tvrulesme Posted January 15, 2023 Share Posted January 15, 2023 We are looking to convert our detached garage into living accomodation. It's part of a Grade II listed building so maintaining the exterior appearance is very important. The current structure is timber rafters, membrane and natural slate. Plan is to deepen the rafters by attaching 50mm timber battens and insulate between and below the rafters with PIR leaving a 50mm ventilation gap. I have wide soffits which I can use to provide an inlet for ventilation but I am wondering how I can provide an exhuast at the ridge without altering the external appearance (which means that ridge vents are out of the picture). Any ideas greatly appreciated. Link to comment Share on other sites More sharing options...
nod Posted January 15, 2023 Share Posted January 15, 2023 A Simple solution is to remove the ridge tiles and mortar and redux with a dry ridge system Link to comment Share on other sites More sharing options...
tvrulesme Posted January 15, 2023 Author Share Posted January 15, 2023 9 minutes ago, nod said: A Simple solution is to remove the ridge tiles and mortar and redux with a dry ridge system Thanks a lot. Looks interesting but... can I reuse existing ridge tiles and would this increase the height of the main ridge at all? Both of these would be deal breakers for the conservation officer. Seem to be the constant mediator between the Conservation Officer who doesn't give a damn about building control and building control who don't give a damn about listed building requirements... Link to comment Share on other sites More sharing options...
PeterW Posted January 15, 2023 Share Posted January 15, 2023 1 hour ago, tvrulesme said: can I reuse existing ridge tiles and would this increase the height of the main ridge at all? Both of these would be deal breakers for the conservation officer. OK so the dry ridge will be slightly lower / same height as mortared ridge tiles however you do need someone to install it correctly. Those ridge tiles could be reused however if they are crock full of mortar you may crack them removing it so worth getting some spares anyway. Looks like you’ve got a couple of slipped slates too so you may have to do some rework on that roof. Is the membrane breathable that has been installed ..?? Makes me laugh though that the conservation idiot is worried about ridge tiles yet is happy with UPVC guttering ….. they really are a strange breed !! Link to comment Share on other sites More sharing options...
nod Posted January 15, 2023 Share Posted January 15, 2023 2 hours ago, tvrulesme said: Thanks a lot. Looks interesting but... can I reuse existing ridge tiles and would this increase the height of the main ridge at all? Both of these would be deal breakers for the conservation officer. Seem to be the constant mediator between the Conservation Officer who doesn't give a damn about building control and building control who don't give a damn about listed building requirements... yes you can 1 Link to comment Share on other sites More sharing options...
Temp Posted January 15, 2023 Share Posted January 15, 2023 If you have the height (propably not?) then put insulation at ceiling height rather than between rafters? Link to comment Share on other sites More sharing options...
Redbeard Posted January 15, 2023 Share Posted January 15, 2023 Eaves-to-eaves ventilation works for a huge number of roofs, but it may not work for your BCO... My house has no ridge ventiulation and no moisture issues (well, not in the roof....) 1 Link to comment Share on other sites More sharing options...
tvrulesme Posted January 15, 2023 Author Share Posted January 15, 2023 24 minutes ago, Temp said: If you have the height (propably not?) then put insulation at ceiling height rather than between rafters? Thank you. I have the height but really really like vaulted/cathedral ceilings Link to comment Share on other sites More sharing options...
tvrulesme Posted January 15, 2023 Author Share Posted January 15, 2023 1 hour ago, PeterW said: OK so the dry ridge will be slightly lower / same height as mortared ridge tiles however you do need someone to install it correctly. Those ridge tiles could be reused however if they are crock full of mortar you may crack them removing it so worth getting some spares anyway. Looks like you’ve got a couple of slipped slates too so you may have to do some rework on that roof. Is the membrane breathable that has been installed ..?? Makes me laugh though that the conservation idiot is worried about ridge tiles yet is happy with UPVC guttering ….. they really are a strange breed !! Yes it's a breathable membrane. Looks pretty recent suprisingly. Tell me about it. There are so many things that suprise me in what they will allow and what they wont. Link to comment Share on other sites More sharing options...
Redbeard Posted January 15, 2023 Share Posted January 15, 2023 If the ridge height is really a problem could you have a mainly-vaulted ceiling with just a tiny dropped area at the apex to allow an air-passage/'plenum chamber'? 1 Link to comment Share on other sites More sharing options...
tvrulesme Posted January 15, 2023 Author Share Posted January 15, 2023 16 minutes ago, Redbeard said: Eaves-to-eaves ventilation works for a huge number of roofs, but it may not work for your BCO... My house has no ridge ventiulation and no moisture issues (well, not in the roof....) Like the idea of this. Are there any holes or anything drilled in the main ridge to allow the moving air to pass through or does it just rely on natural gaps? Link to comment Share on other sites More sharing options...
tvrulesme Posted January 15, 2023 Author Share Posted January 15, 2023 13 minutes ago, Redbeard said: If the ridge height is really a problem could you have a mainly-vaulted ceiling with just a tiny dropped area at the apex to allow an air-passage/'plenum chamber'? Fantastic idea. I hadn’t thought of this. Link to comment Share on other sites More sharing options...
Temp Posted January 15, 2023 Share Posted January 15, 2023 If its a breathable membrane you may not need top vents. See what the BCO says. They are recommended if you have tight fitting slates (rather than say hand made clay tiles) but this is to ventilate above the membrane rather than below it. The void under the membrane can also be reduced to 25mm just to allow the membrane to drape correctly. 1 Link to comment Share on other sites More sharing options...
Temp Posted January 15, 2023 Share Posted January 15, 2023 A vapour barrier below the insulation will also help reduce risk of interstitial condensation. 1 Link to comment Share on other sites More sharing options...
Redbeard Posted January 15, 2023 Share Posted January 15, 2023 @Temp writes: "The void under the membrane can also be reduced to 25mm just to allow the membrane to drape correctly". This is the 'rule' used by many membrane companies and by most roofers retrofitting insulation over a room-in -the-roof, and what I have used for many attic bedroom retrofits, but the most recent government guidance (https://www.gov.uk/government/publications/room-in-roof-insulation-riri-best-practice) suggests 50mm if I remember rightly. Link to comment Share on other sites More sharing options...
PeterW Posted January 15, 2023 Share Posted January 15, 2023 TBH if I was putting additional load on those rafters I would be putting in collar ties and then you get the semi vaulted ceiling by design. You’ll need minimum 100mm between and 35mm over to get close to building regs and may then get away with replacing one or two slates on the hip end with nice lead vents - one at the hip and one high up at the far end should allow airflow and keep BCO satisfied https://www.leadworx.co.uk/shop/lead-roof-vents/lead-flat-top-roof-vents These guys make some lovely stuff..!! Should be a conservation idiots wet dream … 1 Link to comment Share on other sites More sharing options...
Temp Posted January 15, 2023 Share Posted January 15, 2023 4 hours ago, Redbeard said: @Temp writes: "The void under the membrane can also be reduced to 25mm just to allow the membrane to drape correctly". This is the 'rule' used by many membrane companies and by most roofers retrofitting insulation over a room-in -the-roof, and what I have used for many attic bedroom retrofits, but the most recent government guidance (https://www.gov.uk/government/publications/room-in-roof-insulation-riri-best-practice) suggests 50mm if I remember rightly. I've not see that document before. It does indeed say 50mm on page 21. It also suggests a second membrane is needed when fitting some types of insulation, presumably wool like insulation rather than rigid. Never seen that done. Quote Figure 4: Typical through ventilation of rafters and roof void Retrofit designer: A minimum of 50mm ventilation gap should be retained between insulation and membrane which would then mean that a vapour permeable wind-tight membrane may be required to protect some insulation products to avoid thermal bypass from the ventilated cavity . The only exception is warm flat roofs where the insulation is over the joists, these do not require ventilation. Link to comment Share on other sites More sharing options...
tvrulesme Posted January 16, 2023 Author Share Posted January 16, 2023 (edited) 12 hours ago, PeterW said: TBH if I was putting additional load on those rafters I would be putting in collar ties and then you get the semi vaulted ceiling by design. You’ll need minimum 100mm between and 35mm over to get close to building regs and may then get away with replacing one or two slates on the hip end with nice lead vents - one at the hip and one high up at the far end should allow airflow and keep BCO satisfied https://www.leadworx.co.uk/shop/lead-roof-vents/lead-flat-top-roof-vents These guys make some lovely stuff..!! Should be a conservation idiots wet dream … Wow they are lovely. Pricey but lovely. I'll have to consider myself a conservation idiot 😉 Not a bad shout on the collar ties at all. That could work really nicely. Having to meet 0.16 U value now so looking at these options: 100mm between 60mm over 125mm between 40mm over or 150mm between 25mm over Suprising (or maybe not) how the price varies between standard and non standard thickness of PIR Edited January 16, 2023 by tvrulesme Link to comment Share on other sites More sharing options...
PeterW Posted January 16, 2023 Share Posted January 16, 2023 41 minutes ago, tvrulesme said: Having to meet 0.16 U value now Do you have to given it is a listed building ..? I thought there were allowances ..?? 42 minutes ago, tvrulesme said: Pricey but lovely. That’s cheap..! A standard plastic one is £20+ these days and look crap .! Link to comment Share on other sites More sharing options...
tvrulesme Posted January 16, 2023 Author Share Posted January 16, 2023 1 hour ago, PeterW said: Do you have to given it is a listed building ..? I thought there were allowances ..?? You're right, I could pull the Listed Building card and be completely exempt from most Building Control but I've found that it helps if you "try your best". In terms of the Roof/Ceiling height I have a lot to play with so in the grand scheme of things I may as well comply with Building Control on this ceiling aspect in order to play my cards for exemptions elsewhere. It's like a game of poker... Link to comment Share on other sites More sharing options...
PeterW Posted January 16, 2023 Share Posted January 16, 2023 In that case why not look at a scissor truss type setup ..? It would give you the feature ceiling but then give you the rafter depth and allow you to sort out any issues with getting enough insulation in on the sloping sides of the ceiling. Link to comment Share on other sites More sharing options...
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