20mm Drainage Aggregate for Retaining Walls

20mm Drainage Aggregate — The Essential Material Behind Every Retaining Wall

20mm clean crushed drainage aggregate is the backbone of correct retaining wall drainage. Packed behind the wall between the posts and geotextile fabric, it creates the free-draining zone that channels groundwater to your ag pipe and prevents the hydrostatic pressure buildup that causes wall failure.

Without adequate drainage aggregate, even a perfectly built retaining wall will fail over time. This is the material that makes drainage actually work.

How Much Do You Need?

As a guide, a standard retaining wall requires approximately 0.15–0.20m³ of drainage aggregate per lineal metre of wall (based on 300mm drainage zone depth). Use our WallWise Pro calculator to get an accurate quantity for your project dimensions.

Installation

• Place geotextile fabric against the excavation face before backfilling
• Lay ag pipe at the base of the wall on a bed of aggregate
• Backfill with 20mm aggregate to within 150–200mm of finished height
• Wrap geotextile fabric over the top of the aggregate zone before topping with soil

See our full Drainage Guide for installation detail. Pair with ag pipe and geotextile fabric for a complete drainage system.