Phil Addison wrote in message . ..
My daughter's house has had a patch of damp appearing under the upstairs
windows since she bought the house 5 years ago from a builder (I did
warn her!!) who had done it up to re-sale. There was no sign of damp
then, it all being freshly decorated.


Hi

If the builder rendered the whole place that is something of a caution
sign. You dont generally render without good reason - although
unfortunately some fools do. Walls are often rendered when its too
much work to fix problems with them.


Assuming the sills are in fact concrete, my big question now is how do I
prove that it is (or is not) the sill before embarking on replacing
them.

You could just water proof them. I'm doubtful this would be achieved
with silicone fluid. Tarred surfaces stay waterproof far longer than
oil based paints and so on. You cant easily paint over tar though.

Any waterproof paint would do for a while, but really you want
something with good long life, as water penetration again would do a
lot of damage.

And ideally use internal plaster that lets any dampness dry, like lime
plaster. But only once the wall's properly dried.


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 :-).

many are rather gullible unfortunately.


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.

I wouldnt expect either to solve the problem.


I wonder if I might achieve similar on these sills with my small SDS
chisel and/or an angle grinder if its not too hard. Don't know where to
get hold of a stone rasp though.

no


Hmmm... I've just realised that everyone seems to be assuming that the
sill is still porous. Don't forget I have given a very generous sloshing
with silicone fluid. I WAS hoping you'd all say that should be OK, but
the only comment on is from Andy saying his only helped a bit. Is it the
case you all think that stuff was a waste of time?

Penetration is liable to be patchy.


ust admit I tended to discount the external render because on the whole
face of the house there is damp coming through only under the sills of 3
windows (sorry, didn't mention the downstairs one before - that is only
a minor patch similar to the 2nd upstairs one). I imagine if the render
was duff it would come through in arbitrary places. But what you say is
quite possible, and I have rectified the useless seal between frame and
reveal and sill. The problem is finding out which of the possible entry
points is the cause.

There arent by any chance cracks spreading out from some of the
corners of the windows are there? If so, this picture might start
adding right up. 1890s, soft brick, rendered, cracks at window
corners, brickwork in poor condition under the render and wet:
classic.

If that is the deal, a possible explanation may be:

1. Mortar between bricks in such poor state that the wall begins to
move and crack
2. Bad mortar also allows driving rain to penetrate easily
3. Cracks start appearing in wall
4. Quick bodge fix for all of the above is to apply render.

The solution to this is to repair all the pointing - but there are
important caveats with that. If that is your situation do get some
more input before pulling render off, which may cause serious damage.


where should this dpc be? Directly under the sill I suppose. I don't
know if there is one. How can I find out? Good point though - if there
was a dpc it wouldn't matter if the sill was porous, right?

Any water penetration anywhere can be expected to freeze and slowly
disintegrate whatever its freezing in.



It sounds like you're on the right track if you've sealed the cill.
The chances of it being porous are quite remote and if it is, you
could probably cure this with a water repellant.

Not sure about 'sealed', but I hope so. I sprayed them twice with stuff
sold for treating penetration problems in walls. Can't recall the name
at the moment.

Sounds questionable to me.


I used Unibond silicone from B&Q, about £7 a tube.

try toolstation.com, something like 1.50 IIRC

Good luck.


Regards, NT