Tanking is a barrier system: a waterproof coating bonded to the wall and floor that holds water back. A cavity drain membrane is a drained system: a studded membrane that lets water through the structure, collects it behind the membrane and drains it to a sump, so it's managed rather than held back.
One thing that winds me up is how the word tanking gets used for everything, from cellar waterproofing to above-ground walls to membranes. They’re different systems that work in different ways, and knowing the difference helps you understand what you’re being quoted for.
Where the word comes from
Cementitious waterproofing systems for below-ground structures were applied to make the structure watertight. Seal the openings and you’ve effectively made a tank. Hence tanking. It describes a barrier system, not every type of waterproofing.
Barrier protection (Type A, tanking)
A waterproof render, slurry, coating or bonded sheet applied to the walls and floor. It works by holding the water back.
- Pros: no pumps or ongoing mechanical parts, and it can work well on sound, well-prepared surfaces.
- Cons: it has to resist the full water pressure behind it. Preparation is everything, as it has to bond properly to clean, sound masonry. If the structure moves or cracks, or the bond fails, water comes through and finding the leak can be difficult. It also leaves the wall behind it wet.
Drained protection (Type C, cavity drain membrane)
A studded plastic membrane fixed to the walls and laid on the floor, creating a small air gap. Water that comes through the structure runs down behind the membrane into a drainage channel and then to a sump, where it’s pumped out, or to a gravity drain if one is available.
- Pros: it doesn’t try to hold the water pressure back, it manages it. It copes better with old, uneven or salty walls and with some movement. The membrane can be plastered or dry-lined over to give a normal finish.
- Cons: it usually relies on a pump, which needs maintaining, a backup and an alarm. The drainage needs access points for servicing.
Type B
There’s also Type B, structurally integral protection, where the structure itself is designed to be watertight, such as waterproof concrete. It’s mainly relevant to new-build basements.
Which is best?
Neither is always best. BS 8102 sets out how to assess the risk and choose. On many older cellars I prefer drained protection, but each building is different, and sometimes a combination is right. What matters is that it’s designed by someone who understands structural waterproofing and installed properly.
When to call a specialist
If you're being quoted for tanking or a membrane system, have the cellar assessed by a CSSW-qualified surveyor so the right system is chosen. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
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Frequently asked questions
Is tanking the same as waterproofing?
Tanking is one type of waterproofing, a barrier system; membranes are another type.
Does a cavity drain membrane need a pump?
Usually, unless water can drain away by gravity.
Which is better, tanking or membrane?
It depends on the building and risk; BS 8102 guides the choice.