On 22/11/2017 22:44, Roger Hayter wrote:
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?

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)


--
Cheers,

John.

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