Pipe bedding & surround

20 mm single size deep trench surround

20 mm single size is a clean, high-void stone used for deep trench surround, soakaways and attenuation layers. With no fines it drains freely and does not slump into voids, so it is the default where the surround also has to store or convey water.

Get a delivered price Talk to sales

Key specifications

Nominal size: 20 mm single size
Grading: Clean, washed, fines removed
Material: Primary crushed rock or washed gravel
Void ratio: Typically around 30% — attenuation duty
Supply formats: Loose tipper, grab lorry, artic, bulk bag
Typical load: 16–20 t tipper, 28–29 t artic

Typical uses

Deep trench pipe surround
Soakaway and attenuation stone
Free-draining backfill behind retaining structures
Filter layers under geotextile-wrapped drainage

Lead time and delivery

Stocked line — most postcodes in England and Wales are dispatched within 24–72 hours of order confirmation. Scotland and Highland deliveries are typically 3–5 working days. Programmed civils schedules are booked in advance and delivered to your call-off dates.

Tipper, grab and artic haulage. Where we are also removing surplus from your site the same lorry returns loaded, cutting haulage cost and empty running.

Frequently asked questions

What void ratio should I design a 20 mm stone soakaway to?

Clean 20 mm single size is commonly designed at around 30% void ratio, but your drainage engineer should confirm the figure for the material and compaction assumed in the calculation.

Is 20 mm single size the same as 20 mm ballast?

No. Ballast is an all-in mix of stone and sand for concrete. 20 mm single size is a washed, clean stone with the fines removed and is not a concreting material.

Other bedding and capping sizes

Pipe Bedding 4/10 mm Pipe Bedding 10 mm Pipe Bedding 4/14 mm Pipe Bedding 14 mm Pipe Bedding 4/20 mm Pea Shingle 6 mm All aggregates

Everything you need to spec, price and move material — tools, services and the exact material pages they relate to.