Help please!

[Porthole]

Please can I ask an urgent question before I meet builder this morning:

 

Should firings on warm deck flat roof sit on joists or insulation? 

 

If firings are wet when roof installed, does it matter? 

 

Thanks!!!!! 

[Thorfun]

2 minutes ago, Porthole said:

Please can I ask an urgent question before I meet builder this morning:

 

Should firings on warm deck flat roof sit on joists or insulation? 

 

If firings are wet when roof installed, does it matter? 

 

Thanks!!!!! 

the timber firings on our flat roofs sit on the joists. but you can also get insulation that has a built in fall so timber firings are not necessary.

 

as to whether it matters if they're wet or not I don't know the answer to that.

[SteamyTea]

Usually on joist I think.

Everything built in the UK gets wet, most houses stay up.

[markc]

Good practice to put firings on joists to reduce movement.

Most buildings and materials get wet during construction, just don’t seal the moisture in.

if the structure was wet when roof goes on, delay fitting soffits etc to allow drying.

[SteamyTea]

1 minute ago, markc said:

if the structure was wet when roof goes on, delay fitting soffits etc to allow drying.

Does depend if it is a warm or cold roof surely.

[Porthole]

It's a warm roof. He has put them on the insulation as the roof is large and needs to slope both side to side and front to back. Can't change it now if it is wrong unfortunately - says they do it all the time on newbuilds and it is all signed off by building inspector. Fingers crossed! 

[Radian]

If I'm understanding this correctly it means they've created an unventilated void, defined by the firrings, between the top of the cold side of the insulation and the underside of the upper deck. Generally we would not want significant volumes of air able to accumulate water vapour with nowhere to go except for those occasions when temperatures fall below the dew point.

[Temp]

3 hours ago, Porthole said:

It's a warm roof. He has put them on the insulation as the roof is large and needs to slope both side to side and front to back. Can't change it now if it is wrong unfortunately - says they do it all the time on newbuilds and it is all signed off by building inspector. Fingers crossed! 

 

If not already got one..  Consider adding a vapour barrier on the inside before plaster boarding to reduce chances of warm humid air getting from the room into this void.

[Radian]

2 hours ago, Temp said:

 

If not already got one..  Consider adding a vapour barrier on the inside before plaster boarding to reduce chances of warm humid air getting from the room into this void.

 

Maybe but there will always be a certain volume of water vapour in the air above the plasterboard, vapour pressure differentials will see to that. But by the very definition of a warm roof, the area from ceiling up is unventilated so the timescales for vapour equalisation will be greatly extended and therefore  out of phase with outside temperatures and RH levels. I fear there's nowhere you could place a vapour barrier in this kind of build-up other than on top of the insulation - except that that would just promote condensation directly below and collect on the deck supporting the insulation. Hopefully the insulation is tightly fitted together such that very little air exists at the cold side such that the quantities of vapour are small. This circles me back to why a largish void under the firrings sounds like a bad idea.

[Marvin]

14 hours ago, Porthole said:

Please can I ask an urgent question before I meet builder this morning:

 

Should firings on warm deck flat roof sit on joists or insulation? 

 

If firings are wet when roof installed, does it matter? 

 

Thanks!!!!! 

I'm confused with this. I assume your saying build up is joist, flat PIR, firings, boarding and final top coat?  If that is correct, it doesn't sound right to me. I'm with @Radian

 

M