Black mould on underside of roof felt

[Mathew Newton]

No, just paper-backed plasterboard and a skim coat.

The only penetrations are lighting cables and an en suite extractor; all seemingly well sealed.

That too is pretty good - it's an insulated affair with a well fitting compressible rubber seal.

Could well be; unfortunately I can't see if the cavities have been closed at the top due to the design of the roof.

I'll give the felt a good vacuum, and might test some hydrogren peroxide to see if that cleans it up. If I can get it back to a 'mould (or whatever it is) free' state then I can keep a proper an on it, perhaps with humidity monitoring as George suggested along with visible observations of the felt.

[Mathew Newton]

Forgot to comment on this; everything feels bone dry. You would of course expect this with the current weather but there's no sign that anything has ever been damp other than the black marks on the felt.

One thing I haven't done is tried to look on the tile side of the felt - it could well be there's something on there that is causing the marks on the inside. I don't know what, if any, relevance this has but we're in a rural location any there must be a lot of pollen etc floating around which must get trapped on this teabag-like felt. In the past I've found the odd dead fly in the loft dangling from the felt with appears to be a leg stuck!

[George Miles]

there was cold air the other side of the roofing felt which is why condensation happened there, not anywhere else.

[george]

[Mathew Newton]

If it was as straightforward as that I would have expected the overlaps to have suffered similarly as there'd be very little, if any, temperature difference there.

Some have questioned whether it really is mould and I am leaning towards that too; not due to wishful thinking but rather that the basis of the initial hypothesis is not looking strong.

I'll given the felt a vacuum, try some of the hydrogen peroxide that arrived today, and see how things go from there. Finger's crossed I won't see it again or, if I do, it'll be in another few years time as appears to have been the case here which should point to it being something more benign than a ventilation/mould issue.

[Andrew]

Oh dear. Are you having Jeremy Kyle withdrawal symptoms ?.

[Andrew]

well that is part of the problem. Water vapour molecules will migrate through the ceiling and into the loft space. The breathable membrane should cope with this but seems not to.

Modern building regs make foil-backed plasterboard mandatory for upstairs ceilings, for precisely this reason, but older re-roofed properties probably still need some normal vents even if breathable membranes are fitted.

Lofts that have 300mm of rockwool are much colder, so condensation is more of a problem.

<snip>

[Andrew]

If your house was built in 2007 then it should have had foil-backed plasterboard on all the upstairs ceilings.

I wonder if NHBC or similar did the 'building control' ?. Is this a house built by one of the large national builders, or a small local outfit ?.

Look at the pdf called "Re-roofing and Plastering" at Rochdale council.

[Mathew Newton]

Is that 'should' a 'should' or a 'must'? The document isn't clear on that point.

Note also that even if it were a 'must' then if that requirement only came in around 2006/2007 then it still might not apply to a 2007-built house if that house was built against plans agreed before 2006.

Persimmon.... (Enough said! ;-))

Thanks everyone for the comments; I'll keep an eye on things and so what happens.

[Andrew]

Ahh. Building regs are notorious in their choice of words. I think in the world of the BCO, 'should' means 'I won't sign it off until you comply' !!

Oh dear. I bet if you drilled some holes into the cavities you would find :-

1. no insulation
2. inch thick expanded polystyrene slabs chucked in loosely allowing air to circulate on both sides
3. Plasterboard offcuts to avoid paying a landfill disposal tax.

Sadly its too late to make a claim from the NHBC, who probably did the building control. Your local authority BCO would have spotted this and enforced remedial work.

I think the addition of vents into the lower third of the flanks (there are special vents for retrofitting) and maybe a ridge vent would help.

I suspect that painting all the upstairs ceilings with oil-based paint to create a vapour barrier would help, but you would have to move out while the fumes clear. Don't try this without doing some research.

[Mathew Newton]

On the contrary; I've just removed the back wall and was actually quite impressed with the attention to detail: well fitting insulation boards, little in the way of mortar snots; clear cavity down to footings. This was the showhome, or possibly the 'office' next to it, and so was likely one of the first ones built hence the builder's perhaps still had some motivation in them!

The thought never crossed my mind. From what I can gather from flicking through the Approved Documents (acknowledging that they don't hold exclusivity over what satisfies the regulations) I can't find anything that mandates even today the need for foil-backed plasterboard per se; more just that consideration must be made for vapour control.

I won't be taking any action until I know for certain what, if any, problem I've actually got and what caused it.