Driveway drainage explained, how we design water off (and out of) your driveway

Every driveway does one of two things with surface water, it either lets it soak vertically through the build-up (permeable) or sheds it to a controlled outlet (impermeable). Get this decision wrong and the surface is fighting the ground water for its entire life.

Permeable construction

Resin bound over an open-graded Type 3 sub-base is the classic permeable build-up. Water drops through the surface, filters through the sub-base and infiltrates into the ground below when the complete construction, subgrade permeability, levels and discharge route are correctly designed and specified.

Impermeable construction

Tarmac and standard block paving shed water off the surface. That water has to go somewhere, the two acceptable answers are (a) to a soakaway on your property or (b) to a permeable landscaped area. Sending it to the highway or a foul drain is not acceptable.

Falls, gullies and linear channels

• Minimum 1:80 fall away from the property on impermeable surfaces
• ACO or Aquadrain linear channels across garages, thresholds and low points
• Trapped gullies to a properly sized soakaway (BRE Digest 365 sizing)
• Detail around door thresholds and air bricks to keep water off the building

Planning implications

Since 2008 any new front-garden driveway over 5 m² must be either permeable or drain to a permeable area, or planning permission is required. Permeable construction can often avoid the need for planning permission for front-garden driveways, provided the complete system manages water within the property and satisfies applicable planning and drainage requirements. Final suitability depends on the complete construction, subgrade permeability, levels, discharge route and local site conditions, not the surface material alone.