Ok....so yet annother problem with my house. Every single year we get the same issue....faint water lines when its cold coming from the ceiling edge to the carpet floor ( upstairs back bedroom right accros) 3 bed 1960's semi. These marks are also present same location downstairs ( where patio doors are with a lean to conservatory off it) and yes flashing replaced. We have ZERO ventilation in loft, our gutters have no sofit vents either (old concrete gutters where angle grinded off but facia boards too narrow for sofit vents) If It was just a condensation issue I find it very odd it being in the same place downstairs. Oh the felt goes into the gutters so can't be a leek from there. We had a chap on the phone regarding this he mentioned installing a vent system in the attic....costly idea, but Would a few simple roof vents do the trick, with mybe a trickle vent (my newish windows never came with them. It really is the 1 thing driving me nuts and would like just 1 winter without this problem. I'm not very technical but struggling to solve the issue looking online as i dont think its damp. Any advice?