flat roof insulation buildup with deck

[lookseehear]

We are discussing flat roof insulation strategy with our architect for our extension which will have a deck on top of it. We are limited in that we will have to excavate deeper for the extension if we need additional thickness in the build up because the finished deck needs to be at or below the height of the internal floor level.

 

The architect is suggesting steico wood fibre, which I'm supportive of for decrement delay reasons and has suggested the below.

 

The vertical difference between existing ground and first floor levels is 2500mm. Building regs maximum risers for steps is 190 mm , 2 steps equal 360mm. 2500 + 360 = 2860.

360mm + 2300 ceiling headroom  = 2860

2860 – 2300 ceiling height = 560

Within 560 mm we could achieve ;

50- surface of deck below bifold door cill

65 – deck boards on spacers

75 – waterproof membrane laid to falls

80 – ventilation cavity

40 – external insulation layer

              200 – timber joists with full fill insulation

                 50 – service with plasterboard finish

             ------

             560

Using Steico’s technical data sheet 10/11/34 Ver 02 for this build up, giving a U value of 0.148 w/sq.mk.

 

The builder looked at this and said he would want to put in a skin of plywood or osb on top of the joists to lock the roof together, because it adds a huge amount of rigidity to the structure, but I'm apprehensive of this and would want to go with what steico have specced, particularly with respect to any ventilation.

 

Are we missing a trick with other insulation materials or buildups? We likely don't have the depth for a full warm roof because of the size of timbers required. I (and the architect) aren't particularly keen on using celotex post Grenfell if possible, but it might end up making most sense for practicality and cost reasons.

[Temp]

Personally I'm not a fan of timber decking. It invariably ends up wet and slippery unless you go for some sort of man made or recycled plastic product. Others may disagree.

 

Perhaps looks at something like Alupave although I've not used it myself. Bit expensive? Scratches?

 

https://www.toolstation.com/alupave-fireproof-full-seal-flat-roof-decking-board/p82295?store=null&utm_source=googleshopping&utm_medium=feed&utm_campaign=googleshoppingfeed&gad_source=1&gclid=Cj0KCQjwjNS3BhChARIsAOxBM6riUoZlamnaT2zDt4YrT16Ze47ZPqdlwRVJegk_wV_rrm0dDi82lsUaAvOwEALw_wcB&gclsrc=aw.ds

 

Something you need to consider is how to mount hand rails. I think I would look at the type that fix to the outer face of the walls and curve up over the overhanging verge. This avoids any fixings going through the waterproofing layer. Whatever you go for this needs thinking about and designing in before you build.

 

What your architect proposes is a ventilated cold deck. These are going out of fashion in favour of warm deck..

 

https://www.labcwarranty.co.uk/hubfs/Blog/Blog PDFs/Good practice guide for the installation of timber decks for flat roofs - LABCW.pdf

 

Quote

Cold deck roof For Warranty purposes, cold deck roofs are not recommended and an alternative form of flat roof should be adopted. Where cold deck roofs are adopted, they must be limited to 3m².

 

It is possible to build a warm deck with some insulation between the rafters but it typically has to be limited to a ratio of about 1/3rd between and 2/3rd above, something like that. It's also recommended to try and get a condensation risk analysis done. The insulation makers might help with this.

 

Best I could come up with is.. 

 

38 - surface of deck below bifold door cill

50 - 25mm Alupave on 25mm Aluminium battens/spacers

75 – waterproof membrane laid to falls

20 - OSB

190 - insulation

200 - timber joists with 95mm insulation between. Gives a 105mm void parallel to joists only

12 - Plasterboard

 

Total 585

 

Gives you 190 + 95 = 285 insulation but would need to check 190 and 95mm is actually available.

 

Check your choice of insulation can be used under 20mm floating OSB 

 

 

Edited September 26 by Temp

[Temp]

Oops I messed up. Thats more than the 560 you said you had.

 

That would have to be..

 

38 - surface of deck below bifold door cill

50 - 25mm Alupave on 25mm Aluminium battens/spacers

75 – waterproof membrane laid to falls

20 - OSB

165 - insulation

200 - timber joists with 80mm insulation between. Gives a 120mm void parallel to joists only

12 - Plasterboard

 

Total 560

 

Gives you 165 + 80 = 245 insulation but would need to check 160 and 80mm is actually available.

 

 

 

Edited September 26 by Temp

[lookseehear]

Thank you for this. I'll look at it properly this evening. My preference is no drop from internal floor level but architect is keen on a small step down, so saving the 40-50mm could help there