Domakihub How to Tank a Basement: Stop Damp Before It Damages Your Property
A practical guide to basement waterproofing — what tanking really involves, which system to choose, and when to call a licensed specialist.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 6 min read
Tanking a basement means applying a waterproof barrier — usually a cementitious slurry or cavity-drain membrane — to walls and floors so that ground moisture cannot penetrate the living or storage space. The video above walks through the slurry application process in clear detail; this article adds the full decision tree: which system suits your basement type, what surface preparation actually looks like, and at which points you need a qualified damp-proofing contractor rather than a capable owner. Whether you own a stone-built holiday villa in the Peloponnese, a concrete-framed Athens apartment block, or a hotel with a below-grade wine cellar, the principles are the same — but the execution must match the structure and the water table you are dealing with.
What "Tanking" Actually Means
Tanking is the trade term for making a below-ground space structurally watertight from the inside. Unlike surface sealants that simply coat the wall, a properly tanked basement creates a continuous barrier that can withstand hydrostatic pressure — the outward push of groundwater pressing against the outer face of your walls and floor slab. The British Standard BS 8102 (widely referenced by European waterproofing contractors) grades tanking by the final use: Grade 1 tolerates some humidity and is fine for car parking; Grade 2 is adequate for storage; Grade 3 means no moisture in the finished space, required for habitable rooms.
There are two primary systems. Cementitious (slurry) tanking bonds directly to the masonry and works best in new builds or well-pointed brick and block walls where the substrate is sound and the water pressure is relatively low. Cavity-drain membranes (also called Type C or drained protection) physically decouple the wall from moisture by channelling water to a sump and pump — the preferred solution for older stone structures, heritage buildings, or where the substrate quality is poor. Many Greek properties, especially those built before the 1970s, use rubble-stone walls that are too porous and irregular for slurry alone; a membrane-and-sump system is often the only reliable option.
Planning, Permits, and When You Need a Pro
In Greece, converting an existing below-grade storage space (ypogeia) into a habitable room typically requires a building permit (oikodomiki adeia) from the local Planning Authority (Poleodomia), because you are changing the use classification of the space. Work that alters load-bearing walls, cuts new openings, or modifies drainage toward the municipal sewer also requires a licensed civil or structural engineer (diplotaxiologos/politikos michanikos) to sign off the design. Do not skip this step: an unpermitted basement conversion in Greece can void your property insurance and complicate resale.
Even where no permit is needed — for example, simply damp-proofing an existing basement that already has the correct planning use — it is strongly advisable to hire a certified damp-proofing contractor for the diagnostic survey and the choice of system. They carry the specialist moisture meters, endoscopic cameras, and water-table data needed to prescribe the right waterproofing grade. You can absolutely prepare the space yourself and understand the specification before they arrive; the steps below will help you do exactly that.
5 Steps to Tank a Basement (Slurry Method)
- 1
Assess moisture source and water pressure
Before mixing a single bag of slurry, identify where water is entering and why. Use a moisture meter on walls and floor, and tape a square of polythene to the surface for 48 hours: condensation on top means humidity, condensation underneath means water migrating through the substrate. Note any rising damp from the floor slab — if water puddles after rain or the floor stays permanently wet, you almost certainly have a high seasonal water table and should consider a cavity-drain system with a sump pump instead of slurry alone. A professional damp survey at this stage saves significant cost later.
- 2
Prepare the substrate to bare, sound masonry
Slurry tanking only works if it bonds directly to clean, structurally sound masonry or concrete. Hack off all existing plaster, render, or wall coverings to a minimum depth of 150 mm above the visible damp line — in practice, strip the entire wall. Rake out loose mortar joints to at least 20 mm depth and re-point with a hydraulic lime or cementitious repair mortar rated for below-ground use. Fill any cracks, pipe penetrations, and construction joints with a fast-setting hydraulic plug compound, pressing it in by hand and holding against the water pressure for two to three minutes until it sets hard. The floor-to-wall junction (called the cove or fillet) is the highest-risk joint: form a smooth coved fillet of render, roughly 75 mm radius, using a waterproof render mix before applying any slurry coat.
- 3
Apply cementitious tanking slurry in two coats
Mix the tanking slurry (brands include Sika-101, Vandex, or Aquafin) to a thick, lump-free paste following the manufacturer's water ratio exactly — over-watering is the most common cause of failure. Dampen the substrate until it is saturated but not ponding, then apply the first coat at approximately 1–1.5 kg per square metre using a stiff masonry brush, working the material firmly into the pores and the reformed joints. Allow to cure for the time specified on the product data sheet (typically four to eight hours in 20 °C ambient temperature) before applying the second coat in the opposite direction to the first. Total dry-film coverage should reach the manufacturer's minimum for the hydrostatic head your survey identified — more water pressure requires more coats and a higher build. Protect curing slurry from direct sun and dry winds, especially in a Greek summer: mist with water if drying too fast.
- 4
Seal all penetrations and service entries
Every pipe, conduit, or cable that passes through a tanked wall is a potential leak path. Where services penetrate the wall, use a proprietary pre-formed collar or a puddle flange set into the masonry before the tanking coats are applied. Any service entry added after tanking must be core-drilled and filled with a hydraulic plug before being finished with a compatible sealant bead at the wall face. Do not rely on general-purpose silicone sealant for below-ground penetrations — it has no bond strength against hydrostatic pressure and will fail within months. This step is also where a licensed plumber must be involved if you are altering drainage connections to the main sewer.
- 5
Install drainage, ventilation, and finishes
Even a well-tanked basement benefits from managed drainage. Fit a perimeter drainage channel at floor level (especially if a sump pump is part of the system) so any residual water has a controlled exit point rather than building pressure against the tanked face. Ensure the space has mechanical ventilation — in Greece's climate, a basement without airflow will accumulate humidity even through a perfect tanking membrane. Once the slurry is fully cured (typically 28 days for full strength), you can apply a floating floor system or a sand-and-cement screed directly. Do not glue down rigid finishes until you are confident no residual moisture remains: pin the polythene test sheet to the floor again and check after 72 hours. Final habitable-room finishes (insulation, stud walls, electrics) follow standard construction sequence and require the usual permits where applicable.
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FAQ
- Can I tank a basement from the outside instead of the inside?
- External tanking — applying a waterproof coating or membrane to the outer face of the wall — is the most effective approach because it stops water before it enters the structure. The problem is access: in most urban Greek properties and established holiday villas, excavating around the full perimeter to expose the external wall face is extremely disruptive and expensive. External tanking is worth specifying on new builds or major ground-up renovations. For existing buildings, internal tanking (either slurry or cavity-drain) is nearly always the practical choice.
- How long does basement tanking last?
- A correctly applied cementitious tanking system typically lasts 20–30 years before the masonry substrate or mortar joints begin to deteriorate and need attention. Cavity-drain membrane systems last longer in terms of the membrane itself (50+ years is common for HDPE membranes), but the sump pump will need servicing every two to three years and replacement every ten to fifteen years. The single biggest factor affecting longevity is surface preparation: slurry applied over soft plaster or un-pointed joints can delaminate within two to three years.
- Does basement tanking need a building permit in Greece?
- The tanking work itself — applying a waterproof coating to an existing below-ground structure — does not generally require a permit if you are not changing the use of the space or altering the structure. However, converting a below-grade store room (ypogeia) into a habitable room (bedroom, office, gym) does require a building permit and a licensed engineer's signature. Altering drainage connections or creating new openings also triggers permit requirements. Always check with your local Poleodomia or a licensed civil engineer (politikos michanikos) before starting work, particularly for properties in conservation zones (diatirimena) or on registered archaeological-sensitivity land.