New WC drainage

I would like to add a new WC to a loft room in a house with a first floor bathroom with WC, and an additional WC on the ground floor. Both the existing WCs discharge to a soil stack at the rear of the house. It will be possible to connect the new WC in the loft to the existing soil stack, but my questions are:
1. is there a maximum length of soil pipe allowable between the WC and the soil stack?
2. what would be the optimum fall per metre for the soil pipe?
3. is it permissable for a soil pipe to pass through a ceiling void above another room? (The most direct route to the soil stack from the loft is through the ceiling void above the existing first floor bathroom)

Many thanks

Niko

1) I don't know. Hopefully somebody else out there has the answer.

2) A good slope for any drain is one in forty which is about 1.5 degrees.

3) You can certainly put your pipe in a ceiling void but make sure you know which way the joists are running before you start designing. Cutting a 4" hole in a joist is a very bad idea. Also, remember to allow for the slope of the pipe when you check for headroom in the void.

1 in 60 is min drop but you'll find the angles on soil stack components are typically 92.5 and 87.5 so encouraging a 2.5 degree drop (although you can fit the pipe into the 'push fit' connectors at several degrees off the 2.5).

Many thanks for these responses. By 'ceiling void' I mean the gap above a flat ceiling and under a pitched roof - so there is lots of room and no joists to cut. Hopefully this should mean that I can achieve the optimum fall on the pipe.