velux condensation flashing laps - help appreciated

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
jim laid this down on his screen :

If I understand your explanation correctly, a 2" overlap is nowhere near enough to prevent blown rain entering on a normal roof slope angle.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

you would need at least 100mm overlap for a flashing, 150mm ideally.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Rick Hughes formulated the question :

I have just checked mine, which I put in 20 years ago. It has 100mm overlap, and I can see a minimum of two layers at all points in the flashing.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

hi thanks for thoughts - when you say "two layers" what are those two layers of? Presumably velux flashing (from the kit) around the window and what's the other layer? felt, lead,....?
cheers jim
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
jim laid this down on his screen :

A minimum of two layers of the Velux flashing at all points.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Mmmmmmm....Are yours actually "velux" brand roof windows? Re-did the offending one today and there were no " two layers" of velux flashing anywhere!?! Just top flashing of thin "tin" (ali I think - anodised grey)
(NB how do you tell there are "two layers"?- by looking? (at only one at a time I presume)....?
Maybe they have changed (economised) things since 20 years ago?
Problem looks like condensation so extra vapour permeable felt lapped up against top and bottom of the 2 veluxes should cure the problem ..... fingers X'd....!!!
thanks to all - if it doesn't work I'll post back for usefulness..
Jim
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.