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In the two-pipe system, the soil pipe is connected immediately to the drains, and the waste pipe is connected via a trapped gully, which typically takes the waste pipe from the kitchen sink.
Extending a single-stack waste system will mean joining into the main soil pipe. This is commonly fairly simple, provided the pipe is plastic.
REGULATIONS
Most plumbing and waste work is covered by regulations designed to be sure that the results of the work don't seem to be a danger to well-being and don't lead to undue consumption, misuse, contamination or waste of water.
Water By-laws
Because there are differences in this kind of water supplied to different areas of the country, each local water supply undertaking can issue its own water bylaws - though these are normally centered on the Model Water By-laws. You should give notice of your plans for certain plumbing work before it is started, including installing a bidet, flushing cistern or a tap for a hose or making a connection to the rising main.
Two-pipe system
Many older houses have a two-pipe waste system with WCs connected into one vertical soil pipe, and other wastes (baths, basin and bidets) connected into a separate vertical waste pipe. This system calls for less careful design of slopes and connections, but the vertical pipes nevertheless need to be vented to the air.
Building Regulations
The Building Regulations (and their equivalents in Northern Ireland and Scotland) control the way waste systems are designed. You should give notice of your plans to do anything to the waste system in your house (apart from straightforward repair or replacement). Ask the local authority for information about the person to contact-in England and Wales it is the Building Control Officers of your Borough or District Council.
Single-stack systems
Most houses built since about I960 have a single-stack waste system. The branch pipes from the U-bend traps, attached to baths, basins, WCs and so on in the upper storeys of the house, connect into a single pipe usually 100mm in diameter - known as a discharge pipe, soil pipe or soil-stack -which runs vertically down the side of (or through) the house. The prime of this pipe should terminate outside the building, not less than 900mm above the prime of any opening windows (unless a relief valve is fitted). The bottom is connected immediately into the house drainage system - it has no trap in it.
An existing two-pipe system can be extended by allowing extra waste pipes from upstairs rooms to discharge into the hopper head and ground-floor wastes to be led to the gully. This is clearly much simpler than having to cut into the side of the vertical soil or waste pipe - particularly a forged-iron one.
The Water By-laws cowl such things as: the size of storage cisterns and the position of inlets, outlets and overflows: the provision of stop taps and drain taps; the insurance plans of pipes against frost damage, corrosion and vibration; and, most importantly, the design of the system so that there is no possibility of the supply becoming contaminated, particularly by back-siphonage. To be sure that any work complies absolutely, it is important to read (and follow) the local water undertaking's by-laws, which are normally supplied free.
In modern plumbing systems, the pipes which carry used water away from baths, basins, WCs, bidets and showers have traps often known as U-bends entire of water. The water in these traps prevents smells from the sewers getting into the house - in Victorian times, this 'drain air' was thought to be immediately responsible for numerous diseases. Even if this is not the case, smells from sewers are at least unpleasant. On of a toilet, the water trap is part of the fitting; in other situations, it is part of the outlet waste pipe.
WCs at ground-floor level may also be connected to the soil-stack but are more usually connected immediately to the drain. Other ground-floor waste pipes will most probably discharge into a back-inlet gully or through the grid of an open gully. A gully is largely a water trap with the prime open to the air at ground level and an outlet connected to the house drains. The gully should be fitted with a grid to prevent leaves and other things blockading it. The waste pipes enter the gully below the level of the grid but above the level of the water in the gully trap either by simply passing through a hole cut in the prime of the grid, or by being connected to an inlet forming part of the gully. When this inlet is at the back of the gully (the front of the grid is where the outlet is) it is legendary as a back-inlet gully; when the inlet is at the side, it is legendary as, not surprisingly, a side-inlet gully.
When designing a waste system, care has to be taken to be sure that the water in the traps cannot be sucked out so breaking the seal against smells. This can show up if waste water rushes through the branch pipe leading from the trap (or through other pipes connected to this branch) quickly enough to create sufficient suction to pull the water out of the trap. To guard against unsealing, the prime of the soil-stack is left open. It should, nevertheless, be fitted with a cage to stop birds nesting in it and stopping up the open end. (Technically, the length of pipe above the highest branch connection to it is legendary as a vent pipe.) In the single-stack waste system, there are other design constraints - the slope, length and diameter of branch pipes, the position of their connections to the soil-stack, and the radius of the bend at the foot of the soil-stack all have to be worked out carefully in order to meet the requirements of the Building Regulations.
