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.

The property is an 1890 end terrace on a high exposed location facing
the prevailing wind, and rain does drive against this wall at times. The
outside is rendered with small chips (spar?) in a yellowish base. There
are a couple of hairline cracks in the render below the windows, and
that area has been treated a couple of times recently with silicone
fluid without any improvement to the damp patch.

There are two adjacent rooms with windows in identical situations, and
one has a large ugly damp patch below the sill, whilst the other shows
only a very small patch about 3" diameter.

The bad damp patch is about 3 ft wide by 1 ft high, starting 9" below
the sill. I did the 'glass-slip plasticined to the wall' test to verify
that it is damp and not condensation, and this was very clear with gross
condensation on the inside glass surface.

The builder had replaced the original sash windows with uPVC, and the
sills are now concrete, and I suspect the sill heights have been
increased. These sills are not 'bought' concrete ones but appear to have
been cast locally, possibly in-situ. The finish is pretty poor and the
top surface seems to be hand trowelled and not very flat. In fact there
are some hollows that retain little puddles on the sill with the worst
damp problem. I gave both sills a thorough soaking with silicone fluid a
couple of weeks ago and this soaked into them very readily, so I am
wondering if they are porous (or were, before siliconing).

The damp patch has slowly dried back since the winter, and left ugly
efflorescence marks behind. A test with my damp meter (a cheap squawk
type) confirmed the surface to be almost dry compared to the winter
wetness.

Having now siliconed the sills, as well as the walls, I am hoping that
further wet will be kept out, and so last week I decided to hack off the
plaster with a view to making good the interior efflorescence, but I
found some unexpected things.

The plaster turned out to be only a skim of 3-4 mms on a sand/cement
render. I assume that the builder did this to suppress the damp, but it
is obviously ineffectual, so I hacked it all off - that turned out to
quite a test for my cheapo Screwfix SDS+ drill, but was a dream compared
to doing it by hand. I found the render to be covering a real mess of
brickwork, but the main surprise was that the bricks were actually quite
wet, the wettest being right under the sill. I surmise now that the
problem IS that the sill is porous and water is percolating down from
the sill becoming trapped between the (hopefully) waterproof cement
render on the inside and outside surfaces. I suspect that the whole
inner wall is rendered, at least up to the sill level rather than just
over the damp area. Certainly the 4' x 1' area I hacked off did not
reach the limit of the render. Incidentally, i could not detect any
dampness on the outside rendered wall, perhaps not surprising as the
weather has been dry and warm.

I am leaving it for a few days to see how this wet dries out, now that
it can via the exposed inner brick surface.

What I need now is advice on is what to do to make good the inside wall.
I am not convinced that re-rendering it is a good idea - I want to stop
the damp at source, and the original render put on to contain it has not
worked very well. Is a 'waterproof' rendering really waterproof? I
suspect that under the pressure of a couple of feet of water inside the
brickwork wet will seep through; or could it be that the builder didn't
mix the render correctly?

I am also worried about the poor quality of the sills, and suppose that
we may have to replace them if wet continues to enter the wall in spite
of the silicone treatment. In one sense, not rendering the inside will
at least allow me to quickly see if wet is still entering from outside,
whereas I imagine rendering it will delay the occurrence of future damp
but not totally stop it.

What is involved in replacing a sill should that be needed?

Any advice or comments will be much appreciated.

Phil

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Phil Addison
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