What Drainage Gradient Do I Need?

Minimum Gradients by Pipe Type

• 100/110mm foul drain (≥1 WC connected): 1:80 minimum (12.5mm per metre) per AD H Diagram 5 — WC flush provides self-cleansing velocity
• 100/110mm foul drain (no WC, low flow only): 1:40 minimum (25mm per metre)
• 150mm foul drain: 1:150 minimum
• 100mm surface water: 1:100 minimum (10mm per metre)
• 160mm surface water: 1:100 minimum

Understanding Gradient Notation

A gradient of 1:40 means the pipe falls 1 unit for every 40 units of horizontal run. In practical terms:

• 1:40 = 25mm fall per metre
• 1:60 = 16.7mm fall per metre
• 1:80 = 12.5mm fall per metre
• 1:100 = 10mm fall per metre

Why Gradient Matters

Too steep (steeper than 1:40) and the water runs ahead of the solids, leaving them behind to cause blockages. Too shallow and the flow velocity is insufficient to carry solids. The gradient creates a self-cleansing velocity of at least 0.75m/s.

Practical Example

For a 10m drain run from a house to a manhole at 1:40:

• Total fall: 10 × 25mm = 250mm
• If invert at house is 500mm deep, invert at manhole = 750mm deep

Building Regulations

Approved Document H sets out the requirements for below-ground drainage. All new drainage installations require Building Control notification. The gradient can be checked using Maguire’s Rule or the tables in Approved Document H.