Ventillation gaps in firrings

[electrogear]

Yes, you will have to see if a suitable vent can be sourced that lends itself to being fitted on the upstand of the rooflight, or abutment if you like, there are a few abutment vents available, the devils in the detail!

I have a problem there - the outside of the upstand is not flush with the joists below as we've had to triple/quad up the joists (the building inspector visited yesterday and threw that one into the mix after previously saying doubled up joists would be fine!!) so any attempt at an abutment vent would be futile. Besides, I've searched and I don't think one exists, nor can I find an example of any ventillation fitted externally to an upstand on google images. Can I ask is there anything wrong with my suggestion of firrings gaps? I could put a 2 inch gap near to the abutment of each joist with the roof lantern?

 

[^woody^]

This pretty much shows what I was thinking, but I don't fancy notching the joists - I'd rather have gaps in the firrings.

Where is that from? It's quite poor, for a pitched roof let alone flat.

 

[^woody^]

is there anything wrong with my suggestion of firrings gaps

Air won't travel sideways

 

[electrogear]

It's a faff. Fill it all with insulation, non of that cold roof nonsense

I've heard of that before, but aren't you then relying solely on the vapour barrier to prevent condensation build up? Any penetration of the insulation could potentially cause problems?

 

[freddiemercurystwin]

You need to educate yourself a little more, cold roofs went out with the ark!

 

[electrogear]

Air won't travel sideways

Nonsense.

 

[^woody^]

True story bro

 

[electrogear]

You need to educate yourself a little more, cold roofs went out with the ark!

I know the difference between cold/warm deck roofs and filling the joists with insulation doesn't make it a warm deck. As I've said above it's connecting to a garage roof which is cold deck and it would look daft having two levels of flat roof next to eachother (if I were to have a warm deck).

 

[^woody^]

And what if you keep the deck level and fill the void with insulation?

 

[Notch7]

I build orangery roofs all the time, to make them warm roofs, I use 4 x 2 or 6 x 2 ceiling joists. Then firrings, then 11mm osb, then 120mm celetex. The celetex should be worked out so it ends up higher than the main lantern beams ( tripled timbers in your case), that way some, albeit thinner celetex goes over the main beams.

The ceiling joists are so short that 4 x 2 is big enough and leaves enough room for downlighters.

The firrings need careful thinking around a lantern to avoid ponding. The highest point is probably the middle of the lantern between upstand and house.

If it was me, I wouldnt worry about matching the garage roof level unless you are trying to tie in a gutter run.

 

[electrogear]

I build orangery roofs all the time, to make them warm roofs, I use 4 x 2 or 6 x 2 ceiling joists. Then firrings, then 11mm osb, then 120mm celetex. The celetex should be worked out so it ends up higher than the main lantern beams ( tripled timbers in your case), that way some, albeit thinner celetex goes over the main beams.

The ceiling joists are so short that 4 x 2 is big enough and leaves enough room for downlighters.

Never thought of doing it like that!

To be honest I let my builder run with the project and I did discuss the roof at the outset but he said he thought a cold deck would be best to tie in with the garage and so there wasn't a big space above the bifolds. I have trusted his opinion on a few things where he's let me down unfortunately. He seems a bit old school in some respects. In hindsight I should've done more research upfront before all the ceiling joists went in and I might've come across something like you've suggested.

The firrings need careful thinking around a lantern to avoid ponding. The highest point is probably the middle of the lantern between upstand and house.

Yes, that's what I'd figured. With a 1 in 60 fall from the centre of the lantern it starts at 67mm in the centre and I was going to have it falling both ways down to 18mm at the edges which would enable it to meet the garage roof at the same level. The other side of the lantern could be pitched forwards which would mean having gutters on all 3 sides. I don't know if this is the best way to do it but it's the only way I can think of that results in the correct level at one side. Also where the fall is across the joists I was planning to use battens along the top of each joist rather than graduated firrings as I understand the minimum firring size when crossing joists (at 450 centres) is 38mm and that would make the roof higher than I want it to be at the edges.

If it was me, I wouldnt worry about matching the garage roof level unless you are trying to tie in a gutter run.

Yes, I'm tying in a gutter run/fascia.