Basement Waterproofing Types Explained
Tanking, waterproof concrete or a drained cavity system? Here are the three recognised ways to keep a basement dry, how the dryness grades work, and why the design matters far more than the product.
In short: the standard for keeping below-ground structures dry, BS 8102, sets out three ways to do it. Type A is a barrier, or tanking, that keeps water out. Type B uses the reinforced concrete structure itself as the waterproof element. Type C is a drained system that lets a little water in, catches it behind a cavity membrane and pumps it away. On top of that sit three grades that describe how dry the finished space needs to be, from basic parking up to fully habitable. The single most important point is that waterproofing is a design problem: the best workmanship in the world cannot rescue the wrong system for the site.
Waterproofing is a design problem, not just a product
The most common and expensive misunderstanding about basements is that keeping one dry comes down to buying the right product and applying it carefully. It does not. Under BS 8102, the current code of practice for protecting below-ground structures from water, the reality is the opposite: good workmanship cannot rescue an inadequate design. Getting a basement dry starts with a proper assessment of the ground and the water in it, and a waterproofing scheme designed around that from the very beginning, ideally by a waterproofing designer working alongside the structural engineer.
That is because the water conditions drive everything. You have to assume the water table will vary through the year, and on London's low-permeability clay you also get "perched" water, where rain sits against the structure with nowhere to drain. The safe assumption, and the one BS 8102 asks for, is never to assume there will be no water pressure. Design for it, because a burst main, a wet winter or a changing climate will find any system that was designed for a dry hole in the ground.
The three types of waterproofing
BS 8102 describes three forms of protection. They are not ranked best to worst, they are different strategies, and the right one, or a combination, depends on the site.
Type A: barrier, or tanking
A Type A system is a physical barrier applied to the structure, inside or outside, to keep water out. Its strength is simplicity; its weakness is that it only works if it is completely continuous, because any gap is a leak path, and rigid barriers can be cracked by movement. External tanking is hard to repair once buried, since water tracks through the wall and appears inside well away from the actual defect. Internal tanking is more accessible to repair but depends heavily on the wall it is bonded to and has to be designed to resist the water pressure pushing it off the wall.
Type B: waterproof concrete
A Type B system makes the reinforced concrete box itself the waterproof element, with no separate barrier. To keep a basement habitable it needs the concrete designed to hold cracks to a very fine width, well-detailed water-stops at every construction joint (the classic weak point), careful detailing around service penetrations, and good placement, compaction and curing on site. It can really only be proven once ground water comes to bear against it, and a leak is usually repaired by injecting resin at the defect.
Type C: drained cavity system
A Type C system takes the opposite approach: it does not try to keep every drop out. A dimpled cavity drain membrane lines the inside of the walls and floor, any water that gets through is collected in a channel and led to a sump, and a pump lifts it away, or it drains by gravity on a sloping site. The clever part is that because water never builds up pressure against the membrane, small defects in it do not matter. The trade-off is that the system depends on maintenance: the pumps must keep working, so they are designed with redundancy and a battery back-up, and the owner inherits an annual maintenance duty.
On difficult sites, or where the consequences of a leak are serious, two types are often combined, for example waterproof concrete with an internal drained membrane, so there is a second line of defence.
How dry does it need to be? The three grades
Separate from the type is the grade, which describes how dry the finished room has to be. It is worth agreeing this first, because it drives the whole design.
| Grade | What it means | Typical use |
|---|---|---|
| Grade 1 | Some seepage tolerated | Parking, plant |
| Grade 2 | No water penetration | Store rooms, workshops |
| Grade 3 | Dry and habitable, with humidity controlled | Living space, bedrooms, kitchens |
Grades follow the principles in BS 8102. Most basements are designed to at least Grade 2, and any habitable basement to Grade 3. General guidance only; the right specification for your basement should be confirmed by a waterproofing designer and structural engineer.
Why a "dry" basement can still feel damp
Here is the point most people miss. Achieving a habitable, Grade 3 basement is not only about keeping ground water out. A room below ground, lived in and full of the moisture that everyday life produces, will feel damp if that humidity is not managed, because it condenses on the cooler surfaces. A proper Grade 3 design therefore includes insulation, heating, ventilation and sometimes dehumidification to control condensation. If a newly finished basement still feels damp, the first question is whether the problem is water coming in or condensation that is not being dealt with, because the two need completely different fixes.
How EMA Structures can help
A basement project runs through several stages, and we can pick it up at whichever one you are at.
- Planning stage. Many London boroughs require a Basement Impact Assessment to support a basement planning application, covering the ground conditions, groundwater, drainage and the effect on neighbouring properties. We prepare these to help your scheme through the planning process.
- Design stage. We carry out the structural design of the basement and design the waterproofing scheme alongside it, so the two work together rather than against each other. Our team are Delta registered installers, so we can also advise on the waterproofing products themselves, not just the principle.
- Construction stage. Where the planning and design are in place, we can provide a quote with our construction team for the basement construction and the waterproofing together, so the design, the below-ground works and the waterproofing are delivered by one team.
Handling it under one roof is the surest way to avoid the coordination gaps between structure and waterproofing that cause most basement leaks. For what the works cost, see our guide to basement and underpinning costs in London.
Already have a basement with a problem?
If your concern is an existing basement, cracking, signs of settlement, water coming in, or another structural issue, that is a different job from designing a new one, and it starts with a look rather than a design. In that case the right first step is a structural inspection: we visit, diagnose what is actually happening, and advise on what, if anything, needs doing before any work is specified. Our guides to types of cracks in walls and subsidence may help in the meantime.
Planning a basement, or worried about one?
Whether it is a new build, a basement extension, a cellar conversion or a problem with an existing basement, tell us about the site and what you want to do. Send us the details and a structural engineer will advise on the right next step, from a Basement Impact Assessment through to waterproofing design or a site visit. An inspection with a written report costs £495 + VAT, see our fees guide, correct as of August 2026.
Basement waterproofing questions
What is the difference between tanking and cavity drainage?
Tanking, known as Type A, is a barrier applied to the structure that keeps water out entirely, so it relies on the barrier being complete and continuous. Cavity drainage, known as Type C, does the opposite: it accepts that a little water may pass through the wall, catches it behind a membrane, and drains it to a sump so it is pumped away before it can build up pressure or reach the room. Tanking blocks the water; cavity drainage manages it. Both are recognised methods under BS 8102 and the right one depends on the site.
Which basement waterproofing type is best?
There is no single best type; the right choice depends on the ground, the water conditions, how the basement will be used and how it could be repaired if it leaked. Type A barrier systems, Type B waterproof concrete and Type C drained cavity systems each have strengths and weak points, and on difficult sites two are often combined. What matters most is that the waterproofing is designed for your specific basement by someone competent, because the best workmanship cannot rescue a system that was the wrong choice for the site.
Do basement sump pumps need maintenance?
Yes. A drained, Type C basement relies on its pump to keep working, so the system needs regular maintenance, at least once a year, and the pumps should be designed with redundancy: usually two pumps and a battery back-up in case of a power cut, each on its own supply. This maintenance duty passes to the homeowner, so it is worth understanding before you choose a system that depends on pumps. A well-designed system also has an overflow so that if the drains block, water escapes outside rather than into the basement.
What are basement waterproofing grades?
BS 8102 describes three grades of internal environment. Grade 1 accepts some seepage and suits basic uses like parking or plant. Grade 2 requires no water penetration and suits store rooms. Grade 3 is a dry, habitable environment with no penetration, suitable for living space. Most basements are designed to at least Grade 2, and any habitable basement should be Grade 3. The grade describes how dry the finished space must be, which then drives the waterproofing design.
Why is my new basement still damp?
A common surprise is that keeping ground water out is only half of a dry basement. A habitable, Grade 3 basement also has to control the humidity generated by living in it, through insulation, heating, ventilation and sometimes dehumidification, or moisture condenses on cool surfaces and the room feels damp even though no ground water is getting in. If a new basement feels damp, it is worth checking whether the problem is water coming in or condensation that is not being managed, because the two have very different fixes.
Building or fixing a basement?
Send us the site details and how you want to use the space, and we will advise on the right waterproofing type and grade.