Steps to insulate garage roof….

[CotswoldDoItUpper]

Hello all. 
 

I have an uninsulated flat roof on my garage.
 

I also have about 10 spare sheets of 100mm PIR.
 

I want to insulate the garage roof so it’s not so hideously hot in the summer.

 

Walls are already cavity with 50mm full fill rock wool or similar.

 

The pic shows the roof structure. What else do I need (ventilation/breather membrane etc) to do this?

 

Thanks all!

[Marvin]

What's the roof finish felt?

 

[CotswoldDoItUpper]

18 minutes ago, Marvin said:

What's the roof finish felt?

 

Yes.

[PeterW]

You would need to cut back the wall edge and provide cross ventilation above the insulation and below the deck - min 50mm to be safe. Then VCL on inside of insulation and then board /skim. 
 

Alternate option is just put it over the felt, bonded on with any glue of choice, top with 9mm OSB and 150mm screws and washers into the deck and joists below. Extend the fascia’s up to the new level and EDPM over the whole lot. 

[CotswoldDoItUpper]

Thanks @PeterW. I guess option 1 gives a cold roof and option 2 gives a warm roof? 
 

if I were to turn it into a green roof at the same time would option 2 be better?

[Temp]

2 minutes ago, CotswoldDoItUpper said:

Thanks @PeterW. I guess option 1 gives a cold roof and option 2 gives a warm roof? 

Exactly.

 

https://www.cncbuildingcontrol.gov.uk/wp-content/uploads/2019/07/CNCD-053-Warm-Roofs-Easy-Guide.pdf

 

 

[PeterW]

5 minutes ago, CotswoldDoItUpper said:

if I were to turn it into a green roof at the same time would option 2 be better?

 As long as the structure will take the weight yes, but you’re talking about adding 200mm of height at that point - that’s a very big ugly fascia at 400mm or more !

[CotswoldDoItUpper]

2 minutes ago, PeterW said:

 As long as the structure will take the weight yes, but you’re talking about adding 200mm of height at that point - that’s a very big ugly fascia at 400mm or more !

Good point. Can I use a core bit to drill holes through rather then cutting down the whole wall? 2 x 50mm holes per gap? Then I can keep the facias as they are.

[CotswoldDoItUpper]

This is the outside - has had the vegetation cleaned off since this pic!  

[PeterW]

Nice parapet !! Any way you can lift the coping stones and relay them over the top of EDPM lapped onto the blocks. 

[CotswoldDoItUpper]

On 09/08/2022 at 15:49, PeterW said:

Nice parapet !! Any way you can lift the coping stones and relay them over the top of EDPM lapped onto the blocks. 

Possibly. I’m hoping that drilling out a few 50mm holes between each joist means I can insulate internally and then I can bung my sedum on top? 
 

how many holes between each joist; and how should I stop bugs/birds etc nesting in there?!?