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:

http://www.kitchenappliancecentre.co...100-160mm.html

http://tinyurl.com/ksnjjk

I 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.