A sump is a pit or chamber built into the lowest point of a basement or waterproofed underground space to collect groundwater that enters or infiltrates through the structure. Water channelled to the sump by perimeter drainage channels or a cavity drain membrane is pumped out automatically by a submersible pump (sump pump) when the water level in the sump rises to a trigger level. Sumps are essential in Type C (drained) basement waterproofing systems where the approach is to manage water ingress rather than physically prevent it.

How does a sump pump work?

A sump pump is a submersible electric pump installed in the sump chamber. A float switch activates the pump when the water level in the sump rises above a set point. The pump discharges water upward through a pipe to a discharge point - typically the surface water drain, a soakaway, or a watercourse. Dual pump systems (with a standby pump that activates if the primary fails) are recommended for basements in high water-table areas. A battery backup system or mains-fail alarm is recommended to provide protection during power outages - the highest risk of basement flooding often coincides with storms that can cause power failures.

What is the difference between Type A, B, and C waterproofing?

BS 8102 classifies below-ground waterproofing into three types. Type A (barrier protection) attempts to physically prevent water entry with tanking - cementitious render, bituminous membranes, or crystalline materials applied to the structure. Type B (structurally integral protection) uses a waterproof concrete structure (with carefully designed and waterproofed joints) to resist water entry. Type C (drained protection) accepts that some water will enter, collects it via cavity drain membranes and perimeter drainage channels, and pumps it out via a sump. Most specialists recommend combining Type A and Type C for habitable basements - physical barrier plus active drainage as a backup.

Where does the sump pump discharge to?

The sump pump discharge must go to a surface water drain (not a foul sewer), a soakaway (if ground conditions allow), or a watercourse with appropriate consent. It cannot be discharged to a foul drain, as this mixes clean groundwater with sewage and overloads sewage treatment capacity. The discharge pipe should include a non-return valve to prevent backflow into the sump during high water-table conditions. Discharge to a soakaway requires checking that the soakaway is in ground permeable enough to accept the pumped flow rate, particularly during the high-groundwater periods when the pump is working hardest.

Foundations noun

Sump

/ sʌmp /

Also known as: sump pump, sump chamber, pump chamber, basement sump

Definition

A sump is a pit or chamber built into the lowest point of a basement or underground drainage system to collect groundwater that cannot drain away by gravity. A submersible pump automatically pumps the collected water to a discharge point above ground when the water level reaches a trigger point. Sumps are essential in Type C (drained protection) basement waterproofing systems, where cavity drain membranes channel any water ingress to the sump rather than allowing it to flood the basement floor.

In practice

A standard basement sump installation consists of: a preformed plastic or concrete sump chamber (typically 200-300mm diameter, 500-750mm deep) set into the floor at the lowest point; a submersible pump (0.3-0.5 kW for most domestic applications) with automatic float switch; a discharge pipe (25-32mm) rising from the sump to the drain or soakaway outside; and a non-return valve in the discharge line. Dual-pump installations are strongly recommended for habitable basements - if the primary pump fails during heavy rain, the backup activates automatically and an alarm alerts the occupant.

Sumps require annual maintenance - the pump should be tested (lift the float to verify it activates), the chamber cleared of any silt, and the non-return valve checked. In cavity drain membrane systems, the perimeter drainage channel (usually a proprietary slot-drain profile) feeds groundwater to the sump from around the entire basement perimeter. The cavity drain membrane (a studded HDPE sheet) is fixed to the walls and floor, with the studs creating a drainage void behind the membrane through which any water tracks down to the perimeter channel and into the sump.

Building Regulations

StandardBS 8102:2022 - Protection of below-ground structures from water

Type CDrained protection - manage ingress rather than prevent it

DischargeSurface water drain only - not to foul sewer

BackupDual pump or battery backup recommended for habitable basements

BS 8102:2022 (Code of practice for protection of below-ground structures against water from the ground) classifies basement waterproofing by type and recommends combining systems for Grade 3 habitable use. The standard recommends that Type C (drained) systems always include a standby pump and high-water-level alarm. Building Regulations Part C requires basement floors and walls to resist moisture - a properly designed and installed Type C system with sump and pump satisfies this requirement. The structural engineer and waterproofing specialist should both be involved in basement conversion design.

Full Building Regulations guidance

Sump

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