We've had 3 large velux installed above each other up the slope of the
roof above a new staircase.

Between them (horizontally) run the main beams supporting the roof.

the roof has been insulated with 100mm kingspan leaving a 50mm gap to
the felt at the back oif the slates, foilback plasterboarded and
plastered.

Problem :- The beam between the topmost and middle velux has a weepy
wet water mark that doesn't actually drip off but has not gone away
yet (such that we can't varnish that side of the beam or the small
6inch piece of plasterboard that spans between the beam back to the
roof.

The beam between the middle and lower velux has no problems
whatsoever.

The only difference we can see (without starting to rip it all down)
is the lap of the flashings - the problem area has a 2inch lap between
the bottom flashing of the top window and the top flashing of the
middle window, the other velux has a 6inch lap at this area.

Is this the problem i.e. water penetration ? how would I tell if it
were condensation from the inside?
Anyone know the minimum lap for veluxes installed above each other?
Any suggestions for what to do next?


thanks in advance

Jim