Leak in roof - so what is this mould/rot please?

[nemo2000]

The cold water tank in my loft developed a slow leak, which only became apparent a few days ago when drips came through the bedroom ceiling. The leak has been repaired, but it is clear that there has been a slow leak there for some time, which soaked though the insulation and into the plasterboard ceiling, which has sagged away from the joists. Mostly repairable - remove the damaged ceiling, dry out the area and replace - just awkward to work under the tank.

BUT.....there is this white strand-like growth started on one of the joists (see picture attached). What is it please? Clearly it needs to be cleaned off, but might it be a symptom of a more serious problem eg dry rot (I hope not!).

 

[TedO]

That looks like mold to me. Showed pic to neighbor who is a builder and he agrees.Spray it with bleach to kill. What ever it is that area has been weakened. I would suggest attaching another 2x4 across the entire area in case it starts to rot, then you would have additional support.

 

[Johngo]

The bleach will only kill the active mould you need to get some copper sulfate powder and rub it around the affected area to kill the mould spores in case it gets damp again