Warm roof construction

[kaydeejay42]

I am currently building a single story extension that will eventually run across the back and down the side of our semi-detached property. I have been looking at the construction of the warm roof and am a little confused as to the construction.

Does the insulation need to be 'sandwiched' between 2 sheets of ply or does just 1 sheet directly on top of the joists suffice, and what thicknesses do the ply sheets need to be?

I have seen that kingspan do an insulation with ply attached which is 6mm is this sufficient to then be able to walk on?

I need to be able to have access to the roof for general maintenance and to render the first floor exterior.

Can anybody confirm the ideal construction of this type of roof for me please?

 

[DevilDamo]

With 50 x 150mm rafters, you'd look at placing 100mm between the rafters and 52.5mm below the rafters. This will achieve a u-value of 0.18 in accordance with the building regulations.

There are other alternatives and some offer smaller thicknesses, so less impact below the rafters.

By the way, you asked about 6mm being sufficient to walk on. If that's what you're aiming for, then that's a cold roof and you would need approx. 300mm insulation at ceiling level to achieve a building regulation compliant u-value of 0.16.

 

[noseall]

A 'warm roof' is where the insulation sits wholly above the roof joists or rafters and is not vented. What you described i.e. 6mm ply bonded insulation boards are often used for flat roof warm deck applications and are weight bearing.
You will prolly be asked to fit a further 25mm pushed up tight against the underside of the insulation or fit the full 125mm across the joists.

What DOH designs is describing is a cold roof and would require venting.

 

[kaydeejay42]

I have been looking at a warm roof construction that has a ply sheet sitting on top of the joists and then the insulation on top of that with a second (optional layer?) of ply on top of that. There is also a vapour barrier in there somewhere?

Can anyone enlighten me on this type of construction?

 

[kaydeejay42]

Noseall,

Think I was posting at the same time as you!

What you describe is what I was looking at, what is the best way to construct this type of roof?

 

[noseall]

Look here.

 

[DevilDamo]

What DOH designs is describing is a cold roof and would require venting.

I described both. Warm roof... insulation at rafter level (either above or between/under) and cold roof... insulation at ceiling level.

 

[noseall]

What DOH designs is describing is a cold roof and would require venting.

I described both. Warm roof... insulation at rafter level (either above or between/under) and cold roof... insulation at ceiling level.

A warm roof does not have the insulation between or under. B.C make an exception to allow 25mm to be pushed up tight against a 100mm warm roof deck. End of.

If the entire required thickness of insualtive material is between or beneath the joists or rafters then it is not classed as a warm roof.

As i have said a 'warm roof' is where the insulative material sits wholly above the rafters or joists.
You need to stop misleading the o.p. and swot up.

 

[noseall]

I described both.

No. You did not.....

With 50 x 150mm rafters, you'd look at placing 100mm between the rafters and 52.5mm below the rafters. This will achieve a u-value of 0.18 in accordance with the building regulations.

There are other alternatives and some offer smaller thicknesses, so less impact below the rafters.

By the way, you asked about 6mm being sufficient to walk on. If that's what you're aiming for, then that's a cold roof and you would need approx. 300mm insulation at ceiling level to achieve a building regulation compliant u-value of 0.16.

 

[DevilDamo]

You've quoted it too! The first line... a warm roof. The third/last line... a cold roof.

 

[jeds]

You've quoted it too! The first line... a warm roof. The third/last line... a cold roof.

You are misunderstanding. A 'warm roof' is a specific type of construction that cannot have insulation under or between rafters. It can ONLY have insulation wholly over rafters (or joists) in which case the whole of the roof structure remains warm and therefore does not need ventilation.

 

[noseall]

I hope you (DOH designs) do not design buildings!

 

[DevilDamo]

Typically for a warm off, the insulation goes above the structure, but it can also go between...

"Lay insulation between the rafters, leaving a minimum gap of 50mm above the insulation and below the roof underlay. This then allows a ventilation path between the eaves and the ridge. This detail is sometimes also referred to as a "warm roof".

I have also had Building Control check it as I raised it a hell of a long time ago and on 99% of my jobs, that is where it goes, which... gets approved, built and signed off.

 

[freddiemercurystwin]

If the insulation is going in between the joists such as in a garage conversion then its not a warm roof but a hybrid.

 

[noseall]

It is my understanding that the o.p. is constructing a flat roof hence the reference to 6mm ply bonded insulation board.

In 100% of new build flatties a warm deck is the simplest way to go as it alleviates the need for awkward venting etc.

Judging by your wording DOH (mentioning rafters and not joists in your posts) i think your mind-set is regarding pitched roofs where it is more common to fit the insulation between the rafters because it is far easier to vent via airgaps and breathable membrane and is the better solution practically speaking.