I have a small amount of water coming through the inside of my bathroom window. The render outside had blown so this was replaced however the leak did not stop. Used a hose pipe to see if we could detect where the water was coming in by but this did not work. Then I went up the attic and found that where two beams meet at the edge of the house above my bathroom window a patch of mould had appeared. Called in a roofer and they seem to think that my air vent on the roof above the bathroom as slipped and the rain is getting in that way. But the air vent in the roof is about a foot higher up on the roof than where the mould is appearing on the beam inside. The air vents were put in 5 years ago and this is the first issue with the roof we have had. So is this the right answer? Also the roofer said he will not be treating the mould because if the leaked is stopped the mould will go away. Is this the case? From the inside it doesn't look like the insulation is wet and there are no marks coming through on my bathroom ceiling. But can water get through a slipped vent and travel down about a foot before mould is created. Unfortunately you can see the mould from the small bathroom attic door but unable to walk over to that area to fully investigate it from inside.
its possible,water will find the lowest point an creep into the building.If the water ingress has stopped then the roofer has found the cause.Also hes right in saying the mould will disapear with time IF there is sufficent ventilation to the affected area.
Rain can blow up a roof and enter under a vent/tile/slate and then run some distance before it reappears. The best bet is to enter the roof and look at the underneath to see if there is water running along the rafters, use a bright light and binoculars if you cannot get close. Note: Running water usually makes wood look black. Mould usually forms as a result of water vapour rising from the bathroom. Does the roof drip during the warm evenings of summer or is this a cold night only problem. It may be that steam is rising from the bathroom, condensing on the timber and dripping.