Constructing a warm flat roof

[Tristar]

My son has a poorly constructed (pitched) roof on a small extension on the rear of his terraced house. We wish to replace this with a warm flat roof.

Warm roof construction requires that both inner and outer perimeter walls meet the insulation layer above the joists to avoid cold bridging. The simplest way to achieve this appears to be to build the joists into the inner leaf of the outer extension wall, rather than sitting on a wall plate as in a cold roof. Am I correct in this assumption?

 

[joe-90]

Flat roofs knock value off the house. Better sorting the pitched roof.

 

[Tristar]

Thanks Joe, but as the lower edge of the ceiling is only 5' 10" above the floor level, it is not the best use of the area as an added part of the kitchen where it is intended to move the sink. (The extension used to house a small bathroom, since moved upstairs.) The extension is only 1.3m to the rear, and 2.5m, the width of the kitchen. Also the roof was very poorly constructed, with a shallow pitch, 2" deep rafters, and no insulation. The joist/rafters are partly supported by a window frame. the whole roof is disgraceful. A properly constructed warm roof with a 10 degree fall will be much better.

Do you agree, in this case, that my proposal to build in the joists is correct?

 

[^woody^]

Will the joists need to pass over the wall to fix fascia to?

Anyway, lay the joists, put some noggins in near the end to prevent twisting, strap every fourth one down, and then fill the ends over the wall with a load of quilt insulation. No need to block them in

 

[Tristar]

Thanks Woody, I am proposing to use Celotex T4000 as the insulation, and no I don't need the ends to fix barge boards to, so I hope to stop them in the inner leaf. Noggins are also required to seal the Celotex joints to.