Any drain experts?

[PCoulson]

Currently the rain drain off my flat roofed garage runs via a hopper and then runs internaly into garage floor. The pipe work goes under the floor and ends up in the inspection chamber in the garden along with the floul drainage.

Ideally i would like to connect a toilet into this. Is that likley to cause me any problems? I guess i would need an air admistance valve.

I know combined systems are frowned upon but i can change to a soakaway for the rain water without reconstructing the roof or digging up my garage floor and driveway.

[PCoulson]

The toilet won't be going into the hopper (why would I want a toilet on the garage roof) but will connect where the exisiting pipe goes through the garage floor.

I'm guessing the pipe from the roof would need a trap.

[PCoulson]

I was think a of putting a trap on the rainwater pipe (so it doesnt act like a soli stack) and an air admittance valve on the pipe from the toilet.