Mystery Damp!

Bunkermagnet

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Why not put a container of some sort over this point in the lift and see what collects.
Personally, I think if you have moisture ingress on the surface of the loft boarding, the roof has to be the prime suspect.
 

road2ruin

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Why not put a container of some sort over this point in the lift and see what collects.
Personally, I think if you have moisture ingress on the surface of the loft boarding, the roof has to be the prime suspect.

Already did, found a plastic sledge up there and put it over the area for a good week which included some of the heaviest rainfall. Absolutely bone dry.
 

Rlburnside

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Already did, found a plastic sledge up there and put it over the area for a good week which included some of the heaviest rainfall. Absolutely bone dry.

Might not show no matter how heavy the rainfall it could possibly only show alongside high winds.

Not saying it will be this but a possibility
 

PJ87

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Just to add another possibility based only on a working hypothesis. Was it very cold IE frosty? I mean when it chucks down the roof does its job water beggers off out the way.

However what about a frosty morning? Where the roof slowly warms up and rather than disappearing quickly the water gets to a place it normally wouldn't?

Like I say could be wrong just something worth adding to your list
 

Pants

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Definitely not my field but trying to think about it logically o_O, if the underside of the board is dry, anything put over the board remains dry, no water tracking marks leading to the damp patch, then the only thing that I can think of is that there is something on/in the board that is hygroscopic i.e. something that readily absorbes water or (more probably in your case) moisture from the atmosphere. Have you tried swapping the affected board with one in a different location and then seeing what happens?
 
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