I have a top-floor flat in a Victorian building that has had a damp patch on an exterior wall since I moved in (unfortunately not picked up in the survey). The stain is diagonal, which suggested to me that water was getting in through the chimney stack above. But I have been battling with my freeholder who thinks it's condensation as this bit of the wall is directly opposite the kitchen (which has no door).
We have tried using a dehumidifier to no avail. Work on the roof showed the chimney stack and roof were largely OK. A vent has been put in and the patch seems to be drying out a bit but ... slowly.
Does anyone know: How long stonework takes to dry out? How I can get somebody reliable to check it out who isn't biased? Whether I can speed up the drying process with any special paints, etc.?
The thing about humidity is, that visible condensation forms on the coldest surface in a room. Usually a window or a thermal bridge round an old window, is this happening?
Therefore, it is unlikely that your patch is condensation.
The most likely culprit is the flashing round the chimney that is probably old, cracked and leaking.
The next most likely culprit is wind driven rain. Either making its way in through the afore mentioned flashing, or making its way between the brick edges and the mortar - possibly due to a poorly mixed batch of mortar.
The thing to do, is get a pair of binoculars and take a careful look at the wall, making a point of studying the pointing -that is possibly old and missing, if the pointing looks good - then get a ladder and look at the flashing.
I've been assured that pointing and the flashing are OK. No condensation forms on the window, which is adjacent to the damp area, but the damp area does extend to the side of the window. My freeholder assures me that I just have to be patient and the stonework will dry out now that there's a vent, but I'm dubious!
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