Plastic roof channels/valleys - condensation issue

[Owen74]

Hi, I'm hoping somebody knows an easy fix for this.

Our house has a small two-storey extension at the front so there are now two adjoining sloped roof sections on the house (see pic).

All is fine but there's one problem. The two roof sections have plastic (not lead) roof valleys/channels adjoining them, running rainwater down into gutter. While perfectly watertight, the plastic roof valley material is thin enough to allow large amounts of dripping condensation to form on the underside. (Grey plastic shown on pic taken inside loft). I notice a few places where 2 layers of the plastic 'overlap' and in those places the condensation is absent - because the double thickness provides insulation.

I'm fairly sure the loft ventilation is OK, ensured no ceiling holes allowing hot air in, and it's nice and cold (and reassuringly drafty) up there. But still a few degrees warmer than the outside world, no matter what I do, especially when freezing when rainwater cools down that plastic surface on the outer side. And no amount of ventilation seems to stop the condensation forming under it. Not surprising when you consider it's an exposed, thin 3mm layer of plastic between loft and outside world, nothing covering it up or insulating it.

The moisture drips down the inner slope of the channel and contacts wooden beams along the way, which bothers me as the wood soaks it up. I also suspect some of the condensation is dripping down into the wall cavity (on right hand side of pic), slowly soaking the cavity insulation over time.

Just to emphasise: This isn't rainwater getting in. It's condensation forming on the inside. I think I need to 'warm up' the underside of the plastic to protect it against condensation.

I've tried taping over the underside of roof channel with some heavy duty aluminium foil tape, but that hasn't really helped. I'm next going to try some of that foil bubble-wrap stuff, to insulate the valley in same way you might insulate a pipe.

Anyone got any ideas or faced this same issue before? Thanks

 

[Bonni]

Loft insulation

 

[Owen74]

It already is insulated. Do you mean remove it and reinsulate from scratch. I don't have a vapour protection layer, just lagging direct on plasterboard. Is that the problem do you think?

 

[Owen74]

I'm now starting to think the problem is that these aren't meant to be exposed, and there was an error in the build.

 

[noseall]

I'm now starting to think the problem is that these aren't meant to be exposed, and there was an error in the build.

Correct. Zero membrane beneath the valleys is bonkers.

 

[Owen74]

Correct. Zero membrane beneath the valleys is bonkers.

Yep that's what I'm realising. Really annoyed about it.

Being practical I can't rip it up and put membrane under it now. I can only try and protect it internally from condensation and hope to God the plastic lasts a few good years. I'll just have to get it replaced (with membrane) before any cracking/splitting occurs, rather than wait for a leak to happen. What sort of lifespan does this stuff have before it fails? I was thinking if I replace in 5 years that'll be a safe bet.

 

[Owen74]

Brief update - the new roof section's "breathable" membrane was I think laid too tight, not floppy/saggy but really tight across the beams. I managed to get some lap vents into it, and I think that's helped a bit, in terms of temperature and humidity in there. 76% RH today and 7.5 degrees C. That's on a sunny day. It's always worse when raining outside.

 

[diynoob]

Since it's condensation rather than water ingress, can you insulate the underside of the valley or apply an anti-condensation paint?

 

[Owen74]

Since it's condensation rather than water ingress, can you insulate the underside of the valley or apply an anti-condensation paint?

Yeah I'm trying to insulate the underside using SuperFoil, which seems to be helping a bit. I was concerned that anti-condensation paint might somehow corrode or weaken the plastic, since it's meant more for walls or ceilings.

 

[diynoob]

Anything that increases the surface temperature should stop the condensation forming. You can get devices to measure surface temperature They are handy for this sort of thing and for flow/return temps on radiators.

 

[tomm12]

When the extension roof was constructed the valley we can see was not constructed at all, it was left open without valley boards etc. No valley boards and the rest of the valley goes haywire. They must have been relying on the GRP valley to do the work of a properly built valley with boarding and the rest of the needed detail?

In the long run maybe its best to lift the GRP valleys and make a well built no condensation, no possible future leaks valley?
For now as suggested seal off the loft floor, make it air tight. Maybe more insulation maybe more ventilation?

 

[Owen74]

Alright, I have an update on this ongoing saga. I got an independant builder to take a look, and he said while it's not an immediate concern with rot or mould, it is definitely not done right, and he reccommended I get the Building Control inspector round to check it, since it clearly wasn't checked at time of build when it was signed off. He reckons I might be able to get builders back at their own cost, to replace the valleys and place proper wooden support and felt under them. Will see what the inspector says but I dread getting the builders back.

 

[aveatry]

Alright, I have an update on this ongoing saga. I got an independant builder to take a look, and he said while it's not an immediate concern with rot or mould, it is definitely not done right, and he reccommended I get the Building Control inspector round to check it, since it clearly wasn't checked at time of build when it was signed off. He reckons I might be able to get builders back at their own cost, to replace the valleys and place proper wooden support and felt under them. Will see what the inspector says but I dread getting the builders back.

Wouldn't that risk you loosing your extension being signed off and still no show from the builders. Can the building controller / regs do that, I mean revoke the certificate of compliance or whatever its called.

There was a thread on here a couple of weeks ago about the same problem and they were trying to insulate underneath the plastic vally - have a search

 

[Owen74]

Wouldn't that risk you loosing your extension being signed off and still no show from the builders. Can the building controller / regs do that, I mean revoke the certificate of compliance or whatever its called.

There was a thread on here a couple of weeks ago about the same problem and they were trying to insulate underneath the plastic vally - have a search

As I understand it, if the Building Control authority deem it unsafe or not up to standard, they would basically order the builders to revisit and fix the issue at their own cost, as they hold that legal authority. The independant builder told me it would be a 3-4 day job max, so I'm just hoping it won't be a big deal.

But what amazes me though is Building Control inspectors don't bother to inspect the inside of roofs before signing off the build in the first place, so it's down to the owners to notice a problem later on (like I have). The roof is kind of important, isn't it? Why on Earth doesn't that work get inspected with all the rest?

Seems unlikely they would revoke the certificate as that would put them liable for wrongly issuing it beforehand. But it does make me worry. I do still have the option of insulating the underside of the valleys but if there's a "proper" fix available I'll take that instead.

I think the other thread might have been mine, too, but I'll check.

 

[aveatry]

As I understand it, if the Building Control authority deem it unsafe or not up to standard, they would basically order the builders to revisit and fix the issue at their own cost, as they hold that legal authority. The independant builder told me it would be a 3-4 day job max, so I'm just hoping it won't be a big deal.

That's if the builders are still running. 3-4 days sounds like a big job maybe that includs a day of putting up scaffold. I am not sure they would retro fit a breathable membrane the tiles either side of the valley will need to be taken off with the cement bedding then the GRP valley then membrane added but it should go under the existing so any water that gets in will run off over it and not under it, so I am not sure how far they need to strip back tiles to get under. Then valley boards then new GRP valley as the old one will be destroyed getting it off then re set tiles.

But what amazes me though is Building Control inspectors don't bother to inspect the inside of roofs before signing off the build in the first place, so it's down to the owners to notice a problem later on (like I have).

I think they do it from the top before the tiles go on.

Seems unlikely they would revoke the certificate as that would put them liable for wrongly issuing it beforehand.

Good point.