Damp patches on wall in bedroom, please help

[Mojo12345]

Hi everyone I brought a house 3 months ago I have noticed damp patches in the upstairs bedroom on the wall, they mainly occur in periods after heavy rain, above in the loft is a black tank that maybe is a cold water storage tank but not sure what it’s used for, possibly the water is leaking from that or possibly it’s leaking from the roof above or maybe there are some pipes in the wall, any advice or helpful tips would be great thank you

 

[chris1982]

Or through the wall. What does the outside look like? Poor pointing, broken gutter, etc ?