A damp proof course stops moisture rising through a wall. It does not stop water being pushed through a wall by pressure. Below ground level, water in the surrounding soil does exactly that — and for that you need tanking.
What cementitious tanking is
A slurry-applied waterproof coating, made from inorganic chemicals, finely graded quartz sands and cement. Applied to wall and floor surfaces, it cures into a continuous barrier bonded to the substrate.
"Cementitious" distinguishes it from the alternatives — bitumen coatings, or cavity drainage membranes that manage water rather than resisting it. Cementitious tanking resists water directly, which is why it suits below-ground masonry under hydrostatic pressure.
Certified operators apply it with high-pressure spraying equipment, which gets an even coat into an uneven masonry surface far better than a brush.
When a wall actually needs it
A cellar, basement or any below-ground room. This is the main case. Soil around a below-ground wall holds water, and that water is under pressure. A damp proof course does not resist pressure — it interrupts capillary action. Different problem, different solution.
Where ground levels are higher outside than in. Common in older Northern Irish property, especially where paths, patios or driveways have been built up against a wall over the years. The wall is effectively retaining earth.
Rooms cut into a slope. Frequent in Newry, around the loughs, and on sloping sites generally, where one wall of a ground floor room is below the external ground level.
High water table. In Fermanagh around Enniskillen, sitting between Upper and Lower Lough Erne, the water table leaves very little margin. So does low-lying ground near rivers and canals.
When it is not needed. If the damp is at the base of a wall above ground level with a tide mark, that is rising damp and a damp proof course is the answer. If it appears after heavy rain on an exposed elevation, that is penetrating damp and needs masonry waterproofing. Tanking a wall that only needed a DPC is money wasted — our guide on rising damp versus condensation covers how to tell.
What the process involves
Tanking is as much preparation as application. The coating has to bond to the masonry, and it will not bond to anything loose.
Preparation
The surface must be completely free from dust, dirt, loose material, plaster, bitumen, oil and paint. In practice that means stripping back to sound masonry. Old paint or bitumen from a previous attempt has to come off entirely.
Levelling
Uneven surfaces are levelled and repointed with a specialised cement and sand mixture. A light screed or scratch coat evens out the substrate and needs a minimum of 24 hours to cure before tanking goes on.
Application
The slurry is spray-applied to wall and floor surfaces. Continuity matters more than thickness — water finds gaps, so the junction between wall and floor is the part that gets the attention.
Curing
The tanking stays soft for around 24 hours and must not be touched during that time. A second coat can follow once the first has hardened. Surfaces can be rendered after 48 hours, and the tanking becomes fully effective within about eight days.
Replastering
We recommend replastering using salt inhibitor products to our specification. Below-ground walls carry salts, and ordinary plaster over a salt-bearing background will spoil.
Living with a tanked room
Two things worth knowing before you commit.
It is a sealed system, so it depends on being continuous. Later work that penetrates the tanking — running a cable, fixing a heavy unit, cutting in a socket — can breach it. Plan fixings before it goes on where you can.
A waterproof room still needs ventilation. Tanking stops liquid water getting in; it does nothing about moisture generated inside the room. A sealed basement used as a utility room or gym, without ventilation, will get condensation. That is not a tanking failure, but it looks like one.
Why the specification matters
Tanking that fails, fails badly — water under pressure finds the weakest point, and remedial work means stripping back to masonry again. The variables that decide whether it lasts are preparation, continuity at junctions, curing time, and the right replastering.
We are Property Care Association members, which means working to a written specification with a guarantee as standard, and our operators are certified to apply these systems. We have tanked cellars, basements and below-ground rooms across the province since 1967.
Getting it assessed
If you have a damp cellar or below-ground room, a survey establishes whether tanking is genuinely what it needs, or whether a cheaper fix — lowering an external ground level, clearing a blocked drain, restoring ventilation — solves it.
Every survey comes with a written report. See wall tanking, or ring 028 9061 8343.