Advise on how to ventilate loft.

[robpowell200]

I had my roof replaced with slate 3 years ago and every winter since there has been a lot of condensation building up on the underside of the roof and dripping onto the ceilings below. I am aware that this is caused by a lack of ventillation in the loft space but I am confused on how to go about fixing this.
Having called the roofer back to look at the problem, he installed 4 low level roof vents (2 at front 2 at back) and this has not made any difference. Shouldnt he have installed some vents near the top also? If so, more or less than he installed at the low level.
The house was built in 1880 and is stone walled and the front section of the roof has a soffit board but the rear has no soffit board as the facia board is fixed directly to the rear wall. This means there is no air gap at the rear of the house between the roof and the wall. I think this isn't helping matters either. Unfortunately this is the way the house was built all those years ago so I cant change it.
Any advise as to which direction I need to take would be very appreciated, before all my timbers rot away!

 

[freddiemercurystwin]

Ventilation, ventilation, ventilation! You can install a few of these : http://www.rooftrader.co.uk/slate-tile-vents/slate-vents/inline-slate-vents.html in the side with no soffit and some of these: http://www.gutteringandcladding.co.uk/0002784/Circular-Soffit-Vents-White.html in the soffit on the side with a soffit. Make sure there is a clear air gap above any insulation you have up there like this this

Did he fit felt or a breathable membrane beneath the slates? Felt presumably?

 

[robpowell200]

He used what he called Breathable Membrane. Its green one side and white the other from what I can remember when it was being done.

Thank you very much for the advice.

I thought that I wouldn't get these issues with 'breathable membrane'?

 

[freddiemercurystwin]

That's the theory! Would be interesting to have know what type it is, I've known manufacturers reps to visit domestic properties to investigate these occurrences.

 

[JohnD]

it might be that there is excessive moisture getting into the loft. Does anyone drape wet washing inside the house? have you got any holes in the upstairs ceilings, for example around pipes or downlighters? how long is the extractor fan run after every bath or shower?

what sort of windows have you got, and how are the rooms ventilated?

are there any water tanks in the loft? are they cold, and have they got well-fitting lids?