Breathable roofing felt

How does it work?

I know that many waterproof jackets operate by "relative humidity" so that if it is more humid on the outside they actually let moisture "breathe in".

I ask because I had the blue breathable stuff put on my roof two years ago and the timbers look worse than before as if they are damper.

They have stains on them and are covered in black powdery stuff and the edges become round when you rub them.

I had another roofer check it out and he said everything was sound and as for the wood "thats just what wood does"

I'd be grateful for any advice any one can give.

But are they damp? get hold of a moisture meter nd see just how damp they are..

water vapour molecules are smaller than water droplets so the breather membrane lets trapped vapour out without letting rain droplets in.

if your timber is visibly wet, but uniform and widespread then you may have a condensation problem.

as long as the roof tiles are intact and even if the odd one were broken, the water ingress would be localised and not widespread.

I never considered any problems with breathable felt, but a few years back my brother worked on a site of 30 odd houses, the majority having condensation problems in the loft. He fitted airbricks and soffit vents on these which did cure the problem. Since then a few people have told me about the same problem with breathable felt.

At the time I was collecting my retention from one of my jobs, after six months, and went to check the loft, there was condensation up there. Now I put some venting in every roof I do, I don't rely on the breathable felt doing it's job

Membranes are not some miraculous product to ventilate the loft.

They are vapour permeable, but do not replace ventilation.

If you have an unventilated loft which is sealed to prevent vapour from the rooms below from seeping into it, and things like pipes and fan/heating ducts are insulated to prevent condensation, then the loft will have low humidity and the membrane will deal with it.

If you have raised humidity in the loft, then you will need additional ventilation to prevent or reduce condensation formation

Some precautions when using breather membranes

All penetrations into the roof space must be properly sealed
Loft hatches must have effective draught seals
The rooms below the ceiling must have a means of proper ventilation - especially extractor fans in rooms that may experience high humidity
All water tanks in the loft space must be covered
Any vent pipes should be arranged so that they do not discharge water vapour into the loft space

All current membranes should be BBA certified, and therefore should function adequately to deal with normal humidity expected in lofts, and limit condensation.

If you do have damp rafters,and mould is forming, then if there are no small leaks from the roof, then consider some ventilation. Check your rafters after a period of heavy rain

non ventilated roof system that will meet with Building
Regulation approval, the underlay must have third party certification which states
that it can be used on a roof without the provision for ventilation and that the
procedures and fitting are carried out in accordance with the conditions stated
in the BBA Certificate.
Permo® air, Permo® forte and Permo® light are BBA certified to be used in roof
structures without ventilation and would therefore meet this recommendation.

breathable membranes will only work if the vapour pressure on one side is greater than the other the water vapour will only move from the higher pressured side to the lower.
So if the outside is more humid than the inside then vapour will move from inside to out, this is why you still need ventilation to carry the "breathed" vapour away.