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We have just offered a proposal to a client to overlay their existing BUR Gravel 30 year old roof with a batten/insulation/ plywood buildup and new torchon membranes to sort out 30years of ponding/flooding.We proposed to lay 50mmx50mm battens, fixed through the existing BUR/Gravel roof into the old original 6x1 substrate, and throw in some 50mm thick polystyrene sheets cut to size for a bit of insulation value while we were doing the roofing work.
I was quite happy with our suggestion until the client told me his Dad worked for the NZGovt. MOW back in the 1960's and his Dad's advice was that we needed to allow a gap between the insulation and the underside of the new plywood to allow for breathing.
We have already allowed for roof top breather vents (mushroom vents) but now are worried if "Dad" is right about this?.
Do you need a breather space above the insulation in a re roofing situation?
It's not really a roofers question now, more a construction question.
Like to hear some opinions on insulations from builders.
MikeNZ
 
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Mike why don't you tear it off and put down a tapered insulation system, then torch it
 

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I am a roofer and on any major reconstruction like youa re planning I would never start ontop of existing layers. I would take it down to the original deck.

I can not answer your question about spacing because I work more with sloped roofing than flat roofing so I am not too famaliar. I do know you can ALWAYS ask the manufacturers and they will know for sure... and what they say carries more weight than dear old dad.

Also I'd say not to use Mushroom vents. Use One Way Breather Vents. They are made for flat roofs, and look like silver bullets. If you mean mushroom vents like typical shingle breahter vents, what I call mushrooms, you are in for trouble with those on a flat roof. Use the one way breather vents.
 
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