On Wed, 22 Nov 2017 22:44:43 +0000
(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?

Many thanks for any comments.



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.