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I do like a bit of plaster in the house. I didn't know there were weather proof ones. I should look out for them!!

Maybe it has another name like "white cement"

That sounds familiar. I will check. A sealing I will go!! 🙂

The best is to apply sanded membrane if you have any.

And this is why I world be useless at doing my own roof as I have no idea what that is!