In article ,
Michael Chare wrote:
My house has has a partly flat roof, when this inevitably failed I used
some high performance roofing felt which I managed to find, rather than
the regular stuff. I also used block board rather than chipboard
underneath.

Exterior ply is the recommended stuff.

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*60-year-old, one owner - needs parts, make offer

Dave Plowman London SW 12
RIP Acorn