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Cross-market term

Tanking and below-ground waterproofing

Keeping water out of a structure that sits below the surrounding ground, where water can arrive under pressure rather than merely running down a face. Achieved by a barrier, by the structure itself, or by draining the water away before it reaches the inside.
  • 4Markets
  • NoSame thing everywhere
  • 3Calculators

What it is called, by market

One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.

  • United Kingdom

    Close, not identical

    tanking

    also structural waterproofing, Type A barrier, cavity drain membrane

    Names the intent rather than the method, and covers three different approaches that BS 8102 separates as Types A, B and C.

  • Australia

    Close, not identical

    tanking

    also below-ground waterproofing, basement membrane

    As the UK, and also used for wet-area waterproofing inside bathrooms, which British usage more often calls a wet-room system.

  • United Statesyours

    Same word, different thing

    below-grade waterproofing

    also basement waterproofing, dampproofing, foundation waterproofing

    Waterproofing and DAMPPROOFING are distinct code terms: dampproofing resists moisture only and is not accepted where a hydrostatic head exists. The two are commonly sold as though interchangeable.

  • Canada

    Same word, different thing

    below-grade waterproofing

    also dampproofing, foundation wrap, drainage board

    As the United States, with the drainage layer against the wall treated as a standard component rather than an upgrade.

The governing standard, by market

  • United Kingdom

    BS 8102

    Protection of below-ground structures against water ingress, defining Type A, B and C protection and grades of internal environment.

  • United States

    IBC 1805 / ASTM D5385

    When dampproofing suffices and when waterproofing is required, and hydrostatic-pressure testing of membranes.

Calculators for this

Each works in either measurement system, and the terminology on the page follows whichever market you have selected.

Basement Wall Waterproofing Coating Coverage Calculator →Below-Grade Waterproofing Membrane Roll Calculator →Cavity Wall Drainage Mat Area Calculator →

Frequently asked questions

What is the difference between Type A, Type B and Type C?
Where the water is stopped, and each fails differently. TYPE A is a barrier applied to the structure — a membrane or a coating — so the water is held back at a surface, and the whole system depends on that surface being continuous; a single defect at a service penetration or a construction joint is a leak. TYPE B is the structure itself: concrete designed and detailed to be water-resistant, so there is nothing applied to fail, but the construction joints and the reinforcement cover become the critical items. TYPE C accepts that water will reach the wall and manages it: a cavity drain membrane holds the water off the internal finish and takes it to a drainage channel and a sump, which is maintainable but depends on a pump and its power. British practice frequently combines two, because the combination fails more gracefully than either alone.
Why does tanking so often get applied to the wrong side?
Because the accessible side is the inside, and water pressure does not care about access. A barrier on the OUTSIDE of a wall is pushed against the structure by the water, so the pressure helps hold it in place. A barrier on the INSIDE is pushed away from the structure by the same pressure, so it depends entirely on its bond and on the strength of whatever backs it — and a tanking slurry on an internal face of an old wall can be forced off in sheets. That is why external tanking is the preferred arrangement and why internal work is usually specified as a cavity drain system instead, which does not try to resist the pressure at all. The practical consequence for pricing: retrofitting external tanking means excavating, and the excavation is usually the larger number.
Does a damp-proof course do the same job?
No — a DPC stops moisture rising through a wall by capillary action, and tanking resists water arriving under pressure. They are different mechanisms with different magnitudes. A DPC is a horizontal barrier a few millimetres thick placed in a bed joint; it deals with water climbing slowly through pore structure, and above ground that is the whole problem. Below ground the water table can stand against the wall with real pressure behind it, and a capillary barrier does nothing about that. This matters when a basement is converted: the walls may well have a functioning DPC at ground level and still be entirely unprotected below it, and a specification that says the walls are damp-proofed has not said they are tanked.