This is the one factor that, more than any other, determines the position of fittings, the ability to alter or add equipment at a later date, and even the sitting of the bathroom itself. The basic problem stems from the necessity, for health and safety reasons, to connect a WC to a 100mm diameter watertight drain. Where the bathroom is on an upper floor a 100mm diameter soil or soil and vent pipe is required. These are awkward and intractable fixtures whatever they are made from. They can be cast iron, plastics, copper, lead or fireclay, but cast iron and plastics are by far the most commonly used materials.
The vent pipe, which is now often combined with the soil pipe in a modern installation, must be open to the air at a high level, above the highest windows of the house. Drainage is one aspect of your bathroom planning where the advice of your local building inspector will be vital. You are, in fact, required to have his approval before making any alterations to the drains. Different situations will require different solutions, but there are some common factors that should be kept in mind. For example, it is now considered a good practice to run soil pipes internally in a duct to avoid freezing in cold weather, and the same applies to other downpipes. This also eliminates open hoppers in the waste system, which can accumulate leaves and old tennis balls, so there may be something to be said for bringing an old drainage system up-to-date as part of a major bathroom planning exercise.
The main soil pipe from the WC will go straight to the drains from the house along with the waste from the bath, with a manhole and cover for access. The joint between the WC and the soil pipe socket should be made with a flexible mastic or a special plastics soil connector to cope with any settlement or movement in the building.