Condensation

Hi all.
I know there are many threads on this subject.
I just would like to know how much water can build up purely through condensation.
I have been roofing for over 20 years now but have come across a bit of a strange one.
I stripped out a lead valley, had to replace the boards with WBP ply due to the amount of rot, re lined it with lead, re beded the tiles and repointed.
All looked good apart from the tiles being horrible single clay pan tiles with a roll on them that took more compo than enough and never seem to sit properly.
Anyway, the bloke rings and tells me the new board is wet, sure enough it was, but only where he has had a dormer bedroom extended, they filled the roof space with insulation boards and rockwool.
I stripped out the tiles adjacent to the valley (the new lead was dry) and removed the lathe and felt, the underside of the felt was soaked. I replaced the felt and lathe with new and asked the chap to dry the boards out. I noticed he had stuffed insulation into the eaves and also blocked up a air vent in the gable. He has mould growing on his rafters and the new WBP valley board also has mould on it after just two months.
Can this amount of wet really be down to condensation alone taking into account what he has done regarding ventilation and the new build into the loft space??
Cheers
BTW, no other part of the valley is showing signs of wet apart from the bit where the new build is.

when temperatures are as low as they have been lately then it comes as no surprise that there are lots of soggy lofts.

valleys are especially vulnerable as they have virtually no air gap above.

Hi. I have come across this and it has resulted in a code 5 lead roof being eaten away on the underside. ( the building had a change of use but condensation potential was not addressed) As a result some bays needed replacing and ventilation provided via lead welded vent pipes at opposing ends of the roof. Good Luck

Thanks for the replies.
A good downpour on Sunday/Monday and his leak is no worse, in fact the boards have dried out already!!
He used a dehumidifier and got a pint of water in two hours, and unblocked all the vents that he had covered.
He was a bit concerned that the loft had gone cold until someone told him it was supposed to be cold!!
Oh well, we shall see what we shall see.
Cheers.