Mystery Damp!

[Bunkermagnet]

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]

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.

 

[Bunkermagnet]

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.

But did the damp batch dry out, or enlarge/ stay as it was?

 

[Rlburnside]

Ours only happened during the 'Beast from the East' Storm where the high winds and rain were from the East as to the normal South Westerly direction. It hasn't happened again.

I’m in Shetland so we get plenty of beasts

 

[road2ruin]

But did the damp batch dry out, or enlarge/ stay as it was?

I'd say it stayed as is.

 

[Rlburnside]

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]

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]

Definitely not my field but trying to think about it logically , 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?

 

[Bunkermagnet]

I'd say it stayed as is.

Chipboard tends to stay stained once water has soaked into it.
I would put the plastic slide back over it and put some paper towels or the like in it. Any water will show up more easily. How sound are your ridge tiles?

 

[SocketRocket]

I’m in Shetland so we get plenty of beasts

In this case the gale force winds were not in the normal direction. We've had plenty of gale force winds and rain since but nothing has got in.