Setting plaster causing wood to warp.

[Roger Hayter]

I have fitted a 150 x 20mm piece of engineered oak plank vertically behind a worktop. (The long axis of 800mm is horizontal.) It is fixed well along the worktop but cannot easily be fixed to the non-flat, non-vertical wall behind it. I therefore thought it would be a good idea to put some plaster behind it, both for neatness (a difficult gap to keep clean) and for support. I used one coat plaster up to 4/5 of the top of the wooden upstand. Much to my shock, by the next day it had warped 6mm out of the vertical, and away from the wall at the top. The bottom is firmly clamped. At first I thought it was a mechanical effect of the weight of plaster (though on reflection this should not be enough to bend it) but on closer inspection the top of the plaster is now 5mm away from the wood. There is some evidence of longitudinal warping, although it only amounts to 1mm in 800 mm, because it is pretty well clamped against the worktop.

So clearly the side of the engineered wood blocks in contact with the plaster has expanded very significantly. Is this a known effect of plaster? Would waterproofing the wood first have prevented it? Will it un-warp in time and is there any way I can encourage it to?

Many thanks for any comments.

[tabbypurr]

Real wood wouldn't have done that, and it's cheaper.

NT

[The Natural Philosopher]

real wood would have dione worse than that

[Tim Watts]

Yes oak warps if wet. I had it happen to me. So next time, I would tape the back of the wood to keep the water from the plaster going in.

[Thomas Prufer]

Of the water, yes...

Probably.

probalby....

You could wet the front, which would cause it to warp the other way.

Best to very firmly do nothing for a few weeks, though, and then see.

Thomas Prufer

[Steve]

If you're going to have another go at it use car body filler instead of plaster. Or fill the bulk of the gap with expanding foam and leave just the last few mm to be filled with plaster to minimise the wetting.

[Roger Hayter]

Thanks for the useful advice, and to everyone else who has confirmed my error. I just didn't realise the wood would absorb water so fast.

I am definitely not going to do it again, but will try to remember to report back in a few weeks if there has been any improvement.

[John Rumm]

The short answer is yes, the dampness from the new plaster would have been the cause. Chances are as it dries out it will unbend somewhat, although it might not do it completely. It will also depend a bit on how much water got through to the oak itself, and whether it was air dried or kiln dried originally - generally its harder to permanently bend kiln dried hard woods using steam or moisture.

Engineered planks are interesting things. The addition of the ply to the back of them, helps reduce the seasonal variation in width (most change in size of real wood will be across the grain and not along it). Bonding a thick layer of ply to a relatively thin layer of seasoned oak will tend to resist that change. However the assumption here is that the ply side is mostly protected from large changes in humidity, and its the visible oak face that will see most of those changes. This works well for a floor, where you don't want your tight fitting boards to shrink and open up gaps.

In your application its the ply side that got the large increase in humidity, which being thicker probably caused more pronounced bending than would have been the case if it were the thinner size that got wet.

Prevention; sealing the back of the board may have helped (although if the front was not also sealed that may also encourage bending naturally later if the board was not fully acclimated to the room first)

[Steve]

Did the plaster sink and so fill the void left by the timber warping?

If so, I can't see how it could dry back to straight.

If you wrapped the timber in clingfilm would it be possible to remove it after the plaster set?

Else, ; say it's rustic shabby chic.

[Roger Hayter]

No it didn't actually, a gap appeared between the set plaster and the wood.

The timber is very securely glued at the bottom edge, so I can't easily remove it.

I think so, it's not important enough to do again, being a removable replacement for a a fixed worktop that had to be removed destructively to allow a new oil boiler to be fitted.

[Rob Morley]

It's a known effect of water.

Probably mostly - I'd have put polythene between the wood and plaster, and only fixed the wood after the plaster had cured. Or oiled/waxed the wood (which would be a good idea in a worktop situation anyway).

Probably, but you'll not easily accelerate it - applying heat or using a dehumidifier will affect the front as well as the back, but you want a differential effect to correct the curvature. Possibly ignore that on the basis that the wet from the back will eventually migrate to the dry at the front, and that may happen quicker than just leaving it to dry naturally.