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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Flat roof ventilation https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1283787836 Message started by Lectrician on Sep 6^th, 2010, 4:43pm

Title: Flat roof ventilation
Post by Lectrician on Sep 6^th, 2010, 4:43pm

I am still looking to make my garage a warmer zone.

The ceiling height in the garage is 3m. It is a flat roof with board directly onto the underside of the 5" joists. No insulation or ventilation currently.

If I remove the boarding currently on the ceiling and place a false ceiling around a foot lower than the joists, would I then be best to insulate with rockwool or celotex? What ventilation would be required? There is only two faces that could have an air bricks - the front and one side, and prefferably I would only but them on the one side and not the front?

The rear of the garage is already a dining room, so there is wall half way up the garage splitting it into two.

Any info would be great!

[gallery fullsize]Lectrician/1283787580.png[/gallery]

Title: Re: Flat roof ventilation
Post by CWatters on Sep 7^th, 2010, 11:08pm

The official answer to the ventilation question is...

http://www.tendringdc.gov.uk/NR/rdonlyres/691A4679-6F62-448D-A0BF-A12842BD63A7/6781/Flatroofguideforwebsite.pdf

http://www.stockton.gov.uk/resources/environment/bldgcontrol/bcguidenotes/condroof.pdf

and they say if the insulation is between the rafters it's a so called "cold roof" construction which should be ventilated to stop condensation forming. If you can't ventilate the eaves on two oposing sides (because one side joins the house) then you should put a line of vents near that side. Something like these..

http://www.areco.co.uk/vents.htm

These are installed so they ventilate the 50mm void above the insulation not the room.

Then there is the "what can I get away with" type of answer if Building Control aren't involved.... Many people have just stuffed insulation between the rafters, not bothered with ventilation and have not have a problem with condensation in the roof. I've no idea what percentage of people get away with out a problem but presumably Building Control were seeing enough to write a guidance note? If you decide to risk it perhaps best put up a vapour barrier under the rafters before the plasterboard goes back.

Title: Re: Flat roof ventilation
Post by Lectrician on Sep 8^th, 2010, 12:58pm

Thanks.

As I am planning to drop the ceiling and create a fairly large void, air will be able to flow below the joists, so I am assuming I would not need to place air vents in the roof in every joist bay?

Where the joists sit on the breeze block wall, they finish flush with the render on the outside, and a facia is then nailed to them - there is no soffit to speak of, so not sure how I would vent that side......

I think I may just do a warm roof again - the BCO asked me to do that last time when doing the dining room in the end of the garage. He was happy with celotex rammed tight to the underside of the roof, between the joists, with mastic run down each side of the celtex too. Probably going to be easier that way, although I know now strictly correct as it should be above the joists really.....

Title: Re: Flat roof ventilation
Post by woodsmith on Sep 8^th, 2010, 6:31pm

Lec as long as you fit a good vapour barrier in then a warm roof should be fine, the only problem would be if your roof leaked, then the water has no way of evaporating away and the vapour barrier would stop you knowing you had a problem until it got drastic.

Title: Re: Flat roof ventilation
Post by 01Sam on Oct 25^th, 2010, 10:15am

If there will be no proper ventilation and there is no way for air to go out than it would be hard to take the fresh air there should be proper ventilation which is good for health and for the property as well. The design that you made is really good. You can follow this for a good building structure.