Damp at side and top of PVC window

31 Oct 2020
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United Kingdom
We live in a 1930s/40s build and have double glazing throughout. Over the last 18months we have noticed a damp patch on one side of our front bay window, this damp patch becomes more prominent depending on how heavy the rain is (plus it's on the prevailing wind side of the house).
We have had the bay tiling replaced as well as the brickwork checked and repointed as necessary, it was also coated in a sealent on that side of house. Earlier this year we also had the bay roof lead flashing fixed, however, after a spell where we thought this had finally fixed the issue, it's reappeared over the last couple of months.
It's now lead me and a relative to think it may be the mastic on that side of the window, to me the sealent between window frame and brickwork looks okay but could this be the issue.
P.s. a relative has taken away a piece of the damp plasterboard to see if the water is coming from above, we don't think it is, as although the wooden beam coming down looks like it has a damp water mark, it isn't it's dry.
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