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?

On Sun, 26 Mar 2006 15:44:28 GMT, John Stumbles wrote:

How to cover top of cavity

That's what the window sill is for.

--
Nigel M

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

Why don't you just clean the slates off and refix them? They have done
alright for 30 years

dg

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.

I'm not up to date with current spec (chances are it's been changed by
2 jags), but it used to be that cavities had to be sealed with a smoke
(or possibly fireproof) barrier.

the reason is to stop smoke penetration through the cavity from another
part of the building.

The method I've seen most often is to lay cut building blocks on their
sides on the inner leaf to bridge the cavity. A dpc (dipping at least
50mm below the block under-surface) is wedged between the block and the
outer leaf. The cill sits on top of the block.

I've also seen the slate method, which I had to use once due to space
problems over a window lintel (it was approved by the LA Building
Inspector).

Slate is fireproof, but perhaps 2 overlapping leaves would be an
improvement to lessen the chance of smoke penetration. So far as
butting to the outside leaf, I can't recall if I used a dpc, but as
slate is a dpc in itself, so long as no mortar bridge the gap between
outer and inner leaf there should be no dpc problem.


snip

I'm thinking along the lines of stuffing a few inches of rockwool into the
cavity,

If you mean rockwool batts sold for building into cavities, go ahead.
But f you mean rockwool used between joists, I'm unsure about that as
rockwool batts used for cavities have a hardened surface texture which
could be a design feature to limit moisture penetration across the
cavity. BTW do not use polystyrene as that requires an air gap to
prevent condensation in the cavity & stop sweating on the insie wall..

IME you will find it difficult to fill much of an existing cavity from
above, but every calorie saved will help your household bill.

fixing a tanalised batten to the outside wall below the windowcill

Would advise against that as although tanalised timber is supposedly
rotproof it is not water resistant. You'll create more problems than
you solve.

Stick to the masonry or slate solution, avoiding bridging the cavity
with mortar unless there is a dpc stop in place.

and putting some artificial slate

Be careful - what are the water resistant & (fire) temperature
properties of artificial slate?

Real slate can withstand the high temps of a house fire and is nigh on
100% water resistant.

HTH

NB 2 jags' present regs may be different - check with building control
if you have any doubts, better still, someone up to date will post!

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 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.

I should cut a slot in the mortar under the window, and replace the
slate that you've removed. Not a whole lot of point in pushing rockwool
down the cavity, and I'd use real slate for the job, as artificial isn't
absolutely rigid. Muck up after to replace what you've dislodged.