If you are waterproofing a basement or any below-ground space, you will come across the term Type C cavity drainage. It is one of three approaches recognised in the British Standard for below-ground waterproofing, and the one that most often relies on a pump. Here is what it means and why the pump is the part that matters most.
The three types of waterproofing in BS 8102
BS 8102:2022, the code of practice for protecting below-ground structures from water, sets out three forms of protection. Type A (barrier protection) applies a waterproof membrane or coating to keep water out. Type B (structurally integral protection) uses the concrete structure itself, designed to be watertight. Type C (drained protection) takes a different view: it accepts that some water may get through the structure, and manages it rather than trying to hold it back entirely.
How Type C cavity drainage works
A Type C system lines the inside of the walls and floor with a studded cavity drainage membrane. The studs hold the membrane off the structure, creating a continuous cavity behind it. Any water that enters the structure runs down into that cavity rather than into the room, is collected by a perimeter drainage channel at the base of the walls, and is directed to a sump. A pump in that sump then lifts the water away to a drain or discharge point. The room stays dry because the water is intercepted, channelled and pumped out, not because the wall is a perfect barrier.
Why the pump is the critical component
In a Type C system the whole basement stays dry only as long as the pump keeps working. That makes pump reliability the single most important design decision, and it is why a well-designed system does not rely on a single pump alone. Best practice is a sump fitted with twin pumps, so a second pump takes over if the first fails, a high-level alarm to warn you if the water rises, and battery or backup power so the system keeps running during a power cut, which is exactly when heavy rain and flooding are most likely. You can see suitable pumps in our basement waterproofing pumps and sump pumps ranges, often installed as part of a packaged pump station.
Maintenance and access
BS 8102:2022 places real weight on maintenance. A Type C system has moving parts and a sump that can collect silt, so it needs a maintenance schedule, with the first service inspection at handover and regular checks after that, and the sump and pump must be accessible for servicing. A cavity drainage system that is never maintained is a system waiting to fail, so design in access from the start and keep records that show the pump has been tested and serviced.
Choosing a Type C pump system
The right setup depends on how much water the system needs to handle, the discharge height and route, and how critical the space is. For a habitable basement, twin pumps, an alarm and battery backup are the sensible baseline. Tell us about the space and we will help you specify it. Browse basement waterproofing pumps, use the Pump Finder, or call 0115 987 0358. For a deeper look at sump pumps, see our guide to what a sump pump is and how to install one.
In practice, the most popular choice is our FPS Iguazu Sump 635 packaged sump pump station, a sealed sump chamber with the pump built in, ready to drop into a Type C system. Depending on the situation, many opt for the twin pump version, which adds a second pump for redundancy so the basement stays protected if one ever fails. In a basement it is also normal to specify the recessed, sealed and locking cover, which sits flush with the floor, seals against odour and moisture, and locks for safety.
Frequently asked questions
What is Type C cavity drainage?
Type C, or drained protection, is one of the three waterproofing approaches in BS 8102:2022. It lines the inside of a below-ground structure with a studded cavity drainage membrane that accepts water ingress, channels the water to a sump, and pumps it away, keeping the space dry by managing water rather than blocking it.
Does a Type C system need a pump?
Almost always. Water collected in the cavity is directed to a sump and removed by a pump. The dry basement depends on the pump working, which is why twin pumps, a high-level alarm and backup power are recommended.
What happens if the pump fails in a cavity drainage system?
If the pump fails and there is no backup, water collected in the cavity has nowhere to go and the space can flood. This is why best practice uses twin pumps, an alarm and battery backup, and why BS 8102:2022 requires a maintenance schedule.
How often should a Type C system be serviced?
BS 8102:2022 calls for a maintenance schedule, with the first service inspection at handover and regular checks afterwards. The sump and pumps must be kept accessible, and service records kept to show the system is working.
