Venting a flat roof

[Peebs]

Hi,

I am converting my flat roofed garage into a games room. It has 6m long rafters at 400mm centres with noggings halfway (at 3m). I plan to line the ceiling with plasterboard and insulate between the rafters (with a min 50mm gap between the insulaton and the flat roof), with a vapour barrier on the warm side.

The question is, do I need to provide cross ventilation in the roof space? The roof has fascia boards nailed onto the side of the rafters (no soffits etc). How do I vent the roof space? Which vents should I use? Is venting the roof space an absolute requirement if I use a vapour barrier?

Would it be better to use insulated plasterboard with an integral vapour barrier?

 

[masona]

Are you doing away with your garage door ?,if so where is the main entrance ?

 

[Peebs]

Hi masona,

The garage door has already gone This expanse (4.5m x 3m) is being filled with glazing; french doors with two large windows on a dwarf wall either side. Theres also a large window being installed on the side of the new room.

I had the BCO around earlier this week - he approved the proposed building work; my brickie then prepared the site ready for the glazing which will be coming on 10 March. He also screeded me a new insulated floor (4 inches thick) with DPM . His mate did his back in lugging a few tonnes of sand - that could've been me

I am trying to think ahead; what needs doing before I fit out the inside!

 

[masona]

The question is, do I need to provide cross ventilation in the roof space? The roof has fascia boards nailed onto the side of the rafters (no soffits etc). How do I vent the roof space?

The answer is yes,you use fascia vent which sit on top of fascia boards then roofing felt and tiles ontop of this vent,so the air flow underneath the tiles and into the roof space.

Not knowing your roof layout you could fit air brick vent if you have a wall at both side.
The reason I asked you regarding your garage door was if you have building permission
See this for more information.

 

[Peebs]

My builder seemed a bit miffed that I contacted the planning department, but then I suspect that he would wouldn't he

Thanks for your advice, after a bit of nifty negotiation, my builder agreed to install new fascia boards complete with vents. But he'd never heard of cross ventilation. Is this a new term or one that I made up?

 

[masona]

No,they been around a few years,see this.

My builder seemed a bit miffed that I contacted the planning department

Make no diifference,the building inspector will not let the builder's get away with it They will need to see how the roof is vented.

 

[nematoad]

sorry to drag this rather old thread up again but Im faced with exactly this situation in converting my new garage into my workshop/mancave/shrine to makita. I need to vent the 50mm gap above the ceiling insulation on a cold deck flat roof.
I would go whole hog and do a EPDM warm roof but the wife wants a bathroom and the roof only got redone 3 years ago.

Roof is flat felted so no over fascia vents like you would do with pitched roof.
The fascia wood is screwed directly onto the end of the roof joists and there are no soffits to put a vent into .
I dont really want to put mushroom vents on the roof as it will mean a lot of holes( between each of 16 joists) and look messy.

what I really need is a louvered fascia board that is weatherproof but allows air in.
Does such a thing exist or do I just fit a load of small louvres - which there isnt much room for as the felt comes down to only 4-5in above the top of the brick even though the fascia continues down below that?
I even considered fitting a extractor fan or do I just cuff it as its not going to be habitable space.

 

[tallrobw]

Hi nematoad, did you get anywhere? I'm now faced with the same issue and (obviously) looking for solutions! At the moment I'm considering extending the roof over a box consisting of vented sofits and fascia boards.

-Rob.