What Size Are House Drains?

House drains in the UK use standard pipe sizes that have remained relatively consistent for decades. Understanding what size your drains are helps when planning repairs, assessing whether pipes are undersized, and determining which repair methods are suitable. Most residential properties use 100mm (4-inch) or 150mm (6-inch) pipes for their main underground drainage.

What to Look For

• Internal waste pipes: 32mm (basins), 40mm (baths/showers), 110mm (toilets)
• Underground foul drains: typically 100mm (4 inch)
• Shared or main drain runs: 150mm (6 inch)
• Surface water drains: 100mm or 150mm depending on catchment area
• Older clay pipes may be imperial sizes (4 inch, 6 inch)
• Modern plastic pipes are metric (100mm, 150mm)

How We Diagnose the Problem

CCTV surveys accurately measure pipe diameter as the camera travels through. We identify pipe material (clay, concrete, plastic, cast iron) and flag any sections that are undersized for current building regulation requirements.

How We Fix It

If your pipes are undersized (rare in main drains but common in secondary runs), we can upsize during excavation repairs. Relining slightly reduces internal diameter (by about 6mm) but this rarely causes flow issues in standard 100mm+ pipes.