Ventillation gaps in firrings

[electrogear]

I'm in the process of building a 5m x 4m extenstion with cold deck flat roof and 3m x 2m roof lantern. It's occured to me today that many of the roof timbers between the roof lantern and the adjoining wall of the house will not be ventillated and the other timbers will only be ventillated from one end. There will be continuous soffit vents on all three sides so I need to find a way of creating a path through the blocked timbers.

The only options I can think of are:-

1) Notch the joist (200mm joist so only 30mm notches allowable)
2) Gaps between the firrings (supporting 18mm ply felt roof). I'll be using firrings from 18mm at the outer edges to 67mm in the centre.

Does anyone have any thoughts?

 

[freddiemercurystwin]

Are you able to fit an abutment ventilator in?

 

[^woody^]

Why have you got soffit vents on three sides? It's either one or two depending on the joist direction. But the lantern may thwart your plans in any case.

 

[Notch7]

I guess its a tiled skirt style roof?

I do them as a warm roof by doing counterbattens below the tile battens. There are detail drawings on the tyvek site.

 

[electrogear]

Are you able to fit an abutment ventilator in?

Thanks! That is what I was looking for, I just didn't know it existed! Cheers It solves most of the problems, but I still need some sort of cross ventillation between the joists around the roof lantern to allow air flow through.

 

[freddiemercurystwin]

You have a flat roof abutting a wall on one side presumably, so your joists should be running in one direction, so you have vents either on each side or at the abutment and the other end, depending on whether your joists run side to side or front to back, no need for three sides of venting as mentioned. So on two opposite sides of the rooflight abutments you have additional vents, I don't see the problem.

 

[electrogear]

Why have you got soffit vents on three sides? It's either one or two depending on the joist direction. But the lantern may thwart your plans in any case.

I haven't yet. I planned to have soffit vents on all three sides to allow ventillation to the eaves overhang dead spots (plus now the abutment vent). Is that not necessary?

 

[electrogear]

I guess its a tiled skirt style roof?

I do them as a warm roof by doing counterbattens below the tile battens. There are detail drawings on the tyvek site.

No it's a cold deck flat roof with bitumen felt. It needs to tie in with the adjacent garage roof on one corner so having a warm deck would mean a drop in level which wouldn't look right. I did consider a warm deck right the way through including replacing the garage roof but a) budget was tight and b) I've seen a few extensions with bifolds and warm decks and I don't like the huge space above the doors. I'm putting 140mm insulation between the joists and 25mm insulation backed plasterboard underneath which is ample considering the old flat roog had no insulation at all!

 

[electrogear]

You have a flat roof abutting a wall on one side presumably, so your joists should be running in one direction, so you have vents either on each side or at the abutment and the other end, depending on whether your joists run side to side or front to back, no need for three sides of venting as mentioned. So on two opposite sides of the rooflight abutments you have additional vents, I don't see the problem.

The problem is air flow. You don't get proper ventillation with just one side of the rooflight abutments vented.

 

[freddiemercurystwin]

 

[electrogear]

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

 

[electrogear]

View attachment 139745

Oh, so you mean 3 abutment vents (one either side of the roof lantern and one against the adjoining wall?) How do the abutment vents work with the upstand?

 

[electrogear]

Thanks for your help by the way.

 

[freddiemercurystwin]

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!

 

[^woody^]

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