I find myself having to install a ventilation for an internal bathroom
I'm building. Unfortunately there's no alternative for the exit
pipe to go straight up through the roof - no direct access to any
external walls as it's a mid-terrace; and because it's also a corner
terrace it would be tough (and unsightly) to reach an external wall
via bozed in ducting.
So how difficult is this to do, and leave a sound, non-leaking roof?
What's the best device for a newbie to use? Had a look on Screwfix
but can't see anything. Roof has flat tiles, think they are slate.
 Just bought the place. This internal room has apparently been
used as a shower-room of sorts. There is an old ventilation grill in
the ceiling, so yesterday I went up to the loft to find what was on
the other side of it; hoping for at least a duct and a broken fan
maybe. I found a large ancient, galvanized steel box about 18" cube,
firmly fixed down over the grille. Very curious. Box had several
other grilles in the sides, and one had been prised open by someone in
order to look inside. The contents... nothing, zip, nada. The only
purpose seems to be to provide an air conduit into the roof space.
Presumably building control must have been OK with that when it was