On 23/11/17 00:20, jim wrote:
(Roger Hayter) Wrote in message:
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.


Let it dry out before you decide to rip it off. Chances are it
will recover in a few weeks as long as environs are dry &
warm.

Next time (if any) use an adhesive & deal with gaps etc afterwards?


Engineered might. Solid oak won't.