Waste Pipe

A waste pipe is any plumbing pipe that carries “grey water” — used water from sinks, basins, baths, showers, washing machines, and dishwashers — to the home’s drainage system. Waste pipes are distinct from soil pipes, which carry toilet waste (also called “foul water” or “black water”).

Common waste pipe sizes

• 32 mm — used for wash basins and overflow pipes
• 40 mm — used for baths, showers, kitchen sinks, and appliances (washing machines, dishwashers)
• 50 mm — occasionally used for longer runs or where multiple appliances connect to the same pipe

In the US, waste pipe sizes are typically measured in inches: 1.25 inches (basins) and 1.5 inches (sinks, baths, showers).

Traps and connections

Every waste pipe connects to the fixture via a trap — a U-shaped or bottle-shaped bend that holds a small amount of water. This water seal prevents foul-smelling sewer gases from travelling back up the pipe and into the room. Common trap types include:

• P-trap — the standard U-bend under a basin or sink
• Bottle trap — a compact alternative used where space is tight (e.g., wall-hung basins)
• Shower trap — a shallow trap built into the shower base or floor

Why waste pipes matter in a renovation

Waste pipes are one of the most important constraints when planning room layouts:

• Kitchen and bathroom positioning — waste pipes need a continuous downhill gradient (fall) to drain properly. The further a sink or shower is from the main drainage point, the more height is needed, which can limit where you place fixtures
• Running through joists — waste pipes on upper floors often need to pass through or between floor joists, which must be done according to building regulations to avoid structural damage
• Access — waste pipes need to remain accessible for maintenance and clearing blockages, especially at bends and junctions
• First fix stage — waste pipe routes are installed during the plumbing first fix, before floors and walls are closed up

Practical tip

When redesigning a kitchen or bathroom layout, ask your contractor about waste pipe routes early. A beautiful design that places a shower at the opposite end of the house from the main drain may require lifting floors or building soffits to accommodate the pipe gradient. Understanding the drainage constraints upfront saves costly redesigns later.