New warm deck flat roof

[Smath22]

Can anyone tell how much warm deck roofs cost per square metre?

We are going to have our garage converted into a study and utility, and want to turn the existing felt roof into a warn deck roof to meet building regs.

The existing felt appears to be in good order as are the boards under it.

The roof is roughly 6m x 3m.

We are located in the southwest (Exeter)

Many Thanks

 

[freddiemercurystwin]

Unless your BCO has been living in the dark ages or there is some other reason the chances are you should just be able to fill the voids between the rafters with Kingspan or Celotex up tight against the deck and that should satisfy Building Control.

 

[noseall]

I've often wondered which is simpler, cheaper and the least hassle?

If the existing felt could stay;
then either adding insulation and felting over this externally...

...or removing the ceiling and filling the void internally?

I hate cutting and fitting board insulation from below. It's dusty, noxious and gets in your eyes and everywhere else.

However, aesthetically speaking a big ugly deep fascia looks pants and you would still need to trim all drip and kerb flashings etc.

 

[Smath22]

Unless your BCO has been living in the dark ages or there is some other reason the chances are you should just be able to fill the voids between the rafters with Kingspan or Celotex up tight against the deck and that should satisfy Building Control.

I thought this would create a condensation problem ? That's why I was thinking warm deck?

 

[noseall]

I thought this would create a condensation problem ?

It has the potential to - yes.

 

[freddiemercurystwin]

Unless your BCO has been living in the dark ages or there is some other reason the chances are you should just be able to fill the voids between the rafters with Kingspan or Celotex up tight against the deck and that should satisfy Building Control.

I thought this would create a condensation problem ? That's why I was thinking warm deck?

Its a slight possibility but very unlikely and only because the joists are bridging, maybe a chance of some possible discolouration of the plasterboard where it fixes to the underside of the joists but as mentioned this is often accepted by BCO as being an economic if not ideal solution.

If the felt was in poor condition it makes sense to insulate over the top but otherwise I'd avoid it if pos or if cash is no issue.

 

[Smath22]

Well I have had one quote so far and that was for £4000 for the 18sqm for a warm deck with single ply membrane. Comes in at £222 a Sqm. Its seems expensive. If I could fill the void without the need to add ventilation that would be good.

 

[catlad]

I would ask your BCO if he would except joists filled with celotex/kingspan, and then insulated plaster board underneath which comes with a vapour barrier to reduce the risk of condensation.

 

[datarebal]

if you simply just fill the voids almost certainly you will get cold bridging.
you would need to consider say 40mm under the joist also.. probably your bco will say this too.
Personally i'd go warm deck ..but get another quote

 

[Smath22]

ok thanks, I would prefer warm deck as it would make life easier for running electric, network and tv cables, not to mention the instaltion of spotlights. However i cant afford 4K i was hoping for around 1.5K (around £85 sqm)but maybe im being too optismistic.

 

[ngk]

4k is too much get another quote.

 

[Roofer]

If as you say the existing felt is in good order then you can just lay (waterproof) insulation on top and weigh it down with a layer of stones. (Google ballasted roof)

 

[Xenon]

Well I have had one quote so far and that was for £4000

Warm roof in High Performance felt would be approx £45-£50 m2 using your existing membranes as a vapour barrier.

You can use inverted insulation as mentioned but will likely get root growth under it long term. Any inverted insulation will need to be closed cell, known as blue board. It can be obtained with a screed already on it and is interlocking.

http://building.dow.com/eu/gbr/en/pdf/291-40925.pdf

 

[Smath22]

Thanks for the reply if I can find someone to doit for £50 psqm that would be great.

 

[Paintguy]

If as you say the existing felt is in good order then you can just lay (waterproof) insulation on top and weigh it down with a layer of stones. (Google ballasted roof)

Sorry to butt in on someone else's topic, but is this an acceptable practice for retrofit too?

I have a flat roof over my old single story kitchen extension that appears to be uninsulated. We had a minor leak a while back so had a roofer check it out and he says the torched felt and boarding is fine, the leak was just from some flashing where it meets the house. I'm looking to insulate the roof and this method seems ideal, and certainly less invasive than ripping down the kitchen ceiling to insulate between the joists. I also like the idea of less possible thermal bridging from the warm roof method.

Would I also need to block up the soffit ventilation so that all the heat doesn't escape since it will now be a warm roof rather than a cold one?

Thanks