John Stumbles wrote:
Chasing out the wall below the window cill in my kitchen (c. 1970s
extension) I dislodged the top layer which turned out to be about
3/4" of plaster sitting on slates sitting on the bricks of the inner
course, so that the slates and plaster covered over the cavity.
Here's a picture, showing the cavity now revealed and a little bit of
slate and plaster still crossing it:
http://82.21.72.246/~john/cavity.jpg

I'm a bit surprised that there was nothing supporting the outside of
the slate + plaster cap so that it was only held by the adhesion of
the mortar to the underside of the slate. I think a bit of levering
down on the outside of it could have broken it away. I don't know if
this was ever legit building practice.

I'm thinking along the lines of stuffing a few inches of rockwool
into the cavity, fixing a tanalised batten to the outside wall below
the windowcill and putting some artificial slate back across the gap
and mortaring it all up.

Does this seem reasonable?

For a while yes, at least until the batten rots, which won't be long
considering it's on an outside wall and the condensation involved in a
kitchen.

Put a proper window board on it, either timber or pvc, this is what the
groove in the bottom of the frame is for.


HTH