DIY Garden Room Cold Roof Enquiry

[Shlumaan]

Hi there, I'm constructing my own Garden Room / Workshop at the back of my garden right up to the boundary under permitted development. My total height is going to be 2.5m at the tallest section, with flat cold roof, but to achieve this height I'm having to use 5x2s for the roof joists. As the span between the front and back wall is short enough (2.5m) it allows me to use 5x2s which will help to carry out this plan. However, I read somewhere that cold roof has to have a minimum of 50mm cavity. Is that required by building regs and cannot be any less than that? It would be ideal to use 90mm PIR boards for the roof which would leave 30mm cavity for airflow below the OSB roof sheets. If building regs require 50mm ventilation then I can either go down to 70mm insulation boards or use 6x2s and reduce height of my walls, the latter being least desirable as plan is already set out and I already struggled with low headers for french doors, window etc. if I go down to 70mm insulation will my roof be actually cold!? Any thoughts? Thanks!

 

[noseall]

Hi there, I'm constructing my own Garden Room / Workshop at the back of my garden right up to the boundary under permitted development. My total height is going to be 2.5m at the tallest section, with flat cold roof, but to achieve this height I'm having to use 5x2s for the roof joists. As the span between the front and back wall is short enough (2.5m) it allows me to use 5x2s which will help to carry out this plan. However, I read somewhere that cold roof has to have a minimum of 50mm cavity. Is that required by building regs and cannot be any less than that? It would be ideal to use 90mm PIR boards for the roof which would leave 30mm cavity for airflow below the OSB roof sheets. If building regs require 50mm ventilation then I can either go down to 70mm insulation boards or use 6x2s and reduce height of my walls, the latter being least desirable as plan is already set out and I already struggled with low headers for french doors, window etc. if I go down to 70mm insulation will my roof be actually cold!? Any thoughts? Thanks!

You will require insulation across the joists (prior to plaster boarding) as well as between. If between insulation height is an issue, it may be prudent to increase the insulation thickness that goes across the joists, to mitigate. Having said that, as long as the space above the insulation can be successfully vented and nothing obstruct the air movement, crack on.

 

[Shlumaan]

Thanks for that. Someone who calls himself 'garden room guru' on youtube uses 5x2s for rafters and insulates with 100mm PIR, but he butts the boards all the way up to the OSB roof sheets and says if there's no air, there's no need to worry about condensation. He claims that he has never gone back to solve damp issues even after years. Any thoughts about this? It would obviously leave me still with 20mm of cavity right above the plasterboards which would need ventilation I presume?

 

[mrrusty]

At that span you could just use 122mm SIPs https://www.simplysips.co.uk/product/2-44m-x-610mm-x-122mm-roof-sip-panel/

 

[Shlumaan]

are you saying i should fill the entire 120mm of 5x2s with insulation and without cavity for air flow?

 

[rhysjc]

What’s the floor area? If it’s 15m2 or less, or less than 30m2 but is a metre from the boundary or is non combustible, you needn’t worry about building regs. This is provided there’s no sleeping accommodation which I wouldn’t expect in a workshop anyway!

That said, either way you should do the right thing re insulation. I don’t think there’s any specific cavity required for ventilation, apply some common sense. Make sure you’re using vented soffits. Important to make sure that the insulation boards you use have a vapour control layer, else you’ll need to add one. Without this, moist air can condense where the insulation meets the cold air and you risk interstitial condensation.