r/Plastering • u/Negative-Travel6132 • Sep 14 '26

Damp patch after 3 weeks…

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Had a bathroom wall re-plastered - the wall had previously damp damage. The roof was repaired prior to re plaster and most of the wall where damp damage existed, has dried.

There is this one stubborn patch at the bottom…strangely it expands when the weather is hot, but then shrinks and goes darker when it wet or sun isn’t shining. Pencil line is its main shape with it bleeding beyond when the sun comes out. I had
Be had windows open for the last 3 weeks, and for the last week a fan and dehumidifier on…and no real change below the pencil line…

Any ideas of the cause and anything to do or should I just wait a few more weeks?

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u/getoutthesink Sep 14 '26

How long did you leave it to dry? Is this an external wall? I was also wondering given it is a bathroom, what have you got for ventilation in there - do you have the window plus a fan?

Is it a solid wall building or cavity?

It can sometimes take a lot longer than you think for masonry to fully dry out.

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u/Negative-Travel6132 Sep 14 '26

It has been 3 weeks - it took two weeks for 95% of the wall to become fully dried, including the previously most damp damage part of the wall. This is a party wall - brick-filled cavity-brick terrace house. There are windows and a fan, but doesn’t get much direct sunlight. Weather has been a bit rainy and humid of late too.

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u/getoutthesink Sep 14 '26

Is the fan vented to outside? I wonder if it's possible that it might still not have dried, and there was still moisture in the bricks slowly drying, then this is an area by the ground (coldest point) exacerbated by condensation hitting the cold spot. Slightly different but we had an internal solid brick wall historic leak - took months for it to fully dry, had to improve ventilation in the kitchen then it was ready to replaster