Damp Proofing
Tanking: Waterproofing Cellars and Basements
Tanking is a waterproof lining applied to below-ground walls and floors. Here is how it works, how it compares to drained membranes, and its limits.
In short
Tanking is the waterproofing of below-ground rooms — cellars and basements — by applying a continuous waterproof barrier (a cementitious slurry or membrane) to the inside of the walls and floor to hold back water and ground moisture. It is a stronger intervention than ordinary damp proofing because below-ground walls face standing water and water pressure. Where water cannot be reliably excluded, a drained cavity membrane system is often preferred to tanking.
Key takeaways
- Tanking waterproofs below-ground rooms by applying a continuous barrier to walls and floors.
- It must resist water pressure, so it is a stronger intervention than wall or floor damp proofing.
- Cavity drainage membranes manage water to a sump and pump instead of holding it back — often preferred where water is persistent.
- Tanking is specialist work; poor detailing or a single weak point can let water through.
Tanking is the process of making a below-ground room — a cellar or basement — watertight from the inside. Unlike a wall above ground, a basement wall is surrounded by soil that can hold standing water, and that water can press against the structure under hydrostatic pressure. Ordinary damp proofing is not designed for that. Tanking, or its alternatives, is.
How tanking works
In its traditional form, tanking means applying a continuous waterproof barrier to the inside faces of the below-ground walls and floor so that water simply cannot pass through. This is typically done with:
- Cementitious tanking slurry — a waterproof cement-based coating bonded to the masonry.
- Sheet or liquid membranes — a bonded waterproof layer.
The defining feature is that it is a barrier system: it is designed to hold water out completely. That makes continuity critical. A single gap, poorly treated junction, or later penetration (a drilled fixing, a service pipe) can become a leak path, and because water is under pressure it will find any weakness.
Tanking versus cavity drainage membranes
There are two broad philosophies for keeping a basement dry, and it is important to understand the difference:
| Approach | Principle | Best where |
|---|---|---|
| Tanking (barrier) | Hold water out completely | Lower water loads, sound masonry, good detailing |
| Cavity drainage membrane | Let water in behind a studded membrane, then channel it to a sump and pump | Persistent water, higher water table, habitable conversions |
A cavity drainage (or "drained membrane") system accepts that it may not be possible to stop water entirely. A dimpled plastic membrane is fixed to the walls and floor, leaving an air gap; any water that gets through runs down behind it into a channel and on to a sump, from which a pump removes it. Because it does not rely on a perfect seal, many specialists prefer it for habitable basement conversions — though it introduces its own maintenance: the pump and channels must be kept working.
Note
Neither system fixes the cause of water pressure outside the wall. Where feasible, external drainage, guttering and ground management should be addressed too, so the waterproofing is not working harder than it needs to.
When tanking is the right call
Tanking can be a sound choice for a cellar with modest water ingress, sound masonry and junctions that can be detailed carefully — for example turning a damp storage cellar into a dry one. It is less suited to situations with a high water table or frequent flooding, where a drained membrane with a pump is generally more forgiving.
Important
Tanking a basement effectively seals the inside face of the wall. On older, solid and breathable walls this can trap moisture within the masonry and push it elsewhere. Historic England advises caution with impermeable treatments on traditional buildings; get specialist advice before tanking a period property.
Practical considerations
- Specialist work. Basement waterproofing is not a general-builder job. Design standards exist for below-ground waterproofing, and good practice often combines more than one line of defence.
- Disruption. Walls must be prepared back to sound masonry; floors are usually involved too.
- Maintenance. Drained systems need their pumps serviced and channels kept clear. Factor that in.
- Guarantees. As with all damp work, understand exactly what any guarantee covers and who stands behind it.
For rough budgeting, our damp-proofing cost guide gives indicative ranges and explains why basement work sits at the higher end. Those figures are indicative only, never a quotation.
Frequently asked questions
A damp-proof membrane resists ground moisture under normal conditions, typically in a floor. Tanking is a stronger, continuous waterproof barrier designed to hold back standing water and water pressure in a below-ground room. Basements generally need the tanking-grade approach, not a simple membrane.
Sources
- Historic England: Energy Efficiency and Historic Buildings · England · accessed 27 October 2026
DampSense Editorial Team
Research & editorial
Our editorial team researches UK building-moisture topics using government guidance, public-health sources and established building-science references. We write to help homeowners and tenants make informed decisions, and we flag where professional assessment is needed.
Credentials & limitations: Research-led editorial team. We are not chartered surveyors; our guides are general information, not a professional building survey.
