How the two differ in kind
Water in the ground beside a basement is under pressure proportional to its depth, and it will exploit any path. The two systems here take opposite positions on whether to fight that.
A BARRIER system — tanking — is a continuous layer bonded to the structure that water cannot pass. It is the intuitive answer and it carries a demanding condition: it must be continuous and defect-free over the whole area, including every junction, service penetration and construction joint. Under hydrostatic pressure a single pinhole is a leak, and remedying it means finding it, which in a buried or covered structure is the hard part.
A DRAINED CAVITY system does not resist water at all. A dimpled membrane is fixed to the inside face, leaving a continuous void behind it; water that penetrates the structure runs down that void into a perimeter channel, to a sump, and is pumped out. A defect in the membrane is not a failure because the membrane was never holding anything back. What the system is holding back is the ATMOSPHERE of the room, not the water.
In exchange it introduces dependence. The pump needs power, needs maintenance, and will one day fail — and the channel silts, which is why it must be accessible and flushable rather than buried behind a finish. Neither of those is a reason against it; both are reasons it is a designed system with a maintenance regime rather than a product that is installed and forgotten.
The factors that actually differ
Show
| Tanking — a barrier system | Drained cavity membrane | |
|---|---|---|
| What it does about water | Resists it. The layer is the defence and it has to be continuous. | Manages it. Water is expected, collected and removed; the membrane separates the room from it. |
| Consequence of a defect | A leak, at the defect, under pressure — and finding it is frequently harder than fixing it. | Nothing. Water behind the membrane is where water is supposed to be. |
| Dependence | None. It works without power, indefinitely. | A pump, its power supply and its maintenance. Backup pumps and battery supplies are standard rather than optional on anything that matters. |
| Demand on the structure | High. It bonds to the substrate, so the substrate must be sound, clean and prepared — and internally applied barriers work against NEGATIVE pressure, which pushes them off the wall. | Low. It is mechanically fixed and does not care whether the wall behind it leaks, which is why it suits retrofit and poor-condition structures. |
| Maintenance | None, if it was right. There is nothing to service. | Real and scheduled: pump testing, channel flushing, and access points that must remain accessible after the fit-out. |
| Retrofit onto an existing basement | Difficult. The internal face must be prepared and the system must hold back pressure from the wrong side. | The usual answer, because it tolerates a wall that leaks and needs no bond to it. |
| Loss of internal space | Minimal — a coating thickness. | The cavity plus the lining in front of it, plus the perimeter channel, all round. |
| Fixing into the wall afterwards | Any fixing through the barrier is a penetration and a potential leak path, so it is a detail rather than a screw. | Any fixing through the membrane compromises the separation, so battened linings and specified fixings are part of the system — and a later shelf fixed by a joiner is the classic breach. |
Which one, and when
Choose tanking — a barrier system when…
- The structure is new and sound, so a continuous barrier can be applied and detailed properly as it is built.
- There is no power, or the basement must stay dry through a power cut — a store, a plant room with equipment, a property left empty.
- The internal space is too valuable to give a cavity and a lining to.
- The grade of use is modest and the water pressure is low.
Choose drained cavity membrane when…
- It is a retrofit into an existing basement whose walls already leak.
- The structure's condition cannot be guaranteed — rubble walls, old brick, construction joints of unknown quality.
- The grade of use is habitable and dry, where accepting and removing water is more reliable than resisting it perfectly.
- Somebody will maintain it, and a pump failure would be noticed rather than discovered.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Which is better?
- Neither in general, and the standards deliberately do not say so — they classify systems by TYPE rather than ranking them, and then specify the GRADE of internal environment required. A Grade 1 basement for storage tolerates some seepage; a Grade 3 habitable space must be dry and will usually need two forms of protection working together rather than one perfect one. The choice follows from the grade, the structure's condition and whether maintenance will happen. The honest statement is that a barrier system done perfectly on a sound new structure is excellent, a drained system is far more forgiving of everything else, and a basement that must be dry is commonly designed with both.
- Can I just paint the walls with a waterproof coating?
- Not against real water pressure, and this is the most expensive misunderstanding in the subject. A coating applied to the inside face is working under NEGATIVE pressure — water pushing outward against it, trying to lift it off the wall — which is the opposite of the condition most waterproofing products are designed for. Cementitious systems formulated for negative-side application exist and work because they bond into the substrate and are part of it; paint-like membranes generally do not and blister off. Even the right product needs a properly prepared substrate, full continuity at every junction and floor-to-wall detail, and no penetrations. A coating on a damp wall is usually a way of hiding the damp for a season.
- What happens when the sump pump fails?
- Water rises in the cavity and eventually in the room, which is why a single unguarded pump is not a design. The standard arrangement for anything that matters is two pumps on separate circuits, a battery or water-powered backup for the power-cut case, and a high-water alarm that tells somebody on the day rather than on the day the floor is wet. The pump also needs to be testable and the sump accessible after the basement is finished — a sump buried under a fitted floor is a maintenance item nobody will ever service. The failure mode is worth naming plainly: a drained basement with a failed pump fills, and the fit-out is what is damaged.
- Does a drained cavity need maintenance?
- Yes, and it is the part most often omitted from the handover. The perimeter channel collects fines and, in hard water, precipitates lime — so it silts up over years and eventually ceases to drain, at which point water backs up behind the membrane. Systems are therefore designed with rodding and flushing access at corners and intervals, and those access points have to remain reachable after the room is fitted out. Pump testing is the other item. A maintenance schedule is part of the design rather than an operator's choice, and a drained system installed without accessible access points has been built to fail quietly at some point in the next ten years.
- Can I use both together?
- Routinely, and for a habitable basement it is the normal specification rather than belt and braces. The reasoning is that the two fail differently: a barrier fails at a defect, a drained system fails at a pump or a blocked channel, and the probability of both failing at once is far lower than either alone. Standards for the highest grade of internal environment generally expect combined protection for exactly that reason. What matters is that they are designed together — a barrier behind a drained cavity is a coherent arrangement; a drained membrane fixed over a failing coating with fixings driven through both is two compromised systems.
- What about water-resisting concrete?
- That is the third type and it belongs in the conversation, because it puts the waterproofing in the structure itself rather than adding a layer to it. A concrete mix designed to resist water penetration, with construction joints detailed with waterstops and a crack-width limit maintained by reinforcement, can form a basement that is waterproof as built. Its weakness is the joints and any crack that exceeds the design width, which is why it is frequently combined with a drained cavity internally — the concrete does most of the work, and the cavity handles what gets through at a joint. It is a design decision taken before the structure is poured, which is what distinguishes it from the two options on this page.