I have to use flashed-in vents to cope with the low pitch of my roof. You can use the soil pipe lead/rubber weathering slates plus a cowl, but would not look as neat as these:
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may want to use these to vent the roof void as well, but I would need the airspace specification in square millimetres for the building regs, however none are specified. I may have to email manrose (not much hope of an answer, they are a funny lot), but anyone used these outlets, or could make a guess ?Cheers, Simon.