Flat roof leak or condensation?

[John D v2.0]

Hi all, thanks in advance for any comments or replies to this worrying issue.

I noticed some mould on the underside of the flat roof part of the loft. It was built and covered in fibreglass 5 years ago.

I thought it was condensation at first but after the warm (and rainy) weather it's still looking damp.
Knowing the builder, it will be a roof leak (in 4 places) but I want an opinion before I specify a replacement.
It's a two storey house with a flat part at the very top of the pitched roof, so access is not possible for me to look outside.


What do people think? And any advice on a specification to replace it?

 

[blup]

Is that blue sky between the end of the joists in pic 2, or roofing felt?

How is the roof ventilated at present, is there any insulation?

Can you get on to the roof through the skylight in case it is cracking or other damage to the deck?

Blup

 

[John D v2.0]

Thanks very much for your reply @blup
And a happy new year to you.

That's roofing breathable membrane in pic 2. The pic is facing towards the front roughly beside the skylight. The other is also close to the skylight.
The ventilation is via the membrane and gaps around the edges, it could be better though and was in the plan to improve. There is no eaves ventilation as built due to it being bricked between the rafters and exposed feet. The original roof was plain tile with no membrane.

The insulation is mostly 100mm wool but some areas of similar thickness celotex. And around the edges has about 300mm wool.

There is a leak of warm air behind the extension plasterboard that I'm going to address, but it's nowhere near this area of mould. Although this area is near the loft hatch which also leaks a bit.

The red flag for me was that this is specific areas in the middle rather than generally so, and it still hasn't dried out fully despite it warming up and drying out the condensation above the extension. And also that the extension builders were not really competent at all the things they needed to be.

 

[blup]

Happy New Year and to all posters and lurkers

Blup

 

[blup]

Its a warm roof from your description and pics so shouldn't need ventilation, the experts say, in the way a cold roof is constructed. That points to cracks or holes, pinholes even, in the Grp deck when installed or which have developed since. As a rigid covering grp is susceptible to this, especially given the hot summers we have been experiencing. So the cheap repair is to treat it as a leak (there may be localised pooling). The comprehensive approach would be to get a building surveyor to carry out a survey (drone, or even get on the roof), which might cost more than the repair. Can an expert really be sure without ripping off the roof?

Anyway a first world problem but a pain for you, now where’s that hangover cure.

Blup

 

[John D v2.0]

Thanks @blup for your extra response

In terms of a warm roof, I'm sure it's a cold roof. Both in terms of the insulation being at ceiling level rather than rafter level, and in terms of the insulation being below the level of the OSB rather than on top. As a result the roof void itself is an unheated space.

Coincidentally a family member visited with a drone about a month ago and took the photo in my original post plus a closer one. The quality of the original was better and I couldn't see anything obvious at the time, although wasn't doing anything more than a general look for obvious issues.

I will take on board your advice which is helpful and look again when I have some time. Thanks and hope your hangover is eased already

 

[oldbutnotdead]

I'd be thinking condensation initially. Even if it is a leak, that fungus wouldn't be growing if there was adequate ventilation.
Though saying that the way the roof is flashed onto the tiles looks a bit sticky back rather than anything proper.....
Have you got a hosepipe long enough to poke through that skylight and give the roof a good controlled soaking?

 

[blup]

I was thinking cold roof vs warm roof in terms of the flat part of the roof i.e. like a single story flat roof extension. The insulation is above the deck, so making it a warm roof, no need for ventilation if constructed correctly. I take your point about the loft void being insulated from below, but isn't it still a room in terms of how the roof is insulated. I acknowledge that the terms are also used in the way you mention... may be one of the forums big beasts will comment...

Blup

 

[John D v2.0]

I'd be thinking condensation initially. Even if it is a leak, that fungus wouldn't be growing if there was adequate ventilation.
Though saying that the way the roof is flashed onto the tiles looks a bit sticky back rather than anything proper.....
Have you got a hosepipe long enough to poke through that skylight and give the roof a good controlled soaking?

Thanks @oldbutnotdead I had another look today and I'm also leaning towards condensation more. Very helpful point about the fungus. The dew point of the air in the house is about 12c at the moment, and it's only 5c outside right now, so any leak of inside air that rises and can't dissapate due to the large deck there would build up until it gets warmer. The infested areas are generally more shadowy areas in terms of the sun.

The flashings at that level are definitely all lead, I saw it with my own eyes before and after it went on. Whether it's done properly to the standards or not, I doubt it, but it's definitely lead.

The skylight isn't an opener due to cost. The opener would have been nearly triple the cost even for a hand cranked mechanism.

I was thinking cold roof vs warm roof in terms of the flat part of the roof i.e. like a single story flat roof extension. The insulation is above the deck, so making it a warm roof, no need for ventilation if constructed correctly. I take your point about the loft void being insulated from below, but isn't it still a room in terms of how the roof is insulated. I acknowledge that the terms are also used in the way you mention... may be one of the forums big beasts will comment...

Blup

Thanks Blup I've seen people use the terms warm and cold roof in both of those ways, but I'm not keen on either of them due to them being imprecise.

I prefer to say, the pitched roof void is unheated and has a flat roof and joists at the top, just to be specific.

 

[John D v2.0]

I took some photos from when the roof was built 5 years ago, here are just a few of them in case it says anything useful

 

[oldbutnotdead]

I'll shut up about the flashings then- whatever they used has gone under the mat, not sure about bonding lead to GRP (never done it).
That bottom course of tile cuts is interesting.....

 

[blup]

I can see how condensation would build up in a cold roof space with a warm deck above trapping the escape of air with a vapour barrier and insulation. Perhaps a cold loft should have a ventilated cold "roof"

Blup

 

[John D v2.0]

That bottom course of tile cuts is interesting.....

Yeah it's out of sight but it could be better. Architect mentioned the whole roof, structure and all, looked like it had been "thrown together" and the structural engineer wasn't so impressed either.

I can see how condensation would build up in a cold roof space with a warm deck above trapping the escape of air with a vapour barrier and insulation. Perhaps a cold loft should have a ventilated cold "roof"

Blup

The warmer the underside of the deck, the less condensation there would be. It's precisely because it's both cold and impermeable that it can condense out the water.
The only way to ventilate the flat part would be mushroom vents or similar, hard to retro fit.

I've ordered a box of the manthorpe g630 felt lap vents and I'll get them fitted at low level when they arrive. I was tempted to cobble my own but I don't want to lose the "drape" in the membrane.
I've also left the dehumidifier pointing at it.

 

[blup]

Yes something like that.

Blup

 

[Koolpc]

Seems like condensation to me. Ventilation is the key.