Cold roof ventilation

[frank999]

Hi,

I am about to start on a loft conversion for a 1930' semi.

I wish to install some windows in the front pitched roof slope.

This part of the roof will be cold, with a 50mm void for ventilation between the breathable membrane and the insulation - allowing a path running up between the rafters, with vents on the softits. My plans state '5mm ridge ventilation'.
Concrete tiles will sit on battens with the membrane directly beneath the batens. (as a side question - I've seen the term counter batten used, is this simply to create a small void between the breathable membrane and the tiles - or is it something to do with sarking, which is on my old roof, but will be ripped off the small part that will stay)

I'm a bit confused by the term breathable membrane ? surely wouldn't the condensation simply dissipate through this - so why do I need a continuous path up to the ridge. An identical development appears to have no ventilation at the ridge.

If windows go in, assuming you need this ventilation 'path' will I need to drill vent holes around the trimmers. (another side question, I have been told there is a simple equation for strengthening trimming rafters when trimming at the head and sill of an opening, something like - for every rafter removed for trimming you add a rafter either side of the opening (bolted together adjacent to the opening). I assume this obviates the need to justify this by calculation.

Thanks.

View media item 48180

 

[freddiemercurystwin]

You're over complicating things again.

If you have a breathable membrane over your entire roof you don't need any other ventilation ridge eaves or daft holes in the rafters. That's the whole point, its breathable!

If you have a non breathable membrane then fit your ridge vents and eaves vents if none are present. Don't worry about the window breaching the ventilation path, its not worrying about.

Add an extra rafter of the same size each side bolted or screwed to the existing.

 

[frank999]

My building control plans state "5mm Continuous Ridge Ventilation", but I see no specification for roof tiles and what goes under them (ie batens and breathable membrane) - should BC plans state that, its very confusing to follow.

So nice and simple then - it needs no venting with a breathable membrane - from what your suggesting I dont even need soffit vents ?

From what I've read - this breathable membrane lets moisture out but no rain in, I assume in a bad storm with strong winds - some rain on rare occasions gets driven up under the tiles, but simply drips down the membrane to the gutter (I assume you drape it out over the eaves slightly ?)

Thanks

 

[freddiemercurystwin]

Oh Frank, you make it so painful!

Are you replacing your roof tiles and membrane as part of your loft conversion?

Or are the existing tiles being left in place?

If you are leaving your tiles in place what material is the membrane if there is one? Is it a breathable membrane or is it traditional felt?

 

[frank999]

Am I - sorry LOL, its just complete lack of practical experience, makes you realise how important apprenticeships are. Without hands on tutoring - the most simple of things have a multitude of questions, getting up on your roof and doing all this for the first time is quite daunting - to a pro - they could do it blind fold, I'm sure.

Yes, all the roof tiles (and sarking - I believe that is the 'boarding' under the tiles) are coming off leaving just the rafters, - from what I understand a breathable membrane is stapled down, then the battens, then concrete interlocking tiles.

I've not seen any loft conversions round here use felt ... is that still used ?

 

[freddiemercurystwin]

Right OK. You're using a breathable membrane so no need to use any other vents in the ridge eaves or anywhere. Ensure you leave a 25mm gap between the top of the rafters and the insulation, this ensures the membrane can drape down a little.

 

[frank999]

Thanks, the insulation is 50mm, so that will leave a 50mm gap, can I assume counter battening is to create that gap when their is no room between the rafters ?

 

[stuartturbo]

wow i had to put in 90mm in mine with a 50mm gap.
Spend the money and go up if you can space wise

 

[freddiemercurystwin]

50mm will not be enough unless your Inspector is blind, Normally its about 90mm between (with additional battens fixed to the underside of the rafters) and then a continuous 30mm layer or thereabouts.

 

[stuartturbo]

50mm will not be enough unless your Inspector is blind, Normally its about 90mm between (with additional battens fixed to the underside of the rafters) and then a continuous 30mm layer or thereabouts.

snap thats what i was told fit with silver foil also.
I was told this, ie, 50mm gap, then the insulation, then foil, then 50mm gap then 9mm plaster board, the a 3mm skim. All this is the cold side of the roof!!

 

[frank999]

My Plans state: "Insulation between rafters to be 50mm Celotex GA3050 with Celotex GA3060 to underside of rafters 12.5mm plasterboard fixed over and skimmed.

Rafters spec'd as C24 100x50mm, span on slope 2319mm, spacing 400mm, 35 deg.

(thanks for the blog ... nice to see others having a go too)