How to tank a basement or cellar from the inside (UK guide)

Turning a damp basement or cellar into dry, usable space means holding back the ground water that constantly presses against below-ground walls and floors. Tanking — applying a continuous waterproof barrier to the inside faces — is one of the main ways to do it. Done properly it's a reliable, long-lasting solution; the detail that makes or breaks it is continuity.

Before you start: is tanking the right approach?

There are two main ways to waterproof a basement: tanking (a barrier that resists water pressure) and a drained cavity system (a studded membrane that collects any water and channels it to a sump and pump). Tanking suits many cellar conversions and lower-risk situations; high water tables or structurally sensitive cases may call for a drained system or a combination. For a habitable conversion it's worth getting a waterproofing design from a specialist — NHBC and British Standard guidance set out how these systems should be specified.

Step 1: Prepare the surfaces

Tanking only bonds to a sound, clean background. Rake out and repair loose or defective render and mortar, remove old paint, laitance and contamination, and address any active leaks or running water first. Corners between walls and the floor are usually formed with a fillet so the membrane can turn the junction without a sharp edge.

Step 2: Apply the tanking membrane

A liquid-applied membrane is brushed or rolled across the walls and floor in coats, building up a continuous, seamless waterproof layer. Waterproof Blue Liquid DPM is well suited to this — it forms a jointless barrier across both walls and floor, which is exactly what tanking needs. Build the number of coats up to the recommended thickness, and maintain a wet, continuous film with no pinholes or missed patches.

Step 3: Get the wall-to-floor junction right

This is where most basements leak. Ground water will exploit any break where the wall membrane meets the floor membrane, so the two must be lapped and joined into one continuous tank around the entire perimeter, carried over the fillet. Treat the junction as the most important part of the whole job, not an afterthought.

Step 4: Protect and finish

Once the tanking has cured, it's protected and finished — typically battened out or dry-lined, or rendered/screeded over, depending on the system. Avoid puncturing the membrane with fixings that breach the barrier; follow the system's guidance for finishing without compromising the waterproof layer.

Common mistakes

• Skimping on the junction. An unsealed wall-to-floor joint defeats the whole tank.
• Tanking over a poor background. The membrane is only as good as what it's bonded to.
• Ignoring the water table. High-pressure situations may need a drained cavity system, not tanking alone.
• Too few coats / pinholes. Under-thickness or missed spots are leaks waiting to happen.

Related reading

• Which waterproofing product do you need?
• How to treat rising damp in walls and floors
• NHBC standards: waterproof membranes as a tanking system

Frequently asked questions

Is tanking the same as a damp-proof membrane?

They use similar materials, but tanking is a full waterproof “tank” designed to resist water pressure below ground, whereas a basic DPM resists ground moisture. Below-ground habitable space needs tanking-grade waterproofing.

Can I tank a basement myself?

Preparation and application are achievable for a competent DIYer on lower-risk cellars, but habitable conversions should have a waterproofing design and ideally specialist installation — the consequences of failure are expensive.

Tanking or a drained membrane system?

Tanking resists water; a drained cavity system manages it to a sump and pump. The right choice depends on the water table and risk — sometimes both are used together.

Planning a basement or cellar conversion? Talk to our technical team about the right tanking specification.