Roof Construction For A Vaulted Ceiling

[Jimmy_k]

Hi everyone, I’m looking for some advice for a single storey extension at the rear of my house. The build will extend 4.3m out the back and be 5.5m wide. The roof will have a ridge in the centre extending out to the back of the build. As this will have a vaulted ceiling, this will determine the way it’s constructed. 
I’m currently trying to figure out what the pitch of the roof will have to be to have the ridge below the upstairs window. This will be determined by the thickness of the roof. 
 

My question is, if I was to construct a warm roof, how much insulation would I need above and in between the rafters? And what size timber would I use? 
 

Please see the attached photo for reference. Also, rather than windows, it’ll be bi-folds at 3.8m across the back. 
 

Thanks in advance

[Lorenz]

Probably shouldn't say, but I see this term used a lot, do you mean an actual vaulted ceiling as in stone-brick arch or some wooden beams?

[Russell griffiths]

Due to the lack of height you might need to change your thinking on the warm roof. 

 

Lets say for arguments sake you end up with 200mm rafters then 120mm of insulation on top, then a counter batten then a roof batten. 

You will be up 370mm from internal ceiling height. 

 

You could go 200mm rafters with 200mm rockwool type stuff In between the rafters and 70mm pir insulation under the rafters. 

 

The easiest way to work it out is to get a long piece of roof batten and some chalk, go outside and draw your walls onto the house, then lay the roof batten against the house at the pitch you think will fit, use an angle finder on the batten to work out the pitch and give you a bit of an idea what you are looking at. 

 

An architect should be able to do this on his cad program. 

[ProDave]

Surely the most efficient (in terms is insulation for the thickness) will be something like I have, 200mm rafters, insulation between them, and 100mm solid insulation (in my case wood fibre) above the rafters, then membrane, battens, counter battens then roof covering.  You certainly don't want any roof structure with uninsulated gaps and certainly not a cold roof demanding a ventilation space.

 

And for the roof covering, box profile steel will give a lower profile and work at a lower angle than tiles.

 

This type of roof would normally be supported on a ridge beam, but your challenge will be engineering support for the ridge beam without a great thick pillar for it to sit on.

 

I just accepted a great big pillar between the windows

 

 

 

[Jimmy_k]

1 hour ago, Lorenz said:

Probably shouldn't say, but I see this term used a lot, do you mean an actual vaulted ceiling as in stone-brick arch or some wooden beams?

Basically that the ceiling follows the profile of the roof rather than being a standard, flat, horizontal ceiling. See ProDave’s picture. That’s the goal. 
 

 

1 hour ago, Russell griffiths said:

Due to the lack of height you might need to change your thinking on the warm roof. 

 

Lets say for arguments sake you end up with 200mm rafters then 120mm of insulation on top, then a counter batten then a roof batten. 

You will be up 370mm from internal ceiling height. 

 

You could go 200mm rafters with 200mm rockwool type stuff In between the rafters and 70mm pir insulation under the rafters. 

 

The easiest way to work it out is to get a long piece of roof batten and some chalk, go outside and draw your walls onto the house, then lay the roof batten against the house at the pitch you think will fit, use an angle finder on the batten to work out the pitch and give you a bit of an idea what you are looking at. 

 

An architect should be able to do this on his cad program. 

Is it necessary to use 200mm timber for the rafters? The structural engineer has said we only require 125mm timber. 
In your warm roof scenario would this have insulation between them or purely 120mm on top? 
 

 

32 minutes ago, ProDave said:

Surely the most efficient (in terms is insulation for the thickness) will be something like I have, 200mm rafters, insulation between them, and 100mm solid insulation (in my case wood fibre) above the rafters, then membrane, battens, counter battens then roof covering.  You certainly don't want any roof structure with uninsulated gaps and certainly not a cold roof demanding a ventilation space.

 

And for the roof covering, box profile steel will give a lower profile and work at a lower angle than tiles.

 

This type of roof would normally be supported on a ridge beam, but your challenge will be engineering support for the ridge beam without a great thick pillar for it to sit on.

 

I just accepted a great big pillar between the windows

 

 

 

Thanks Dave,

the structural engineer has Ok’d this for the steel. We’re just waiting for him to come back to us as to whether it needs to be 152 or 203mm. 
As you can see, there’s a strip of steel running horizontally, preventing the pitched steels from spreading. 
As for the roof covering, it’ll have to be slate to match the rest of the house. 

[nod]

We have a similar setup on one of our builds 

I’ll form the front in either oak or steel 

To stop the rafters spreading 

[Russell griffiths]

I would say you would need 120mm on top of timbers, but you need someone to crunch the numbers for you. 

You don’t have insulation between the timbers on a warm roof, in between and on top or underneath is a bit of a hybrid, my extra insulation is underneath, Dave’s is on top, many ways to skin a cat, you need it to work in your situation with the height you have to play with. 

[Jimmy_k]

21 minutes ago, Russell griffiths said:

I would say you would need 120mm on top of timbers, but you need someone to crunch the numbers for you. 

You don’t have insulation between the timbers on a warm roof, in between and on top or underneath is a bit of a hybrid, my extra insulation is underneath, Dave’s is on top, many ways to skin a cat, you need it to work in your situation with the height you have to play with. 

Thanks Russell. 

 

https://www.uvalue-calculator.co.uk/calculator/pitched roof/insulation between and over rafters/unventilated/no sarking board/125mm/400mm/120/

I came across the above calculator on Kingspan’s website and it calculates the U-value depending on insulation thickness and a few other factors. If I select…

- Pitched roof 

- Insulation between and over rafters (for some reason above rafters alone isn’t an option)

- Unventilated

- No sarking board

- 125mm rafter depth

- 400mm centres

120mm insulation (60mm between and 60mm on top)

I get a U-value of 0.16W/m2.K

 

According to the document below, that’s good enough to comply with building regs. 
 

https://www.kingspan.com/gb/en-gb/products/insulation-boards/insulation-technical-hub/approved-document-l-england

[ProDave]

44 minutes ago, Russell griffiths said:

I would say you would need 120mm on top of timbers, but you need someone to crunch the numbers for you. 

You don’t have insulation between the timbers on a warm roof, in between and on top or underneath is a bit of a hybrid, my extra insulation is underneath, Dave’s is on top, many ways to skin a cat, you need it to work in your situation with the height you have to play with. 

So what do I have, warm roof, cold roof or something else?  Mine has insulation above the rafters AND insulation between them.  It just makes sense to me, part of my philosophy that the entire wall and roof structure should be adding insulation, no "wasted" gaps with no insulation in them.

[Temp]

13 minutes ago, ProDave said:

So what do I have, warm roof, cold roof or something else?  Mine has insulation above the rafters AND insulation between them.

 

Insulation above rafters makes it warm roof. You can have some insulation between them as well but care is needed because this effectively "moves" the rafters nearer the cold side. I think the rule of thumb is no more than 1/3rd of the insulation between the rafters and 2/3rds above? For safety some sort of condensation risk analysis should be done.

[Jimmy_k]

3 minutes ago, ProDave said:

So what do I have, warm roof, cold roof or something else?  Mine has insulation above the rafters AND insulation between them.  It just makes sense to me, part of my philosophy that the entire wall and roof structure should be adding insulation, no "wasted" gaps with no insulation in them.

Based on what I’ve read, Dave, I’d say yours is a hybrid. Although, from what info I can find on the kingsman website. The only roof listed as a warm roof is a flat roof with 120mm insulation above and above only. It says in the pdf for the kooltherm k7 pitched roof board that the thermal resistance of your above rafter insulation should be greater than that of the insulation between them. 
 

This picture is taken from the following document

 

https://www.kingspan.com/gb/en-gb/products/insulation-boards/resources/product-literature/kooltherm-k7-pitched-roof-board

 

[ProDave]

4 minutes ago, Temp said:

 

Insulation above rafters makes it warm roof. You can have some insulation between them as well but care is needed because this effectively "moves" the rafters nearer the cold side. I think the rule of thumb is no more than 1/3rd of the insulation between the rafters and 2/3rds above? For safety some sort of condensation risk analysis should be done.

Yes a condensation analysis was done on my wall and roof build up.

[Temp]

3 hours ago, Jimmy_k said:

My question is, if I was to construct a warm roof, how much insulation would I need above and in between the rafters? And what size timber would I use? 

 

My guess is you don't have a lot of height to spare as making the roof thicker reduces the pitch due to the window. Is it steep enough for the tiles? 

 

I'm thinking build it as a cold roof with all the insulation between the rafters. Fully fill them. Use a vapour permeable membrane that is approved for use in contact with the insulation (think kingspan do one). Then counter battens and tile battens. Ventilation needed under the tiles but not under the membrane if its vapour permeable. Put a vapour barrier on the inside. 

[Jimmy_k]

29 minutes ago, Temp said:

 

My guess is you don't have a lot of height to spare as making the roof thicker reduces the pitch due to the window. Is it steep enough for the tiles? 

 

I'm thinking build it as a cold roof with all the insulation between the rafters. Fully fill them. Use a vapour permeable membrane that is approved for use in contact with the insulation (think kingspan do one). Then counter battens and tile battens. Ventilation needed under the tiles but not under the membrane if its vapour permeable. Put a vapour barrier on the inside. 

I’ve just worked it out on the back wall and with 125mm timbers, 60mm insulation on top, 38mm counter batten, 19mm slate batten and then the slates (20mm), I can get a pitch of 20.5°. I’ll have to check but I think this is ok for slate. 
 

I’d much rather keep it a warm roof if possible as it’s much easier to add spotlights wherever needed. I’ll weigh up my options for a cold roof though and see what pitch I can get away with 

[Temp]

Conventional deep recessed downlights are rather out dated now. There are now quite a few surface mounted or shallow recessed LED lights worth looking at. Stuff like this although not for a sloping ceiling..

 

https://hartingtonheath.com/product/i-lumos-led-6-watt-round-recessed-lighting-panel-ultraslim-ceiling-light/?gclid=CjwKCAjwoNuGBhA8EiwAFxomAz3-cAj4jHcGoAhA-ry4HI2EjMIkKsKUW7BRJ22doJDE3kBz7A0nVhoCPKkQAvD_BwE

 

 

Edited June 26, 2021 by Temp

[Jimmy_k]

10 hours ago, Temp said:

Conventional deep recessed downlights are rather out dated now. There are now quite a few surface mounted or shallow recessed LED lights worth looking at. Stuff like this although not for a sloping ceiling..

 

https://hartingtonheath.com/product/i-lumos-led-6-watt-round-recessed-lighting-panel-ultraslim-ceiling-light/?gclid=CjwKCAjwoNuGBhA8EiwAFxomAz3-cAj4jHcGoAhA-ry4HI2EjMIkKsKUW7BRJ22doJDE3kBz7A0nVhoCPKkQAvD_BwE

 

 

Thanks for that. I had no idea.
We’re still a good way from electrics but I guess knowing about modern lighting at the early stages could influence the way we construct the roof! And, they’re cheap too!

[Temp]

The figure to look at is the lumen output. Around 400-500 lumen is equivalent to a 50W GU10. I usually go for Warm White versions.