Guide
Basement waterproofing: tanking vs cavity drain systems
Updated
Waterproofing is where basements go wrong years after the builders leave, and where the cheapest quote is most often the most expensive decision.
The three types, in plain terms
- Type A, barrier protection (tanking)
- A continuous barrier applied to the structure, internally or externally, that keeps water out. Cheap to specify, unforgiving in execution: a single defect in the membrane or a service penetration done badly can undo the whole system, and it is very hard to inspect or repair once finishes are on.
- Type B, structurally integral protection
- The structure itself is designed to resist water, typically with a properly designed and detailed concrete box. Strong where it is designed and built as such from the start, which usually means a new-build or a full dig-down rather than a retrofit into old brickwork.
- Type C, drained protection (cavity drain)
- Water is allowed to reach the structure, then collected in a studded membrane cavity, channelled to a sump and pumped away. Tolerant of imperfect substrates, which is why it dominates retrofits, but it introduces a pump, a power supply, a backup, and a maintenance regime that the guarantee will depend on.
The British Standard for waterproofing below-ground structures sets out these types and the concept of combining them, and it expects a waterproofing design to be produced by a suitably qualified specialist rather than picked off a price list. That is the single most useful thing to know before you compare quotes.
How each one fails
| System | Typical failure | Consequence |
|---|---|---|
| Type A tanking | Pinhole, poor detailing at junctions or a service penetration | Water appears in the finished room; repair usually means stripping finishes |
| Type B integral | Construction joints, cracking, poor detailing at penetrations | Localised repair, often specialist injection work |
| Type C cavity drain | Pump failure, power loss, blocked channels or silted sump | Flooding within hours; avoided by backup power, twin pumps, alarms and servicing |
What to ask before you accept a waterproofing price
- Who is the waterproofing designer? Ask for a name and their qualification, not just the installer's brand.
- Which type is proposed, and is a second type combined with it? On habitable basements, combining is common practice for a reason.
- What does the guarantee actually cover, who backs it, and does it survive the installer going out of business?
- What maintenance does the guarantee require? Cavity drain guarantees typically depend on documented servicing; miss it and you may have nothing.
- Where does pumped water discharge to, and is that discharge lawful and consented?
- What happens in a power cut? Battery backup, alarms and a second pump are cheap next to a flooded basement.
Verify the specification with a qualified waterproofing designer for your property. This page describes the system types and the questions to ask; it is not a design, and the right answer depends on the water table, the structure and the intended use of the space.