Warm deck roof question on single storey extension

[ey143]

When constructing a flat roof single storey extension, exactly what part is defined as the 'warm deck'?

Secondly, how much space a) must there be and b) is there usually, between the internal ceiling and the outer weatherproof covering?

 

[denso13]

When constructing a flat roof single storey extension, exactly what part is defined as the 'warm deck'?

All of it. It's a term for the insulation being above the deck rather than below it (a cold deck).

how much space...is there usually, between the internal ceiling and the outer weatherproof covering?

The depth of the joists plus the insulation.

 

[ey143]

How many cm is that usually in total and the individual parts?

 

[AronSearle]

The type of insulation will affect the depth of insulation required, rigid foam would be typical, polystyrene is cheaper but needs to be thicker to meet regs.

Of the top of my head I think you need about 70-100mm of foam insulation to meet regs.

 

[DevilDamo]

If you insulated between and under the joists (i.e. cold roof) then you probably be looking at 100mm Celotex (for example) between the joists and 50mm under the joists. You need to provide min. 50mm air gap so would require min. 150mm deep joists. When you go under the joists, this can sometimes cause issues with fixing the plasterboard to the underside due to longer screws, etc...

Insulating above the joists as in a warm roof, you do not require any ventilation and you would be looking at 130mm Celotex above the the joists on ply with a single ply membrane (for example) in order to meet the regulations.

 

[ey143]

Is there a good link to diagrammatically show the layers and bits you mention as I'm having difficulty visualising.

If cost wasn't an issue (within reason ) what's the most efficient way or amount to insulate the ceilings eg 120mm instead of 100mm?

 

[noseall]

Is there a good link to diagrammatically show the layers and bits you mention as I'm having difficulty visualising.

Try here.

If cost wasn't an issue (within reason ) what's the most efficient way or amount to insulate the ceilings eg 120mm instead of 100mm?

The simplest way and one that is specced the most often is a warm deck, mainly due to the fact that no venting is required. As DOHdesigns said, 130mm above the joists is required to satisfy latest standards. BCO's will accept 100mm + a further 25mm pushed up tight against the underside of the deck.

 

[ey143]

My original question should have read which part of the flat roof is the 'deck' because I didn't know the whole thing wasn't a 'warm deck'.

Is the deck therefore quite simply the wooden joists running parallel with the goal post metal frame?

Then, as you say, any 130mm insulation above the joists/wooden beams makes that a warm deck and then immediately above that goes plywood or whatever the outer hard material is followed by weatherproofing?

Anything under the wooden joists is known as the cold deck and will consist of plasterboard attached to it?

Sorry to ask the basic questions, just want to make sure I've got my head around this which I think I am now.

Also, rigid foam = kingspan/celotex? I'm assuming this is not polystyrene as surely this stuff was banned because of fire/smoke risk right?

 

[Nakajo]

There are loads and loads of details available online - just look around