How deep should the sub-base be for permeable paving?

Sub-base depth depends on your soil's infiltration rate. For clay soils with very low infiltration, you may need 350-500mm of clean angular aggregate (Type 3 or 4/20mm open-graded). For sandy soils, 150-200mm may suffice. The sub-base acts as a temporary storage reservoir, so the depth is calculated from the required storage volume divided by the void ratio (typically 30%).

What is the void ratio for permeable paving sub-base?

The void ratio for clean angular aggregate (Type 3 sub-base or 4/20mm single-size stone) is typically 30%. This means 30% of the sub-base volume is empty space that can store water. Do not use standard Type 1 sub-base — it compacts with very few voids and won't provide storage.

Does permeable paving count as SuDS?

Yes, permeable paving is a recognised SuDS technique. Under permitted development rules in England, driveways using permeable surfaces or draining to a permeable area don't need planning permission. Permeable paving can help meet SuDS requirements under Schedule 3 of the Flood and Water Management Act.

Do I need geotextile under permeable paving?

Yes, a geotextile membrane is essential. Use a non-woven geotextile on the base and sides of the excavation to prevent fine soil particles migrating into the aggregate and clogging the voids. Do NOT use impermeable membrane (polythene) — the system needs water to infiltrate into the ground below.

How much does permeable paving cost?

Permeable block paving typically costs £40-60/m² for materials, plus £20-35/m² for sub-base aggregate. Installation labour adds £25-40/m². A typical 50m² driveway might cost £4,000-7,000 installed. While more expensive than standard paving, you avoid the cost of separate drainage and may not need planning permission.

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Permeable Paving Calculator — Sub-base Depth & Storage

Calculate sub-base depth, aggregate and geotextile for permeable paving based on soil infiltration rate and void ratio.

Paved Area

Total area of permeable paving

Soil TypeInfiltration: 0.001 m/hr

Paving Type

Rainfall Intensity

1 in 100yr + 40% CC ≈ 50mm/hr

Paving Price

Material cost per square metre

Sub-base Aggregate Price

Clean angular aggregate price

How We Calculate This

This calculator determines the sub-base depth needed to provide adequate stormwater storage for permeable paving, based on soil infiltration rate and rainfall intensity.

Storage formula

Storage Needed = (Rainfall Volume - Infiltration Volume)
Sub-base Depth = Storage Needed / (Area x Void Ratio)
Void ratio for clean angular aggregate is typically 30%.

Materials

Sub-base aggregate density is approximately 1.8 tonnes/m³. Geotextile membrane covers the base and sides of the excavation with 150mm overlaps at joins.