Subsoil Drainage and Land Drainage

What This Requirement Covers

Approved Document C requires that subsoil drainage is provided where the site is subject to high groundwater levels or waterlogging that could affect the building. Subsoil drainage reduces hydrostatic pressure on below-ground walls and prevents moisture from rising through the ground floor.

Key Requirements

When Subsoil Drainage Is Required

1. Sites with a high water table (water table within 1 metre of the ground floor level)
2. Sites with clay or silt soils that are prone to waterlogging
3. Sites where surface water naturally drains towards the building
4. Basements and below-ground construction
5. Where groundwater could compromise the integrity of foundations or floor slabs

Types of Subsoil Drainage

1. French drains: Trenches filled with granular material (gravel) with a perforated pipe at the base to collect and convey groundwater
2. Land drains: Perforated or porous pipes laid in granular-filled trenches at a gradient to a suitable outfall
3. Drainage blankets: Layers of granular material or geocomposite drainage layers placed against below-ground walls to relieve hydrostatic pressure
4. Sump and pump: Where gravity drainage is not possible, a sump pit with a submersible pump removes collected groundwater

Design and Installation

1. Subsoil drains should be laid at a minimum gradient of 1:200 (steeper is better)
2. Drains should be positioned below the finished floor level and below the lowest habitable room
3. The granular fill should be clean, free-draining material (10-20 mm gravel)
4. A filter fabric (geotextile) should wrap the granular fill to prevent soil fines clogging the drain
5. The outfall must discharge to a suitable location (soakaway, ditch, or surface water drain)

Practical Compliance Tips

1. Investigate the groundwater conditions as part of the ground investigation before designing the building
2. Install subsoil drainage at the same time as the foundations; retrofitting is difficult and expensive
3. Ensure the drainage has a positive fall to the outfall; ponding water defeats the purpose
4. Use a geotextile filter fabric to prevent the drain from silting up over time
5. Provide inspection chambers or access points for maintenance
6. If a sump and pump system is used, provide a high-level alarm and a backup pump
7. Keep drainage records and as-built drawings for future maintenance