Clean Stone — The Complete Guide

Clean stone is a single-sized washed aggregate that's essential for drainage applications. Unlike MOT sub-base, it contains no fines, which means water flows through it freely.

What is Clean Stone?

Clean stone (also called single-sized aggregate, clean crushed stone, or drainage stone) is a washed crushed rock or gravel with particles of a consistent size — typically 20mm, 40mm, or 75mm. The washing process removes all dust and fine particles, leaving a clean, single-sized stone that drains freely. This is what makes it fundamentally different from MOT Type 1, which contains fines specifically to compact and bind.

Common uses

French drains and soakaways
Drainage blankets around foundations
Septic tank drainage fields
Permeable paving sub-base
Pipe bedding and surround (larger sizes)
Sustainable urban drainage systems (SUDS)

Sizes & grades

20mm (most common for drainage), 40mm (larger drainage and soakaways), 75mm (heavy duty drainage and oversites).

How much do you need?

For a soakaway crate surround, fill the void around the crate with clean stone. For a French drain trench: length × width × depth × 1.5 tonnes/m³ (clean stone is lighter than compacted aggregate).

Pro tip

Always wrap clean stone drainage in geotextile membrane to prevent fine particles migrating in and blocking the drainage over time.