Damp management and remediation
Structural waterproofing
Structural waterproofing, usually below ground, protects against penetrating dampness. It forms a 3-dimensional watertight 'box' by applying materials to the outer or inner sides of walls and floors. It is known as tanking. This is applied as new work when the building is built (internally, externally or sandwiched) or as a retrofit application inside existing walls.
Great skill and care is required in selecting what product and method to use for structural waterproofing. The wrong choice of tanking system could, for example, result in moisture being pushed to the above-ground walls and habitable space higher in the building. The breathability factor is as important for below-ground walls as it is for above-ground walls.
Remember to consider management or cure. In basement walls in older properties you never cure the damp, but you can manage it. Forming ventilated cavity walls, the use of ventilated physical vertical and horizontal dpms, and the use of below-floor drainage channels and pumps are some examples of management. However, the application of directly applied asphalt or cementitious systems could upset the equilibrium and divert the damp to other parts of the building. Useful guidance on basement water proofing can be found in BS 8102: 2009.
Wherever possible the primary defence against ground water entering a building should come from applying the damp-proofing systems on the outside of the building – only this offers a total cure. Any system applied to the inside can only ever manage the problem.
There are cementitious based products that incorporate additives, which result in the cured concrete becoming watertight. There are also prefabricated interlocking concrete panels that are factory made and assembled on site to form a waterproof box.
One of the most difficult problems is the routing of service pipes and drainage entering the building below ground and directly through the earth-retaining walls. Gas pipes, water main supply pipes and electrical cables often enter buildings via basements, particularly in commercial and domestic buildings where there was never an intention to use the below-ground areas for either storage or occupation.
Changes in the use of a building pose the greatest challenges to keeping out damp. Many older properties within the central business districts of cities like London and Paris are valuable real estate. They have brick-vaulted ceiling basements and supporting columns that often extend under the pavement areas and part of the highways. These basements pose great challenges for specialist consultants and damp-proofers to find solutions to keep out the damp (enabling clients to realise a newly discovered asset).
One of the biggest threats to basements is flooding. While the erection of the Thames flood barrier has given greater confidence to London property owners, other cities and towns often suffer from catastrophic flood disasters. Basement rooms do not stand a chance from such a force of nature.
What to consider for tanking
Below-ground structural waterproofing should ideally form a watertight and 5-sided compartment or box. The waterproofing should continue up to 150mm above external ground level, at which point it should link to a horizontal dpc. This is often hard to achieve due to the presence of changing ground levels or complications of building detailing.
Waterproofing should be continuous at junctions of walls and floors and such potential stress points should be reinforced to reduce the risk of cracking or damage to the structural waterproofing.
Test underground services and repair any leaks to give the waterproofing the greatest chance of success. Remember that escapes of water from defective underground drainage and potable water main supplies could become a problem to neighbouring properties and this should be factored into any design scheme. Always assume that the local water table could rise to a height of ¾’s of the structure below ground (BS:8102). Consider steps to reduce the hydrostatic pressure of water by installing subsoil drainage where practicable or cut-off drains on the higher side of the basement to divert water. Take care to reduce the likelihood of condensation internally by selecting suitable heating and ventilation regimes, and carefully managing the generation of moisture that could create high-humidity conditions. Where a waterproofing system allows the transfer of water in vapour form, choose appropriate breathable internal finishes (that also meet the guarantee conditions).
Structural waterproofing is specialist and demanding work. Pay careful attention to:
- use of specialist approved and trained contractors;
- selection of the generic systems;
- choice of materials;
- workmanship;
- detailing; and
- insurance backed guarantees that often require independent insurance inspections during the course of the installation of the water proofing scheme.
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Types of structural waterproofing to consider:
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Carefully scrutinise a contractor's guarantees before accepting tenders to ensure that the client's interests will be looked after. Guarantees may not, for example, cover the client for any damage caused by defective water-proofing to internal finishes or client's effects or furnishings, etc. There may also be costly re-inspection fees, payable up front by your client and only reimbursed if the contractor considers that the actual waterproofing has failed. Such unfair terms can be distressing to clients.
For a general background to the systems available, refer to chapter 8 of Dampness in buildings (Ref. 1), the BCA design guide: Basement waterproofing (Ref. 2) and the Code of Practice: Remedial Waterproofing of Structures Below Ground.
Before deciding on a structural waterproofing system, decide which grade of basement environment you want to achieve. BS 8102:2009 divides basements up into 4 grades:
- Grade 1: basic utility, e.g. car parks;
- Grade 2: better utility, e.g. workshops;
- Grade 3: habitable, e.g. residential; and
- Grade 4: special, e.g. archive storage.
Grade 3 is usually the aim for residential space. Some waterproofing systems lend themselves more readily to an 'upgrade' from one category to the next.
After choosing what grade you require, consider how each system under consideration meets the following criteria:
- the degree of hydrostatic pressure predicted;
- adhesion/bonding/fixing capability to the substrate;
- ability to cope with structural movement;
- expected design life of the system in relation to years of service required;
- ability to be formed/installed to intricate/complex configurations or detailings;
- ability to achieve a good seal around penetration of services;
- the finished depth or thickness of the system and how this might reduce the internal size of the basement space;
- ease of repair; and
- other general performance requirements such as condensation performance, acoustic performance, thermal insulation, water vapour resistance, chemical resistance, fire resistance, structural integrity, and maintenance management issues.
Where it is not possible to affect a reliable tanking system below ground due to the prevailing site conditions, or the configuration or construction detailing of the existing building, your client must be fully informed concerning the risks involved and the importance of 'managing' dampness in the subject space rather than expecting a 100% cure. Creating a perfectly 'dry' below-ground habitable space is not straightforward nor is it easily achieved over the longer term.
There are 3 categories (A, B and C) of basement water proofing listed in BS 8102:2009. One of the most effective systems is a type C drained cavity membrane system for managing dampness to existing buildings, which can also be used in new constructions below ground. These types of system often require installed sumps with pumps and perimeter drainage channels, fitted with battery back and alarm systems. Type A and B systems involve cementitious systems incorporating additives and reinforced water proof concrete, which offer good protection from ground water intrusion.
Considering the intended use and occupation of the area is key when selecting which system to use. Fixings onto walls (e.g. for heavy duty shelving, installation of sockets, routing of drainage) often emerge as a key factor in determining which to choose. Space will be another consideration and can often compromise the choice of tanking system.
Ground gas Radon contamination is another consideration when tanking out existing and newly constructed basements. Areas known to be potential sources of Radon gas can be checked with Local or District Council engineers departments, Building Control or through the Environment Agency.
Nearness of large trees and vegetation, including damaging Japanese knotweed, can undermine structures below and above ground and damage type A and B water proofing systems. Root systems can penetrate into drainage pipes underground, disturb potable water mains supplies and even penetrate through type C cavity membrane systems.
The intended use and occupation of a building determines whether the goal is just reduction or complete elimination of damp. For example, it would be essential to prevent groundwater entering into the basement of an industrial building storing electronic products or operating computer systems; the same is true for basements intended for human occupation. Conversely, it would not be necessary to achieve a totally dry environment if the intended use was to store plastic crates.
In areas at risk of flooding modern buildings are often purposely designed with the basement acting as a storm drain, especially where the building is near a river or flood plain area. Here the purpose of the basement is entirely to manage the excess of water in a worst-case scenario. It is the only type of basement in occupied buildings designed to actually take in water.
In extreme cases, and where the local topography allows, planting several willow trees may be a more practical measure than it sounds and may even offer a permanent cure. (However, you then run the risk, if the cure is too effective, that desiccation of the ground would cause an entirely different problem.)
Further information
- Ref. 1: Oliver, A. (revised by Douglas, J. and Stirling, J.S.), Dampness in buildings (2nd edition), Blackwell Science, 1997 (ISBN 0632040858)
- Ref. 2: BCABCA design guide: Basement waterproofing, British Cement Association, 1994 (ISBN 9780721014753)
- BS 8485.
- BS EN : 1992-3.
- Building Research Establishment. Radon Guidance on protective measures for new buildings, BR211, BRE Press, Watford 2011.
- RICS Guidance on Japanese knotweed.
- Damage to Buildings Caused by Trees, BRE Good repair guide 2, BRE Press, Watford.
- Institute of Civil Engineers; Specification for pilling and embedded retaining walls, second edition, Thomas Telford publishing, 2007.