Ventilation for cold flat roof

[Alex Buxton]

I am facing with the situation in converting part of my new garage into my office. I need to vent the 50mm gap above the ceiling insulation on a cold deck flat roof. I only know that during the last building regulation inspection.

Roof is flat fibreglass 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 10 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?

Read more: https://www.diynot.com/diy/threads/venting-a-flat-roof.7063/#ixzz65NFV1v8p

 

[Notch7]

Drill out for circular vents is the quickest solution:

https://www.amazon.co.uk/White-Plastic-Round-Soffit-Fascia/dp/B00GOUG0X6

Please make sure you dont have downlighters protruding through into the void -Ive seen that on jobs, not good

 

[Alex Buxton]

many thanks, Notch7. Is it watertight? will rainwater sip in when strong wind?

I dont understand "Please make sure you have downlighters protruding through into the void", what does that mean?

Read more: https://www.diynot.com/diy/threads/ventilation-for-cold-flat-roof.533378/#ixzz65NSdLH3u

 

[Notch7]

many thanks, Notch7. Is it watertight? will rainwater sip in when strong wind?

I dont understand "Please make sure you have downlighters protruding through into the void", what does that mean?

Read more: https://www.diynot.com/diy/threads/ventilation-for-cold-flat-roof.533378/#ixzz65NSdLH3u

Sorry it should dont -edited.

I mean downlighters in the ceiling.

Im not sure about strong winds blowing water in, you have a point -I dont suppose your roof has gutter all round? -if so they could go just under.

 

[Alex Buxton]

on one side I have guttering, but not the opposing side.

Does over fascia vents work for a flat roof (FibreGlass) ? Does it cost a lot more work to retrofit?

 

[^woody^]

You need through ventilation, not just on one side.

 

[Alex Buxton]

Hi woody, correct, both sides are needed.

What would be the best option to do that retrospectively?

 

[Alex Buxton]

You need through ventilation, not just on one side.

Hi woody, correct, both sides are needed.

What would be the best option to do that retrospectively?

Read more: https://www.diynot.com/diy/threads/ventilation-for-cold-flat-roof.533378/#ixzz65YNsvK1x

 

[noseall]

Christ! Would it not be simpler you bite the bullet and have a warm roof?

 

[^woody^]

You will need soffit vents, or anything fitted in fascia risks letting rain through.

You will also need to be sure that the air is ducted across the undeside of the deck not all over the place in the roof and wall cavity

 

[Notch7]

You will need soffit vents, or anything fitted in fascia risks letting rain through

Yes thats the problem Ive found when trying to sort out incorrectly build cold roofs.

Back in the good old days flat roofs were 8 x 2 joists and a bit glass fibre pushed up.....none of this new fangled ventilation stuff.

 

[^woody^]

Yes thats the problem Ive found when trying to sort out incorrectly build cold roofs.

Back in the good old days flat roofs were 8 x 2 joists and a bit glass fibre pushed up.....none of this new fangled ventilation stuff.

Yes I often wonder whether this "ventilated cold roof condensation" malarky is actually made up, as I dont ever remember seeing roofs wrecked by lack of ventilation and as opposed to by some ongoing leak or felt failure.

 

[noseall]

Yes I often wonder whether this "ventilated cold roof condensation" malarky is actually made up, as I dont ever remember seeing roofs wrecked by lack of ventilation and as opposed to by some ongoing leak or felt failure.

We have stripped a few. Oddly, the ones with the old wooden boards seemed ok. The ones with ply deck had a lot of black spot mould on the back of the ply. It may be that you could not see the mould on the back of the timber boards but I'm not convinced. One of the consequences of the mould is that when the property is empty and unheated for any length of time, there is an unmistakeable mouldy pong in the air, which could be attributed to the flattie.

Other than mould nuisance, I'm not sure that there is any other detriment....

 

[Leofric]

There was a publication called something like ' Thermal insulation and condensation, avoiding the risks ' published by BRE I think which gave the principles of vapour barriers/thermal insulation in different sorts of construction ,unfortunately I don't have a copy now .

 

[Hash123]

Hello and Happy New Year.

I am facing a similar predicament - limited by 2.5m height restriction so went with the cold roof option. Walls are built from 140mm dense concrete blocks and will have 50mm insulated plasterboard stuck to them on the inside.

6x2 roof joists have just been added together with OSB decking. Waiting for the epdm roof to arrive to install. Will install 100mm PIR insulation in the joists, leaving 50mm gap between insulation and roof deck. There is no overhang so will not have anywhere for vented soffits.

I want to avoid putting vented mushrooms through the rubber room if possible.

My question is, how important is it to have this ventilation? If I use a good quality vapour control layer, such as the YBS foil tec, would that suffice? Would butting the insulation against the roof deck be another option?