Black mould in roof valleys

I've just had a survey done on a 9 year old bungalow.
The survey has highlighted black mould on both valleys on the breathable membrane but everywhere else appears ok.
Does anyone have an idea of whats causing it?
Is it a design fault or bad workmanship?
and whats it going to cost to put it right

"breathable " membrane unable to breathe @ those points - bloody stuff is a waste o time There is only one way to treat a loft - ventilate over the insulation - with proper airflow . But then I`m a bit of a dinosaur

Nige F said:

"breathable " membrane unable to breathe @ those points - bloody stuff is a waste o time

Click to expand...

Like all things it has its place, in a terrace for example it is fine where you have nothing against the outside of the membrane to restrict its breathability. Where you have it its not really a design fault more of an oversight. But to repeat Nige f ventilation will sort it out. Not a deal breaker and if I was the seller and you asked for recompense in the sale price Id tell yu where to go (if I wasn't a desperate seller that is).

Nige F said:

"breathable " membrane unable to breathe @ those points - bloody stuff is a waste o time There is only one way to treat a loft - ventilate over the insulation - with proper airflow . But then I`m a bit of a dinosaur

Click to expand...

The design of a valley - timber boards and impermeable lead or valley trough directly on the membrane, means that the membrane will never breathe beneath valleys and there is no way around it.

By design, the valley is a thermal bridge and more prone to condensation ( and black mould) than other parts of the roof void - and that surveyor should have known this and explained it.