Advice - Roofing Air Gap Issues - Cold Roof

[Jake9]

Looking for some advice on roofing if I may:

I’m building a cabin with cold roof in SW UK and concerned about the air gap between insulation and OSB deck.

The roof comprises (inside to outside):
12mm Q mark plywood
Thick vapour barrier (black DPM) that is fitted best I could but without butyl tape to seal screw holes and stapler holes.
170mm deep rafter filled with 150mm PIR insulation which only gives me a 20mm air gap (50mm is recommended building regs). I messed up the materials order.
That’s where I’ve left it.

Roof is 10 degrees single pitch roof
Spans 4.2 meters
I’ve already ordered 32/1000mm box profile metal roofing sheets
High usage holiday cabin

I can see 3 options going forward:

A) No OSB on top of rafters - just sarking felt, counter battens then metal roofing
Q) is this weather proof enough? Warm enough? Too much airflow?


B) OSB fixed to rafters (20mm air gap), breather membrane, cross batten then metal box profile roofing. Gaps on front fascia and eaves for airflow.
Q) enough airflow? 50mm is building regs minimum

C) option B but with battens on top of rafters to lift OSB and increase air flow (cross and through ventilation)
Q) is this too much airflow?)

Any advice or thoughts much appreciated. I know cold roofs are prone to moisture build up so I want to get this right.
Cheers,
Jake

 

[DAZB]

Option C to increase the airflow would be my choice but you may also need to insulate below the OSB internally or you may get thermal bridging occuring.
Far better option is to build a warm roof and there are plenty of methods on the web such as this :

Warm and Cold Flat Roof Construction | Diamond Fibre Glass

Provider of warm roof construction & cold roof construction in west midland for over 25 years, Aspect has the experience to deliver a lightweight, extremely strong, robust material. Get your quote today!

diamondfibreglass.co.uk

ignore the fact it is a fibreglass website it is the methods shown which demonstrate the build up of each type.

 

[Jake9]

Thanks for that DAZB,

I've already fixed the ply and vapour barrier with no thermal bridging consideration for the rafters. It had to suffice due to budget restrictions. I've stopped after insulating between the rafters.

Warm roof would have been too tall and I wanted the internal headroom.

What's the need for OSB? I can only really think that it serves to help with anti-racking, but the battens for roofing and 12mm ply ceiling ply are making it strong and rigid. Can't see it being much of an insulator.

Thanks,

J