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Advice for vapor permeable roof sheathing on a flat roof

user-3916423 | Posted in General Questions on

We live just out of Melbourne in Australia. For reference, our climate zone is Cfb (temperate, without a dry season and with warm summers). Annual HDD (Farenheit, with base 65 degrees) is 2,838 degrees. Annual CDD is 450 degrees. It very rarely goes below freezing temperature.

We’re renovating our house, and I’m stuck with what to do with our roof. It’s a VERY flat metal roof. By design the pitch is 0.5 to 0.8 degrees. That’s not a typo. Current structure is 6 inch hardwood joists at 16” centres, that function as both ceiling and roof joists. Original T and G timber lined ceilings from the 1960’s below, battens above to give it its minimal fall, and then the metal cladding above. There is some aluminium sarking below but it’s in pretty bad shape. So technically, a ventilated roof, but probably ineffective. 

Realistically we’re trying to just make the thermal performance OK, and to not burn bridges with dealing with water vapour. We have significant limits as to how much higher we can make the roof because of other arhchitectutal details, parapet walls, etc.

currently, I’m thinking (from inside to outside) 6 inches of glass wool insulation between joists, battens to increase the fall to around 2 degrees (about max we can achieve. This is definitely doable from a metal clad roof specification point of view. Often done in Australia), a vapor open WRB (ie: Pro Clima Mento Plus), ventilating battens above, then metal cladding. Hopefully this will provide a drainage plane to deal with condensation on the underside of the metal, even if it’s pretty ineffective at ventilating (no stack effect).

This house will never be a great performer, nor will it be highly airtight (concrete masonry unit walls without an airtight layer on them. I thought the Mento Plus could be used as the airtight layer for the roof, more so to protect the timber roofing structure from moisture rather than achieving any serious airtightness from a thermal performance perspective. 

Rigid roof sheathing below metal clad roofs are almost never used in Australia (usually screwed onto battens, but given the very flat roof (even at 2 degrees), I think it’s a must to support the flexible WRB and stop it ponding. I’d rather not go the vapour impermeable foam sheathing route, which will commit me to a warm roof sheathing approach, which I don’t think we will achieve with our height limitations.

has anyone got advice as to a sensible vapour permeable rigid/ structural roof sheathing we can use to support the WRB? The vapour permeability of OSB and Plywood seems to be unknown by most local suppliers. I’m concerned about the sheathing not being able to dry in both directions and ending up wet from water vapour accumulation below the WRB.

Also, do people have a preference for vertical versus horizontal battens above the WRB, or opinions on some of the ‘open’ plastic battens, in order to allow condensation dripping down on the WRB to drain out unimpeded?

Thanks in advance.

George

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Replies

  1. user-2310254 | | #1

    George,

    I lived somewhere that had homes with parapets and low-slope metal roofs. These were usually four-bucket houses. That is, you generally needed to keep four buckets on hand for the inevitable leaks.

    Would a membrane roof be a better fit? Maybe with R-30 or more of tapered rigid insulation underneath?

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