I'm currently renovating a house and we're coming to the end of the project.
The original kitchen extension had a flat leaking roof, which was replaced about 2 months ago but the bricks were very wet internally after removing all the old internal plaster (about 6 months ago). A month ago the internal walls were plastered with a water proof membrane behind the boards on parts of the wall that were originally a brick shed. Around a month ago the outside exterior was sand/cement rendered. Over the last few days the heating has been on and there's been quite a bit green mould growing on different parts of the wall, some by the original window (parts that haven't been externally rendered) and growth on the opposite side too.
My hope is that there's water in the brick that's struggling to get out, but once it's all out it'll be fine,
The house is 1940's ish and I'm pretty sure it wouldn't have been built using lime and so not needing a breathable render.
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