"Al Reynolds" wrote
| While thinking idly about extending my house,
| I happened on the idea of having a patio on the
| flat roof of my extension. ...

| I lived in a student hall of residence with concrete flat roofs. I didn't
have water stains on my ceiling but some did.

Have a felt or membrane roof with upstands sufficiently closely spaced for
decking to go over the top.

Two concerns may be guarding (building control) and overlooking neighbours
(planning).

Owain