Damp above UPVC french doors - new extension

We have a steel-framed, lean-to extension with a Metrotile (lightweight steel framed) roof. The extension is about 3 years old.

There is a damp patch above our UPVC patio doors in the extension. It's been gradually building over months and months, no obvious drip/leak as such.

I've had a couple of roofers look at it.

One suggested perhaps we need deeper guttering on the outside (not convinced, it never overflows and never feels full). Seems an odd suggestion.

The other suggested it may be condensation and recommended popping a couple of air vent tiles in the roof to help. We currently don't have any.

Another thing we notice is a draft along the seal where the doors close into the frame. i wonder if that could create the problem?

We don't get any condensation on the windows or doors.

What could be causing this?

Any advice as to what sort of tradesman i should get to look at it?! The original installer no longer exists and not sure who to ask.

 

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