Firings & voids in flat roof build up

[GlynM]

Hi, I am out for tender for my flat roof currently, its on two levels designed as a warm roof behind parapets and contains rooflights and a sedum roof to parts. The house is a timber frame with timber I joists supporting a flat ply deck and then I plan on waterproofing over the insulation (either EPDM, Felt or Liquid - still undecided).

 

My question is in part around my debate to go with insulation cut to falls or firings and a level insulation to create a fall. In either scenario I have been looking for the roof to fall to one side and then run in a shallow gully to at least 2 outfalls per level.

 

I've queried with the frame manufacturer whether firings could be installed on the roof joists with the deck above this then creating the required falls. Their response has been that they need the ply directly onto the joists to prevent against racking and that should I go with a firings option then these should be fixed on top of their flat deck with another ply deck to support the insulation placed on top of this. My concern is that I'll be creating an un-vented void here and whilst I believe the dew point will not be within this void, I have concerns about building in this detail.

 

Are the frame manufacturers correct in the racking comment and this build up with a void is sensible (apart from the cost of the additional deck) or would I be building in problems?

 

The other option is I go for tapered insulation directly on the flat deck and avoid this void completely, but as I understand it, this is a more expensive option than firings?

 

Any help/ opinion would be much appreciated. 

[Mr Punter]

I have a similar roof buildup. I joists installed flat, chipboard decking, firrings, OSB, VCL (polythene), insulation, OSB, waterproofing, tiles on adjustable pedestals.

 

Tapered insulation is expensive.

 

Unvented voids are not an issue.  That is what you have in joist zones of upper floors.

[ToughButterCup]

4 hours ago, GlynM said:

[...]

go with insulation cut to falls or firings and a level insulation to create a fall.

[or]

[...]

go for tapered insulation directly on the flat deck

 

Ring Chris Perfect (I kid you not: his name is as written) at Kingspan and ask for a tapered insulation design and quotation. All he'll need is a plan and your target U value.

Smashing bloke. But costly but ...... 01544387333.

Once you have that you'll be able to compare prices. Loads of others do tapered insulation - Kingspan's was the best overall offering for what we needed.

[GlynM]

Thanks for your assistance and for the contact at Kingspan - I'll follow it up.

 

Good to know the unvented voids are not an issue - one of the potential roofers was expressing some concern about this arrangement saying he'd never seen this type of arrangement.

[Temp]

On 30/10/2019 at 11:38, GlynM said:

 

I've queried with the frame manufacturer whether firings could be installed on the roof joists with the deck above this then creating the required falls. Their response has been that they need the ply directly onto the joists to prevent against racking and that should I go with a firings option then these should be fixed on top of their flat deck with another ply deck to support the insulation placed on top of this.

 

I think they are (over) concerned the ply wouldn't be well fixed to the joists due to firings in between. Perhaps could be solved by tapering the joists or raising one end of the joist (firings on the underside to level the plasterboard?). 

Edited November 7, 2019 by Temp