flat roof

[electrified]

constructing a warm roof will not be possible due to the height of the roof.

small joists prevent a cold roof.

so i was wondering if i could have an empty void with no insulation, moisture plasterboard if it helps.

I understand that i may need ventilation, and i would do this via fascia board vents is this right?

if there is anything else i may need could you let me know

thanks

 

[mikeey84]

If you are building this under a building notice, your inspector will take a particularly dim view of your plan.

 

[SpaceWorm]

What are you building? Extension, garden room etc

 

[electrified]

What are you building? Extension, garden room etc

extension

 

[electrified]

If you are building this under a building notice, your inspector will take a particularly dim view of your plan.

well warm roof will bulk it out about a foot, and cold roof you have to leave a gap, can’t remember how much. I can easily add vents on fascia board though.

 

[mikeey84]

well warm roof will bulk it out about a foot, and cold roof you have to leave a gap, can’t remember how much. I can easily add vents on fascia board though.

You could do a hybrid, or inverted roof. There is a thread a bit further down.

Basically insulation pushed up to underside of roof, level with bottom of joists, no gap, and a vapour
membrane across, then pb.

If you are getting it signed off, they won't let you not put insulation in.

 

[Notch7]

constructing a warm roof will not be possible due to the height of the roof.

small joists prevent a cold roof.

so i was wondering if i could have an empty void with no insulation, moisture plasterboard if it helps.

I understand that i may need ventilation, and i would do this via fascia board vents is this right?

if there is anything else i may need could you let me know

thanks

What is the roof size and span

maybe you could put in 2 steels at 1 third and 2 thirds, then say 4 x 2 joists and insulation over the top

 

[electrified]

You could do a hybrid, or inverted roof. There is a thread a bit further down.

Basically insulation pushed up to underside of roof, level with bottom of joists, no gap, and a vapour
membrane across, then pb.

If you are getting it signed off, they won't let you not put insulation in.

Hi i did put a post on the roofers section, and i said i would do a cold roof with 50mm insulation flush with the bottom of the joists, this leaves a 50mm air gap that i will ventilate with fascia vents on 3 sides, on the bottom of the joists i will put a vapour barriers and top it off with a vapour plasterboard. Does that seem okay to you or do i need to make any adjustments? Thanks

 

[electrified]

What is the roof size and span

maybe you could put in 2 steels at 1 third and 2 thirds, then say 4 x 2 joists and insulation over the top

what do you mean by two steels? how far thick will this make the ceiling?

if you read what i said to mikey just then, do you think that will be okay? Cheers

 

[Notch7]

what do you mean by two steels? how far thick will this make the ceiling?

if you read what i said to mikey just then, do you think that will be okay? Cheers

steel size depends on span.

 

[electrified]

steel size depends on span.

span of 4x2 from wall plate to wall plate is about 2 metres.

 

[Notch7]

Hi i did put a post on the roofers section, and i said i would do a cold roof with 50mm insulation flush with the bottom of the joists, this leaves a 50mm air gap that i will ventilate with fascia vents on 3 sides, on the bottom of the joists i will put a vapour barriers and top it off with a vapour plasterboard. Does that seem okay to you or do i need to make any adjustments? Thanks

yes it’s possible - cold roofs suffer condensation if ventilation is not good enough.


I’m not sure what insulation thickness would be - maybe 150mm


Why is height a problem if it’s a warm roof? - is there a low 1st floor window or planning height restriction

 

[electrified]

yes it’s possible - cold roofs suffer condensation if ventilation is not good enough.


I’m not sure what insulation thickness would be - maybe 150mm


Why is height a problem if it’s a warm roof? - is there a low 1st floor window or planning height restriction

inside floor of house is about 15cm higher. need to match up with this, also house has a pitched roof and we want the height of the extension to be at the last slate tile, so we can have the guttering same level, any higher will have a flat roof sticking out and will make the side profile look a bit weird. 4 inch is 100mm so hoping for 50mm insulation. Also it is going to be a bathroom so is going to have an extractor fan.

 

[Notch7]

span of 4x2 from wall plate to wall plate is about 2 metres.

That span only needs 4 x 2 anyway.

warm roof would be 95mm + 11mm OSB + 120mm insulation + 18mm OSB then add on firrings if joists are set plumb

is that too much roof thickness, 245mm

 

[Notch7]

inside floor of house is about 15cm higher. need to match up with this, also house has a pitched roof and we want the height of the extension to be at the last slate tile, so we can have the guttering same level, any higher will have a flat roof sticking out and will make the side profile look a bit weird. 4 inch is 100mm so hoping for 50mm insulation. Also it is going to be a bathroom so is going to have an extractor fan.

unfortunately the minimum flat roof insulation is 120mm for a warm roof to achieve 0.18 u value.

cold roof is more


Your only other option is to get a SAPs calculation for the whole house and prove any other house improvements done at the same time ( or at least recently) add up to the notional elemental value of the extension.