On 22 Jun 2004 23:08:51 GMT, (Andrew
Gabriel) wrote:

In article ,
Phil Addison writes:
On 21 Jun 2004 00:38:51 GMT, (Andrew
Gabriel) wrote:

Cement goes back to roman times. However, prior to Portland cement,
it was more expensive, so it wasn't used for everything like it is
today.

Thanks Andrew for clearing that up. I haven't been able to get up there
on a ladder yet to have a closer look, but last time the sill material
looked pretty recent.

So does mine;-)

;-)

I think the only way the sill will be letting water in is if
there are cracks in the render.

If it IS rendered like yours!

OK, or cracks in the casting;-)

I meant I'm not convinced it is that construction, but I haven't had a
chance to test it further for brick innards.

If you stop the water entry and let it all dry out, you can use
pretty much anything (personally, I use bonding coat as the scratch
coat). However, none of the gypsom-based plasters are suitable if
it's damp.

I understand why you can't use these if it is permanently damp, but do
you mean you can't use them if there is still residual damp that is
expected to dry out through the plaster. In other words must it be
completely dry before using plaster as scratch coat?

The problem is if it takes weeks to dry out, as it well might.
That's long enough to damage the plaster. You end up with the
finish coat looking permanently dark (like it was wet), and the
rear turns to powder. You'll probably also get white powder salts
growing on the surface as the water carries salts to the surface
and evaporates leaving them behind (looks like white mold), but
that just brushes off. If the wall is decorated, it doesn't come
off so easily, and left in the decorations, I've heard it said
it can have a hygroscopic effect, keeping the area slightly damp.

That makes sense.

Is that why cement render often seem to be used after damp treatment, so
that it can be applied straightaway to the still wet brickwork?

Maybe.

More likely is that because damp treatment usually doesn't work
because it's wrongly diagnosed, you have to make the finish
waterproof to hide the damp. Most damp treatment companies seem
to only have equipment for injecting a damp proof course, so the
only thing they're likely to diagnose is rising damp, which mostly
isn't the cause. The sand and cement render will project against
damp wall (unless it's soaking wet), without injecting it.

Oh aren't we cynics here :-). My daughter had two quotes from builders
before I got volunteered to look at it. First guy said to hack off the
plaster, and render it with sand/cement. He didn't notice apparently
that it was already done and the plaster skim so thin in places you
could actually eyeball the cement!! Second guy was sure it just needed
silicone fluid treatment outside.

Another factor may be to prevent the plaster bridging the new
injected damp course. Sand and cement should be OK for this (as
always, assuming it has waterproofer added). It may not be possible
to inject low enough to avoid bridging by internal finishes due to
outside conditions.

True. I suppose there are some outside conditions that can't be
rectified, but on here we would rectify them and not need the injection.

I have done a scratch (base) coat of cement (and lime if you like it)
and sand in a 1:1:6 ratio, with waterproofer added. Follow up 24 hours
later with regular plaster finish coat (no PVA required unless you
leave it much longer and the scratch coat has dried). The sand and
cement is very effective at keeping the water away from the plaster
finish coat. However, you'll probably need to let the wall dry out
first, at least so it doesn't feel and look wet.

Are you saying if it is not totally dry I should use that sand and
cement mix rather than bonding plaster as the scratch coat? What is the
problem if it is too wet; won't stick I suppose?

I haven't tried when its very wet, but that might be an issue.
The suction might be non-existant, and the PVA coat might not soak
in to the brickwork (could dilute it less perhaps and hope it mixes
with the moisture at the surface).

I see. I'm now thinking of leaving it open for a few weeks or till its
obviously drying before doing more.

Thanks again everyone for such excellent details.

Phil
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