Upstand on cold flat roof where to insulate

[rssteve]

Hi, i am planning my flat roof extension with roof terrace, the part that is not cover by the terrace will have an upstand for a roof window. I am going for a cold roof as I am very limited on the height of the roof, and also think it is much easier to fix the balustrade without cold bridging if the insulation is below the joist. I've got 30mm going below the joist then 100 between.

What I plan on the upstand is 4x2 to make a frame, osb on the outside, 100mm insulation between the 4x2, then on the inside another 30mm to join onto the 30mm on the ceiling, then line with plasterboard. Is this acceptable on the cold roof? Should I join onto the vapour control membrane on the ceiling and run it up to the roof window under the plasterboard?

 

[noseall]

Anyone mentions roof terrace and balustrade I shudder. The detailing and carrying out of such detailed work requires much effort from both the designer and the installer.#

If you think you can get permission and you can find a reputable experienced builder to put your drawings into practice - then good luck. Post the designers' and the builders' number on here.

Steer well clear of cold flat roofs too.

 

[freddiemercurystwin]

Anyone mentions roof terrace and balustrade I shudder. The detailing and carrying out of such detailed work requires much effort from both the designer and the installer.#

If you think you can get permission and you can find a reputable experienced builder to put your drawings into practice - then good luck. Post the designers' and the builders' number on here.

Steer well clear of cold flat roofs too.

Christ on a bike nosey in the grand scheme of top end architectural detailing a handrail atop a terrace is not that hard.

 

[noseall]

Christ on a bike nosey in the grand scheme of top end architectural detailing a handrail atop a terrace is not that hard.

The connection detail, i.e. where the guardrail fixing brackets pass through the roof material, is design and attention to detail - critical.

They have a similar failure rate to box gutters.

 

[freddiemercurystwin]

Not when I detail 'em!

 

[rssteve]

Thanks for the reply. I currently have planning permission so no worries there. The balustrade manufacture I plan on using seem to have the details sorted. Each post will have a 10mm plate that sits across 2 joist, there is then 4 angled brackets that sit tight against the joist, these will be bolted through with m12 bolts and big square washers. On top of the plate is an upright 60mm galv box section, this will protrude a minimum of 150mm above the finished roof height. The roof is then finished tight up to box section and the balustrade posts which are 48mm drop into the box section, I believe they use a chemical resin inside to fix the posts tight. As for waterproofing the posts have a welded collar that sits over this box section and diverts any rain over and onto the roof. I'm happy with the balustrade design.

My reasons for the cold roof are

1. my ground floor ffl to my upstairs soffit is only 4.6m, so using a warm roof construction leaves me very short of height for the door onto the terrace.
2. the warm roof would add a minimum of 80mm to the internal height of the roof window compared to the cold roof.
3. if the balustrade fixing came through the insulation of the warm roof I guess the cold metal on the warm side would be a huge condensation risk.
4. i'm having a twinwall flue fitted through the flat roof. regs state that if the twinwall through passes through a unventilated roof void, then a ventilated fire stop needs to be fitted above and below the void, this is to stop heat build up. This would mean a direct path from the room, out to the lead flashing above the flat roof leaving me with a draught. If however the roof void is ventilated like a cold roof is, the firestop plate fitted below the roof is unvented, so although the metal plate has outside air the other side, at least it will be airtight.

I understand the preferred choice is a warm roof but I can't seem to make it work, I am however grateful for any suggestions if I have missed something that may help me

 

[freddiemercurystwin]

Thing is, in nosey's defence, to make all this work and ensure the detail will not leak and the drunken rabble of students who have a party on your deck in twenty years time will not rock the balcony loose once you've moved house, a designer needs to be considering all of these issues before a manufacturer of the balustrade has been selected and the structural engineer has designed the roof structure almost before panning really – a completely coherent approach is required. Do stuff in the wrong order and you end up compromising somewhere.