Roof Ventilation - What is Sensible?

[malcolm]

We're having the tiles lifted and refitted on the roof in May, and as part of the work we'll have a permeable membrane fitted to the roof. The roof does not currently have a membrane and is very well ventilated. The wood structure looks good as a result and I would like to keep it that way.

The roofer suggests we don't need to include any eves or ridge ventilation because the membrane is breathable. The internet seems to support this and building regs annoyingly reference a British Standard which is expensive to look at.

What have you guys done for roof ventilation when you have a permeable membranes?

 

[MatthewC]

I fitted a membrane at my roofer's suggestion and then insulated the joists in the loft (all with LBC). Like yours, the loft was also fairly well naturally ventilated, even with new soffits and fascia.

Later on I found out that I needed building control approval as well, but the officer was very pragmatic about that; I had thought that one application was sufficient! That was in my naive and innocent days...

 

[Feltwell]

Our Building Inspector insisted on vent tiles being fitted even though we had breathable membrane.

The roofer was surprised - said that usually the membrane alone was enough. Before the tiles went on, you could feel the air coming through the membrane but with no water getting through even in very heavy rain - so I think the roofer was right.

I wish the building inspector hadn't insisted on the vent tiles as whilst they were a good match to the normal tiles when they went on, they've weathered very differently and now stand out.

 

[MatthewC]

Our Building Inspector insisted on vent tiles being fitted even though we had breathable membrane.

The roofer was surprised - said that usually the membrane alone was enough. Before the tiles went on, you could feel the air coming through the membrane but with no water getting through even in very heavy rain - so I think the roofer was right.

I wish the building inspector hadn't insisted on the vent tiles as whilst they were a good match to the normal tiles when they went on, they've weathered very differently and now stand out.

In which case I'm glad that I didn't have a BCO present at the time! I remember the discussion about vent tiles and my roofer said they weren't needed so I agreed with him. The roof space is well-ventilated and has never leaked (yet) - even before the last of the slates were fitted when there was an area of membrane exposed to the sky...

 

[a twig]

Another vote here for no venting required if breathable membrane is used. When ours was re-done, it was just the membrane with clay double romans on top, no issues at all one year in, to the extent that I've stopped bothering going up in the loft to check!

 

[Feltwell]

I'm actually contemplating blocking up a couple of my tile vents. They vent into a roof void which has since been opened up as a built-in cupboard in one of my attic rooms - it was a criminal waste of space before, two areas about 3' wide by 6' long by 4' high just as blanked off voids.

The tile vents into a space behind some Kingspan insulation. This part of the roof is very high up & very exposed - Despite the insulation being fitted tightly and sealed with aluminium tape as best as I can, there's still enough wind pressure during storms through these tile vents to cause a cold draught out of the cupboards.

I do believe the membrane breathes enough to make any more ventilation unnecessary. One point to remember is that the membrane is supposed to be laid loosely on the roof prior to being stapled in place, so it sags slightly between the rafters - this stops any water that gets past the tiles being able to puddle behind the roofing battens. Many roofers stretch it taught which is not good practice.

 

[malcolm]

Thanks, I'll aim for a permeable membrane with no additional ventilation as a starting point. I ought to mention the roof to the building regs bloke at some point before I start and see if he agrees.

Building regs documents suggest minimising the paths where warm air from the house can get into the roof space to minimise condensation. They mention sealing the loft hatch but that's the least of the worries here. I can seal most places when I add insulation, but what would be the best way to treat a 17th century timber frame hidden behind a 20th century brick and timber frame façade. The old timber frame has clearance around it and vents into the roof space at the moment. I'm thinking let it vent into the roof space but block air paths from the house to the old frame and as best as possible try to seal up the exterior.

 

[Feltwell]

The membrane I had recommended to me, which I'd recommend to others, is Proctor Roofshield. It's very, very strong - much more resistant to tearing than the cheaper stuff my roofer showed me which he used for temporary covering only. Someone else on here, can't remember who, has used it as well.

Only downside is it's bright green on top - should all be hidden though! The underside is a less offensive white.

http://www.proctorgroup.com/products/roofshield

 

[a twig]

Add a second vote to the Proctor Product - that's what our roofers recommended, and they do a lot of old/listed buildings to boot!

 

[malcolm]

Thanks, I'll ask the roofer to use Proctor. I had seen a green membrane recommended on here before (but couldn't find the post as usual).

It surprised me that there is no need for additional ventilation when using a breathable membrane. It might be a good idea to also fit it to the barn which currently has a non-vented non-brethable covering and a fair bit of rot to fix. I would be happier fixing the rot without 2 tonnes of tiles in place.

 

[Mickey]

Hi All,

Just curious about the use of membrane especially on listed buildings, we had to have a bat survey before undertaking any work to our roof which is normal for listed buildings I believe and were advised we could not use breathable membrane because it can lead to bats dying because they get caught up in it and can't free themselves, presumably by their claws. We had to use normal old fashioned roofing felt.

Mickey

 

[Flyfisher]

Whereabouts in the country are you?

Seems that these things vary quite a lot. The architect who drew our kitchen extension plans said we'd probably need to have newt fencing erected around our house (because of the proximity to a river and ponds) as well as a bat survey, but he was basing that on his experiences in Hertfordshire. However, in my three LBC, one PP and one BC applications here, there was never any discussion or requirement for such things.

Meanwhile, on the recent A11 dualling work between Mildenhall and Thetford, straddling the Suffolk/Norfolk border, the authorities have required 'bat bridges' to help prevent bats being hit by cars.
http://www.telegraph.co.uk/news/politics/8827505/Bridges-too-far-The-500000-bat-crossings.html

Also, a friend's neighbour in Hertfordshire was recently refused permission to increase the height of their rural bungalow and one of the cited reasons was that the house is in a known 'bat area' and the change in height could cause casualties.

Now I'm not a bat expert so I'm probably talking rubbish, but my suspicion is that bats are pretty good at detecting and avoiding stationary objects and are also pretty good at detecting and avoiding moving objects. But hey, I'm not an expert bat consultant, though I'd suggest they are a species best avoided if at all possible.

 

[MatthewC]

Hi All,

Just curious about the use of membrane especially on listed buildings, we had to have a bat survey before undertaking any work to our roof which is normal for listed buildings I believe and were advised we could not use breathable membrane because it can lead to bats dying because they get caught up in it and can't free themselves, presumably by their claws. We had to use normal old fashioned roofing felt.

Mickey

The membrane I'm thinking of is a smooth sheet and I can't see how a bat could get caught up in it. It might stop them entering the loft through a gap in the tiles but otherwise I don't see the issue.

There was no requirement in my house for a bat survey (and I did get LBC for the roof) - I think there has to be some reason for suspecting their presence.

 

[atticman]

But hey, I'm not an expert bat consultant, though I'd suggest they are a species best avoided if at all possible.

Do you mean bats or expert bat consultants that should be avoided as a species, FF?

 

[Flyfisher]

Er, you're not a bat consultant are you? :wink:

 

[Mickey]

Hi

We are in Cornwall we were advised by building conservation team that we would need a bat survey in connection with the proposed works for which we would apply for LBC (lifting a plasterboard ceiling back up to original height), the conservation consultants come out and you have an initial survey to check for any bat activity £500. Then if there is any they come out again to conduct emergence surveys x 2 another £1000 if you are lucky or unlucky depending on your view and probably your wallet and you have an active roost or rare species you would need to have the consultants in to conduct a 'watching brief' whilst you are carrying out the work, god knows what that would cost.

Don't get me wrong I like bats and enjoy watching them in the garden at dusk in the summer but I can think of many things I'd rather have spent £1500 quid on!

Mickey

 

[Flyfisher]

I also like to see bats at dusk and have watched them for many years on warm summer evenings, both before I stripped our roof and since, and I've not noticed any decline in their numbers. We've also had Swifts nesting under our pantiles for many years and they also don't seem to have been affected. I suspect wildlife is more adaptable that we might think. After all, where did bats live before we built all the houses they are now deemed to be dependent on?

 

[DJH]

Transylvania?