Venting through a ridge beam - cutting slits

11 Feb 2009
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United Kingdom
Man with all the steels here.

Its a hip to gable loft extension, pitched cold roof on front slope, flat warm roof at rear.

My BC Full plans application has been approved.

I only have 2100mm ceiling height in this configuration, I can regain another 100mm if I change the flat warm roof deck to a cold roof deck, but I need advise on the venting, the steel ridge would be directly under 150x38mm ridge timber beam, the 175x50mm flat roof joists bear into the webbing of the beam - with the odd cut to allow this, firring then 18mm WBP ply on top of the joist. Plasterboard under the joist. PIR insulation between the joists - leaving a 50mm void on top for venting.
On the Pitched side a LR breathable membrane will be used, with a 50mm void - a half/third round ridge tile will be mortared at top (the breathable membrane means their is no requirement for a 5mm vented ridge tile).

If I cut between the pitched roof rafters, a 25mm deep slit would that be acceptable by BC - this would allow condensation to vent from the flat roof. I have shown this slit just between the first few rafters.

Would there be any objection to the solid block struttingbeing just slightly lower to maintain a 50mm gap, obviously this would only occur to half of the strutts, where the firring is less than 50mm

I understand people have suggested stuffing the whole void between the joist with insulation - so their is no void for the condensation to occur, but reading has informed me this is only applicable to existing , not new roofs.

Some images attached.
The material to the bottom left of the steel is 25mm PIR.
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reading has informed me this is only applicable to existing , not new roofs.

Do some more reading.

What is the different between an old and a new roof?

Again, this is a result of using a designer who has not got a clue how to design for the customer .. and will just do something crap and easy to get the plans passed
Thanks for the strong tie tip - the regs state you need a 25mm continous gap or equivalent either end, or near enough if engineering does not allow. So that would maybe do the trick.

I read BS5250 for guidance - but there is nothing in there to help me design this.

I am sure the tip to fill the void with PIR - only applies to existing roofs, new roof must either be warm or cold decked roof designs.

Someone today thought that the breathable membrane would not allow enough condensation to premeate through - and the flat roof would fill with condensate from the pitched area. My plan was for condensation to travel into the pitched void and out.
Is there a limit to how much condensation a 'vapour permeable' breathable membrane can emit ?
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You do not need a breathable membrane under your flat roof and you can buy vapour proof plasterboard.

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