counter-examples:

Am I right in saying, if I buy some MDF from a hardware store, it is much more likely to be flat than ordering pineboard from a timberyard?

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Oddthinking
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2 Answers2

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MDF is practically purpose-made for this application. It is very dimensionally stable, one of its prime selling points. And while it's not as stiff or strong as ply it is as a rule more stable these days (because of the falling standard of plywood, while MDF is easier to make to a reasonable standard consistently).

As often mentioned MDF can bow under its own weight (a point regularly used with glee by its detractors). But that is when it is used flat as in a shelf, not edge-up. When it's oriented vertically it is much stronger and far less likely to sag, which is also true of any solid wood and ply incidentally.

3.6m (that's nearly 12 feet for Imperial users) is a very long span though, I hope the plan wasn't just to attach it at the ends and let the middle float. It shouldn't need much, just one or two points of attachment should do it.

An additional concern here is whether you can source MDF in a 3.6m length. If that's not the case the span won't be a single piece of material anyway, so a join of some kind will have to be devised and implemented. And of course any such joint will have to be factored in in how you attach the pelmet.

I've seen suggestions here that MDF is less likely to be warped (as long as it is kept dry) but I've also seen counter-examples:

Please note those don't give counter-examples to the quoted statement. Storage conditions that make for bent wood or manmade boards where gravity is a factor are completely separate to any tendency in a material to change dimensions or warp with changes in humidity.

Am I right in saying, if I buy some MDF from a hardware store, it is much more likely to be flat than ordering pineboard from a timberyard?

I wish for all our sakes that the answer to this was a simple yes, but unfortunately it's more complicated than that. It's not just the material qualities of the wood/board itself that is in play here, they way it's stored is hugely important.

Even stuff naturally un-inclined to bow can bow given half a chance by poor storage conditions. One great example of this is full sheets of ply only supported at or near both ends, even plywood 19mm and thicker can readily bow if stored this way, particularly in a stack.

And conversely weaker wood (e.g. long lengths of thin softwood) can remain just as flat as they came out of the kiln if stored well, i.e. fully supported along their length.

Graphus
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  • Thanks, @Graphus. The plan is to attach it from 6 brackets on the ceiling (mainly to distribute the weight on the ceiling). I am over-engineering to compensate for my inexperience in estimating how much the ceiling and brackets can hold. I expect I will need to join two pieces - the best method of doing that is an open question I need to research further. I appreciate the MDF might be warped if stored badly, but I am trying to work out if I am just buying myself the same problem if I try again with MDF. I am getting the impression it is more likely to succeed with MDF. – Oddthinking Jun 14 '16 at 15:20
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    @Oddthinking, six brackets sounds perfect. I too take the over-engineering approach any time I'm uncertain, better to be safe than sorry. Re. joining two strips like this, the ideal edge for each piece would be a scarf joint (a shallow angle formed on each one) but this is both difficult to form without the right power saw and in MDF the sharp corner would be very weak and prone to damage until the two pieces are glued. So you may be forced to use a simple butt joint (use plenty of glue) and reinforce across the back somehow for added security, e.g. with mending plates. – Graphus Jun 14 '16 at 15:33
  • Depending on how much spare room you have in your construction you can add an aluminium L beam to stop sag in 2 directions. – LosManos Jun 14 '16 at 19:57
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MDF is much more likely to be flat and is relatively dimensionally stable - however it's not overly strong when running long unsupported distances.

If the pelmet is attached at its top along the length then it should be fine. If you plan on only attaching it at the ends it will probably sag over time.

(...usual concerns about formaldehyde off-gassing and MDF dust go here...)

Dave Smylie
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