DIYbanter - Flat roof

I assume we still have search issues with the Google archive? I can't
believe there are only 4 hits for "flat roof".

My query concerns an extension that I'm considering building, some
time in the future, on the back of my dormer bungalow, to replace a
run of conservatories and porches of varying degrees of dilapidation.
It will be about 12 feet out from the house, by 20 feet along it. My
intention is to have a flat roof (with a fall, obviously), not least
for the purposes of sitting on (therefore, a parapet).

So, a few practical questions, as I get my head round the design of
the thing. Firstly, is it OK to go with joists onto the existing wall
plate at the bottom of the sloping roof of the bungalow (probably
removing the bottom row of tiles), so that the top surface of the flat
roof meets the bottom of the next row of tiles up, and flash up under
the tiles? if so, how far up do I need to flash? Should there be an
upstand between the edge of the sloping roof, and the flat roof (there
are ceiling height issues for the extension that make an upstand
undesirable).

Currently a gutter runs along the bottom of the sloping roof. I was
thinking of doing away with this, rather than have a valley, and
letting the sloping roof drain on to the flat roof, and thence to a
gutter at the outside edge of the flat roof. Is that a conventional
way to do it?

Finally, if I'm thinking of using the roof (officially or
unofficially) as a sitting area, what should I be finishing the roof
with? A rubber system with lightweight roof terrace tiles on top?

Pardon the vagueness of my questions, but I'm just starting to get my
head round a project that I plan to undertake in 2 or 3 years time
(after I've finished renovating the rest of the house!). No doubt more
questons will follow.

Regards
Richard