10 Type C (drained) protection


Type C waterproofing protection manages water that penetrates the external shell of a structure, by collecting it in a cavity formed between the external wall and an internal lining/wall. There is permanent reliance on this cavity to collect groundwater seepage and direct it to a suitable discharge point, e.g. drains or a sump for removal by gravity drainage or mechanical pumping.

New construction generally incorporates a cavity drain membrane. However, the use of other products and techniques, such as drained voids constructed in masonry, can also be considered. Traditionally, the cavity in floor construction has been formed by the use of either no-fines concrete or ceramic tile systems. These are rarely used in new construction, but might be encountered when refurbishing existing structures (see 5.2.3).

Schematic illustrations of Type C protection are given in Figure 2c).

10.1 Structural aspects

The outer leaf of the exterior wall should be capable of controlling the quantity of water that can pass through it, in order not to exceed the drainage capacity of the system. Water entering a drained cavity system is regulated by the structure, so defects that might result in unacceptable leaks should be remedied before the system is installed.

10.2 Cavity drain systems

NOTE Cavity drain systems do not change the loadings due to water on an existing structure, other than where remedial measures are taken to control water ingress.

10.2.1 Cavity drain systems with membranes Cavity drain membranes

NOTE Where cavity drain membranes are used, the membrane forms a permanent cavity between the external elements of the structure and the internal wall/floor finishes. Such cavities vary in width, depending on the stud height or profile of the membrane, but are usually up to 20 mm.

Cavity drain membranes should be used in accordance with the manufacturer's instructions. In particular, the stud height or profile of the membrane should be selected in conjunction with the manufacturer's data and after considering the external hydrostatic pressure, the porosity of the structure and the predicted rates of water ingress though the structure's external fabric.

Cavity drain membranes can be used on surfaces that have been contaminated with impurities. However, in these situations, consultation should be undertaken with the local environmental agency regarding discharge from the system.

Before a cavity drain membrane is laid or fitted on walls and floors constructed of new concrete, the concrete surface should be treated to reduce the risk of leaching of free lime or mineral salts and to avoid the obstruction of the drainage system. Floor cavities

Where the floor cavity incorporates perimeter channels that discharge into a sump(s), both the channels and the sump(s) should be cleaned before, during and after installation of the membrane to allow uninterrupted drainage (see also 10.3).

Before the cavity drain membrane is laid:

  • a) the floor should be flood tested to confirm that all water runs freely to the points of collection; and
  • b) the base slab should be cleaned to remove all debris that might cause blockages.

Sections of the membrane(s) laid across the floor should be jointed and sealed.

Once laid, the membrane should be protected against damage caused by following trades.

The membrane should be inspected for damage and any defects should be remedied before floor finishes are applied.

NOTE The cavity drain membrane may be covered with a variety of floor finishes dependent on the design requirements of the structure. Wall cavities

The wall cavity should be constructed so that it remains free draining at all times.

Where the wall cavity incorporates perimeter channels that work in conjunction with a drained floor cavity, the drainage, which is common to walls and floors, should conform to

Before the cavity drain membrane is fitted, in situations where the cavity is to be constructed or installed over existing walls, all wall coverings that might decay, or become loose or friable, should be removed.

NOTE 1 If they are not removed, such wall coverings can cause the blockage of the cavity or drainage channels and impede free drainage.

Sections of the membrane(s) fitted to the walls should be jointed and sealed ensuring adequate laps.

NOTE 2 The cavity drain membrane may be covered with a variety of wall finishes dependent on the design requirements of the structure.

10.2.2 Cavity drain systems without membranes

Where a drained cavity is formed by a masonry cavity wall, the inner leaf should be either built off a concrete upstand, constructed integrally with the slab, or built of engineering bricks to a minimum of 150 mm above channel level.

Materials should be selected which are appropriate to the environment on both faces.

Where the cavity is constructed against an embedded retaining wall, allowance should be made for the permitted construction tolerances, in order to maintain the necessary channel width.

Care should be taken during construction to keep the cavity clear of debris and mortar droppings.

In order to allow free drainage from the channel and access for maintenance, it is recommended that this channel is laid nominally level but with adequate access points for maintenance through the inner leaf of the cavity wall. Where possible, the drainage channel should be formed within the depth of the slab. Where this cannot be achieved, the detailing should be such that water cannot migrate from the cavity across the slab.

A drained cavity to the roof should be considered either as part of the base construction or, where necessary, as part of the remedial measures.

10.2.3 Cavity ventilation

It is not usually advisable to ventilate the cavity; however, it might be necessary in certain circumstances, such as where there is a potential for radon, methane or other ground gases and contaminants to be present. In these circumstances, specialist advice should be sought during the design phase.

10.3 Maintenance and commissioning

10.3.1 Design

In order to maximize the long-term integrity and effectiveness of a waterproofing system incorporating Type C protection, the waterproofing system should be designed to be maintainable.

Access points that allow routine maintenance of channels and outlets should be incorporated into the design of the waterproofing system.

10.3.2 Installation and inspection

Immediately after the installation of a cavity drain system, drainage channels and sumps should be cleared out and tested. Pumping devices should be checked, tested and fully commissioned in accordance with the manufacturer's instructions.

NOTE In circumstances where pumps are running for long periods of time, or where the system is subject to silting or the deposition of free lime, more frequent servicing might be necessary.

The servicing requirements for the waterproofing system should be clearly set out in the documentation supplied by the designer to the client, including the need for regular planned maintenance of the drainage and/or pumping systems not less than once a year.

The client should be informed that any failure to adhere to the maintenance schedule could result in a failure of the waterproofing system.

11 Remedial measures

11.1 General

NOTE There are many causes of seepage in new and existing structures, principally poor design and/or specification, defective materials, defective workmanship, deterioration of the structure, or a change in the external environment (e.g. rising groundwater or locally leaking sewers or water mains). Adjacent construction works can also affect the pattern of groundwater flow and surface water run-off. A number of these factors can also combine to cause problems.

Before any remedial action is taken, defects should be diagnosed to determine the cause and extent of failure. The correct diagnosis of the fault is of vital importance, to establish whether faults exist with the system as a whole, or whether faults are localized (see also 5.2).

Where remedial work is required, the following measures should be considered:

  • a) the installation of a tanking system or a drained cavity;
  • b) the installation of external drainage; or
  • c) localized works to the fabric of the structure, such as:
    • 1) pressure or vacuum grouting (see 11.2);
    • 2) crack sealing with resin or cementitious mortar (see 11.3);
    • 3) crack filling by pressure or vacuum injection (see 11.4);
    • 4) the replacement of locally defective material (see 11.5).

These should be considered irrespective of whether planned remedial treatment has been included as a contingency measure in new construction or for maintaining or improving the internal environment of an existing structure.

Repairs to concrete should be carried out in accordance with the relevant part(s) of BS EN 1504.


If the location of a defect in an existing external membrane can be established, it is possible in some instances, where access is not a problem, to expose the membrane and carry out repairs by excavating locally, adjacent to the walls of the structure.

Where the internal membrane is fully bonded to the substrate, the defect is generally located at the position where dampness or seepage occurs. Once the internal finish has been removed to reveal the defect, the membrane can be locally repaired using compatible materials.

Where the internal membrane is only partially bonded to the substrate, the defect might be more difficult to locate. Even if the defect can be found, further migration to other defects can follow. In such cases, consideration needs to be given to removing and replacing the entire membrane with a new bonded system.

Locating defects in a sandwich system can be particularly problematic and localized repair is seldom viable.

Where access is not feasible, grouting in accordance with 11.2 and 11.4 ought to be considered.

11.2 Pressure or vacuum grouting

Grouting to cut off seepage might repair isolated defects. However, it should be noted that, where a large number of defects occur, it is more effective to prevent water ingress by other methods.

There are a number of grouting materials available for use based on:

  • a) cement;
  • b) bentonite;
  • c) chemical (e.g. acrylic);
  • d) resin (e.g. epoxide, polyester and non-expansive polyurethane);
  • e) expansive polyurethane;
  • f) modified rubber latex.

Holes should be drilled through the walls and floor of the structure adjacent to the defect (sometimes inclined to intersect seepage paths) and the pressure or vacuum grout should be driven into the material behind to gel and seal the leak. As grout selection and application are specialized techniques, advice should be sought from manufacturers and experienced applicators prior to use.

NOTE Often more than one phase of grouting is needed as the seepage might be moved to defects elsewhere in the structure or higher up in the walls.

Care should be taken when grouting larger areas to avoid blocking any external drainage systems.

11.3 Crack sealing with resin or cementitious mortar

Where structural continuity is not required and there is no hydrostatic pressure against the adhesion of the repair, cement grout, neat cement grout or low viscosity latex emulsion should be brushed into cracks and porous areas to seal them against water ingress.

Suitable materials include:

  • a) cementitious slurry;
  • b) cement/silica-fume slurry;
  • c) polymer-modified cementitious slurry;
  • d) polymer resin;
  • e) cementitious crystallization systems;
  • f) epoxide putty.

11.4 Crack filling by pressure or vacuum injection

Pressure or vacuum injection can be used to fill and seal cracks and joints, particularly at kickers where a waterstop has become displaced and cutting out and replacement is not practical.

NOTE 1 Porous areas of concrete can sometimes be injected successfully, but severe honeycombing might require the defect to be cut out and replaced.

NOTE 2 Injection techniques have been used to seal very fine cracks.

The selection of grout systems should take into account:

  • a) the likelihood of structural movement;
  • b) the nature and size of the defect;
  • c) the moisture content of the substrate;
  • d) the temperature of the structure;
  • e) the injection method to be used.

Where the wall or floor is expected to remain damp, a water-tolerant grout should be used. Suitable materials include:

  • 1) epoxy resin;
  • 2) polyurethane resin;
  • 3) acrylic resin;
  • 4) polyester resin;
  • 5) styrene-butadiene rubber (SBR) and acrylic emulsions;
  • 6) polymer-modified cementitious grouts.

11.5 Replacement of locally defective material

NOTE 1 Where a relatively small number of well-separated defects in the walls or floors result in seepage (e.g. poorly compacted concrete), adequate repairs can be achieved by cutting out and replacing the defective area. Achieving a water-resistant joint between the substrate concrete and the repair material is the most critical feature of this method. The installation of temporary drainage points, in the area to be repaired, might be necessary in some situations to control the seepage. However, some repair materials are formulated for application where running water is present.

NOTE 2 Surface preparation and the compatibility of the physical and chemical properties of the repair material are important. Many types of repair materials are available, including:

  • a) concrete compatible with that used in the original construction;
  • b) polymer-modified cementitious mortars and concrete;
  • c) polymer-based mortars;
  • d) sprayed concrete.

The properties of the repair material should be selected to match the substrate as closely as possible, particularly the shrinkage and thermal behaviour. Specialist advice should be obtained to select the most suitable concrete repair system and to specify the required performance of the material to suit the size, depth and location of the area to be repaired.

Where proprietary mortars or concretes are used, they should be applied in accordance with the manufacturer's instructions, and any repair should be undertaken by experienced contractors.


Standards publications

For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

BS 5454, Recommendations for the storage and exhibition of archival documents

BS 8485, Code of practice for the characterization and remediation from ground gas in affected developments

BS EN 934, Admixtures for concrete, mortar and grout

BS EN 752, Drain and sewer systems outside buildings

BS EN 1011, Welding- Recommendations for welding of metallic materials

BS EN 1992-3, Eurocode 2: Design of concrete structures - Part 3: Liquid retaining and containment structures

BS EN ISO 15614-1, Specification and qualification of welding procedures for metallic materials - Welding procedure test - Part 1: Arc and gas welding of steels and arc welding of nickel and nickel alloys

Other publications

[1] INSTITUTE OF CIVIL ENGINEERS. Specification for piling and embedded retaining walls. Second edition. London: Thomas Telford Publishing, 2007.

[2] GABA, A.R., SIMPSON, B., POWRIE, R. AND BEADMAN, D.R. Embedded retaining walls - Guidance for economic design. C580. London: CIRIA, 2003.

[3] GREAT BRITAIN. Building Regulations 2000 and subsequent amendments. London: The Stationery Office.

GREAT BRITAIN. Building (Scotland) Regulations 2004, as amended. Edinburgh: The Stationery Office.

GREAT BRITAIN. Building Regulations (Northern Ireland) 2000, as amended. London: The Stationery Office.

[4] BUILDING RESEARCH ESTABLISHMENT. Radon - Guidance on protective measures for new buildings. BR211. Watford: BRE Press, 2007.

[5] BUILDING RESEARCH ESTABLISHMENT. Radon - Guidance on protective measures for new dwellings in Scotland. BR376. Watford: BRE Press, 1999.

[6] BUILDING RESEARCH ESTABLISHMENT. Radon - Guidance on protective measures for new dwellings in Northern Ireland. BR413. Watford: BRE Press, 2001.

[7] GREEN, B.M.R., MILES, J.C.H. AND REES, D.M. Radon in Dwellings in Scotland: 2008 Review and Atlas. HPA-RPD-051. Didcot: Health Protection Agency, 2009.2)

[8] BAMFORTH, P.B. Early-age thermal crack control in concrete. C660. London: CIRIA, 2007.

Further reading

BS EN 1991, Eurocode 1: Actions on structures

BS EN 1996, Eurocode 6: Design of masonry structures

BS EN 13252, Geotextiles and geotextile-related products -Characteristics required for use in drainage systems

BS 8102:2009 Code of practice for protection of below ground structures against water from the ground