Warm or cold roof

[Silage]

Currently at structural engineering stage for building regs submission and looking at roof covering loading. The choice of waterproofing is subjective and well documented; single ply, bituminous, EDPM, GRP - seems that everyone has a preference or horror story. Anyway more fundamentally my design is showing a cold roof design for a new build flat roof extension (parapet roof), however having researched flat roofs the majority of advice is to go for a warm roof to eliminate interstitial condensation (eg Bauder, NFRC) . As I chose the timber frame provider to avoid this issue at the sole plate I'm thinking that I'm walking into this problem up above. 

 

Any experience or thoughts welcome. 

[nod]

29 minutes ago, Silage said:

Currently at structural engineering stage for building regs submission and looking at roof covering loading. The choice of waterproofing is subjective and well documented; single ply, bituminous, EDPM, GRP - seems that everyone has a preference or horror story. Anyway more fundamentally my design is showing a cold roof design for a new build flat roof extension (parapet roof), however having researched flat roofs the majority of advice is to go for a warm roof to eliminate interstitial condensation (eg Bauder, NFRC) . As I chose the timber frame provider to avoid this issue at the sole plate I'm thinking that I'm walking into this problem up above. 

 

Any experience or thoughts welcome. 

 

[nod]

Warm roof all day long

I'm not a roofer but ive seen loads of condensation problems with cold roofs

With all the above mentioned

If you look at my back posts ive just completed a GRP roof First attempt

I used the multi layer insulation as I was trying to keep the roof lantern as low as possible   Otherwise I would have used 150 kingspan and as much as I could cram in on the underside

     Hope this helps

[lizzie]

59 minutes ago, Silage said:

Currently at structural engineering stage for building regs submission and looking at roof covering loading. The choice of waterproofing is subjective and well documented; single ply, bituminous, EDPM, GRP - seems that everyone has a preference or horror story. Anyway more fundamentally my design is showing a cold roof design for a new build flat roof extension (parapet roof), however having researched flat roofs the majority of advice is to go for a warm roof to eliminate interstitial condensation (eg Bauder, NFRC) . As I chose the timber frame provider to avoid this issue at the sole plate I'm thinking that I'm walking into this problem up above. 

 

Any experience or thoughts welcome. 

Warm roof. We have mbc frame and are putting on a single ply membrane as final finish. My bro in law is commercial roofing contractor he advised warm roof.

[Temp]

+1

 

Building control recommend warm where possible... 

 

https://www.tendringdc.gov.uk/sites/default/files/documents/planning/building control/Flat roof guide.pdf

 

With a warm deck roof there is less chance of thermal movement and condensation, it should therefore be considered as the standard form of construction for flat roofs.

[Silage]

@nod, @lizzie, @Temp, thanks for the response, great info. You've confirmed my thoughts and I'm pushing for warm over the proposed cold. Should still have 300mm posijoist that we can insulate below but tapered  pir on top of deck now to create a warm roof. Let's see if TF company can agree to that. I'm talking to Bauder regarding bituminous roofing system although they can supply single ply. 

 

Lizzie, I'd be interested in your MBC detail   Would you be able to pm me?

[Temp]

Need a bit of care if you plan to put putting some insulation above and some between joists/rafters. If you put lots of insulation between the rafters the temperature at the top of the rafter will fall possibly leading to condensation at that point. I forget what the rule of thumb is but it's something like put 2/3rds above and no more than 1/3rd between.  Perhaps someone else can comment.

[Silage]

Thanks @Temp, in that case I'll  look to revise, i guess that mean no/partial fill in the posi-joist and a decent pir thickness above. Not clear on how I'd get partial fill though. As above any other comments welcome. 

Edited July 18, 2017 by Silage

[jack]

If there's any insulation between the rafters, I believe it's no longer a cold roof. As Temp says, the danger is a condensation point that falls anywhere near the rafters.

 

Should be fine as long as there's enough insulation above, but get it modelled if you're seriously thinking about it. 

[Crofter]

Is this condensation risk a particular problem in flat roofs, where you have an impermeable outer membrane?

Just asking because both my wall and roof buildups have got insulation between frame studs/rafters as well as inside, but none outside. However I have a vapour barrier on the inside, and a breather membrane on the outside, so presumably any condensation on the outside is free to leave?

[jack]

Short answer, @Crofter, is yes for the reasons you mention.