Warm or cold flat roof for concrete deck with overhang?

[timbr]

Hi,

I have a single-skin brick extension with a concrete flat roof from the 50s/60s that I’m getting re-covered. The concrete roof overhangs the exterior by 100-150mm.

I’m trying to decide whether to go warm or cold roof. I originally thought warm but now I’m concerned about potential cold-bridging via the sides/underside of the concrete overhang, as I’m working on the assumption insulation will only go on top.

Given that the extension is single skin and I’ll be internally insulating the walls, am I best off treating the roof the same and opting for a cold roof?

Many thanks.

 

[Mr Chibs]

You could insulate the perimeter and underside toe of the concrete roof externally, anyway.
If you are insulating the interior of the walls, you may be better off insulating the inside of the roof.

Are you having an office/gym/workshop?

Any heating you apply inside will get to temperature sooner via internal insulation.

 

[timbr]

Thank you. I was wondering whether the sides/underside could be insulated but naturally this would increase the size of the overhang and I wasn’t sure if this was going to look silly and/or be problematic with the addition of fascia/guttering. Roofers we’ve spoken to only mentioned insulating the top and couldn’t provide a definitive answer with regards my query here.

Room will be a utility + bathroom w/ shower.

 

[^woody^]

You are much better off having a continuous lining of insulation on the walls and ceiling, otherwise its a major condensation risk on the inside.

You could potentially line the soffit and edge with the same thickness as the roof surface, but that will require good detailing and workmanship.

 

[timbr]

Thank you, yes that makes sense.

I just need to understand how to account for spotlights in the ceiling, which would have been easier with a warm roof. A false ceiling possibly and then air bricks is seemingly one possibility.

 

[Mr Chibs]

Don't chop holes in insulation for downlights, try to pick some other form of lighting or surface mount ones.