Cold Pitch roof to warm flat roof detail

[Andrewb]

Good evening,

I've got a slightly tricky detail at the junction from a flat roof to a pitched roof, where I have a couple of height constraints. The floor level under the flat roof will step up to the floor level under the pitched roof, so I need to maintain head height on the couple of steps up. I also am trying to not make either roof any higher than they are.

My detail laps the warm flat roof PIR insulation over the beam / wall (depending where you are) and the mineral wool between the pitched roof rafters overlaps this.

In both the roof areas I am wanting to make the condensation risk low and avoid cold bridges between the 2 roof types.

Any feedback or suggestions on my design as it stands would be much appreciated!

[Tims]

I've got a very similar detail though mine is probably worse as it's a junction between a warm flat roof and an existing cold roof loft space with an existing Steel perlin that the warm roof is tied into (dormer bungalow). I had big worries about the steel being a cold bridge and a condensation magnet especially as its above the family bath room. In the end I wrapped the warm side of the steal in a VCL and used silly expensive Primoclima Tesconvana tape to seal the VCL to the steel and around all of the joists the VCL for the warm roof also attaches to the steel and wraps up over the PIR of the warm roof which is cut to butt right up to the steel. On the loft side (loft has VCL at just above ceiling level) 400mm of rock wool fills sides of steel and I've got some going over it. As its an area of concern I've looked at the loft side on cold days and there are no signs of condensation (non on the warm side either). With a warm roof to warm roof it should be be much easier to continue insulation over or at least mostly up to your steel at both cold faces and keep a continuous unbroken VCL under and around the steel. It was a faff for sure on mine. If as your detail shows your steel/beam is fully on the side and under full depth insulation you can possibly juts your VCL over the top of it, though I imaging it has the pitched roof rafters on it.

Edited February 19, 2023 by Tims
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[Andrewb]

Thanks for this, some useful things to think about there.

The VCL over the beam will probably be easier and I could continue it along the wall that the beam will be built into.

I'll actually have almost the same as you on the right hand side of the drawing where I join the existing pitched roof, so your detail is very good to see, thanks 

[Dave Jones]

id add some mushroom vents at the base of the pitched roof/flat roof junction to get some air flow.

[Andrewb]

On 20/02/2023 at 08:16, Dave Jones said:

id add some mushroom vents at the base of the pitched roof/flat roof junction to get some air flow.

good call, added to the drawing!

[Dave Jones]

id also thoroughly recommend using Kingspan Thermataper on the flat roof. Send them your plans and where you want the outlets and they will make the insulation with the correct falls. No need to mess about with firring strips etc. They also can supply it with glass fibre top which you can directly bond EDPM too saving a layer of OSB which isnt cheap these day.

 

https://www.kingspan.com/gb/en/products/insulation-boards/roof-insulation-boards/thermataper-tt47/