I wonder if anyone has experience of placing fibreglass over an
existing flat roof?

I have a building which is simply a flat roof fixed between rendered
brick walls. The problem is the steel profile sheeting was laid too
flat and the gutters of the adjacent roofs overflow, the subsequent
deluge overwhelms the flat roof and water gets in via the overlaps. I
have painted these with resin sealant but this hasn't survived the
movement of the panels over time, I also think some water is getting
past blown render.

I am considering just tanking the roof and up stands with fibreglass
but wonder if this should be applied directly or a membrane used to
allow for differential movement?

AJH