Damp patches around rooflight?

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After having an extension built about 4 years, I've just noticed some damp on the ceiling around one of the rooflights. (Its a single storey lean-to extension with a pitched tile roof). We are having a lot of rain and wind right now, however I can't understand why all of a sudden there is something wrong - unless something is failing. My first thought is a lot of moss has built up around the edge of the rooflight and flashing, and whether this could be retaining moisture and some how water finding its way in? Any assistance would be most welcome
 
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Thanks, in my haste never thought to do that
 

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I cleared a load of moss off that was built up around the roof light flashing, and all tiles look sound and unbroken etc. Not checked the flashing at the top of the roof, so will look to see if that is compromised. I’m hoping the build up of moss has something to do with it
 
We conducted an experiment recently (6 days ago) whereby we looked at how the lack of a high level rain diverter affected how the roof windows perform during a rain shower, whilst the roof was untiled above the roof windows.

Surprisingly, 4 out of five managed to keep 99% of the water out simply with the correct stapling back of the felt. One failed miserably. We then installed our own diverters over each of the windows using a convenient lap in the membrane to tuck under some 450mm DPM and lap it onto the head of the tiles directly sitting on the top edge of the roof window. Keylite don't supply a 'J' diverter like Velux (sometimes) do.

Worked perfectly.

The moral of this story is that (as Datarebal stated) look at the tiles above the roof windows first.

 
Just to be 100 % safe before the ridge or roof flashings go on we usual turn the underlay back on its self by about 25 mm to divert any water either side of the velux .
We also wipe our boots off before walking on the tiles...:whistle:
 
Thank you everyone for your comments. What does puzzles me is how after 4 years or so without a problem, suddenly we appear to have a leak? Nothing has changed, and other than possible deterioration of the felt or flashing I would have thought it would have leaked before (when we first had heavy rain) after installation. That’s why I wondered if the build up of moss it to blame
 
Looking at the pictures it shows damp at the top corners. This indicates to me the underlay isn't totally perfect on the corners of the frame. This then will allow ingress from faults above... lead flashings etc . any pictures looking above?
 
Just to be 100 % safe before the ridge or roof flashings go on we usual turn the underlay back on its self by about 25 mm to divert any water either side of the velux .
We also wipe our boots off before walking on the tiles...:whistle:
When I say turn the underlay, I mean the nearest next lap above the velux, which could be anything up to a metre up slope.
 
Ok well they are not Velux, probably Keylite.
All the tiles are too tight to the frame, moss will restrict flow and cause dam effect.
the tile course behind the window is incorrectly fitted
what is the flashing below the house window like?
 

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