Basements waterproofing
Typical basement failures and remedial options
- Failure of the waterproofing system.
- Structural failure of the foundation or slab.
- An increase in hydrostatic pressure.
- Poor workmanship, leading to poor lapping of the waterproofing.
- Condensation occurring on buried pipes or cold bridges, or failure of mechanical extracts.
- Poor natural cross-ventilation.
- Leaks in pumped waste pipes that can create offensive smells.
- Careless installation of mains services.
- Basement seems to be tanked or lined but only ordinary render has been applied (removal is a simple solution).
- Damp penetration through front steps (a particular problem with half-basement designs).
Remedial options
- Injecting grouts and chemicals in the soil around the foundation walls, under slabs, behind walls, and into leaking cracks and joints.
- Alleviate the water pressure.
- Alter topographical elements and the use of internal and external drains.
- Apply additional internal waterproofing materials.
The threat of rising groundwater
Basements are considered by the BRE to be energy efficient, as a greater proportion of a property's outer walling is insulated against heat loss by surrounding ground. But set against the undoubted advantages of utilising below-ground space is the considerable risk of flooding from rising water tables. Due to a lack of good building plots, we are turning to inferior building land that was given a wide berth by our forebears – even using flood plain sites.
We are also facing problems from rising groundwater levels in several major UK cities, for example parts of Birmingham and London. Major cities have experienced a retreat of industry, resulting in less extraction of water from deep wells. During the years of water extraction, the water table in London gradually lowered, up until around 1965; from then on we have seen a continuous rising of groundwater.
Rising water threatens the continued use of below-ground space, and could damage building structures and services. The new British Library was built with pumps to protect the building and its contents.
Waterproofing an existing basement: general considerations
Faced with a damp basement, first think of how best to keep out penetrating dampness. But bear in mind that other issues will need to be addressed too.
A means of reducing or eliminating condensation needs to be designed in. No matter how well you succeed in combating moisture from outside the habitable space, the condensation enemy within could defeat all your efforts to create a dry living space. Design standards for basement waterproofing flag up the need for condensation control to eliminate dampness. Suitable heating, ventilation, insulation and lifestyle can eliminate or reduce the risk, as well as help provide a more comfortable and enjoyable living experience. With a lack of openable windows it is most likely that mechanical ventilation is needed – almost inevitably if the basement incorporates kitchen, bathroom, WC or utility room space.
A structural engineer's advice at an early stage is recommended if you plan to increase existing basement ceiling height, or carry out any alteration with structural implications. Work near to property boundaries could require preparation of a Party Wall Award under the Party Wall Act 1996.
Means of escape in case of fire (or even flood) must be designed for – and the relevant Building Regulations and codes of practice consulted and complied with. There will need to be consideration of lighting, and how best to use both artificial and naturally available light. These are some of the important design issues to address for a good quality of basement space to be achieved.
Basement damp patches
Often a damp patch may look nothing more than a £50 plaster repair, but a damp patch in a basement is a little more ominous and expensive, and going down the localised repair route could damage your professional reputation, rather than impress the client.
So a site meeting might not end on a good note. Instead of suggesting a localised repair, you will more often than not have given the client the painful truth: that the basement needs gutting.
The client will need to make a decision to 'manage' the dampness (i.e. fire-fighting damp) or install a comprehensive waterproofing scheme; that is, a 'cure'. Managing could just mean making good damp patches as they arise and redecorating. However, that could be a risky route to take, as there could be an underlying development of rot or mould that would be best tackled sooner rather than later.
Ten reasons why basement damp patches demand our respect
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Typical basement waterproofing works
- Removing occupiers.
- Removal/storage of all furnishings, personal effects, etc.
- Strip out of bathrooms, kitchens, services.
- Removal/adaptation of internal partitioning.
- Hacking off/general preparation of walls and floors.
- Removal/setting aside joinery items.
- Formation of drains/sumps as necessary.
- Application of waterproofing to walls and floors.
- Application of any loading coats/walls/slabs.
- Refitting kitchen/bathroom/WCs.
- Final finishes/decorations.
- Associated external works.
The scope of waterproofing required depends to some extent on the degree of 'dryness' required in the subject basement. Obviously, where valuable archive material is to be stored, a totally dry controlled environment is required. In a cellar under a house, used only for storage of non-valuable items, limited dampness may be acceptable.
The table below is an excellent starting point for waterproofing design. It illustrates how waterproofing strategy might need upgrading if we change use of the basement; for example, from occasional storage to residential use – Grade 1 to Grade 3. Seepage of water into the habitable space, or damp patches, would not be tolerated in a Grade 3 basement.
| Grade | Basement usage | Performance level | Form of construction |
|---|---|---|---|
| 1 | Car parking, plant room, workshop | Some seepage and damp patches | Type B |
| 2 | Workshops and plant rooms requiring drier environment, retail storage | No water penetration but moisture vapour tolerable | Type A, B |
| 3 | Ventilated residential and working areas including offices, restaurants | Dry environment | Type A, B, C |
| 4 | Archives and stores requiring controlled environments | Totally dry environment | Type A, B, C |
The concept: you may design basement waterproofing to meet a desired grade, or alternatively upgrade a basement to a higher grade. Certain forms of construction are more suited to upgrading. You could, for example, upgrade a Grade 1 environment to Grade 2 by installing a drained cavity system or by application of internal waterproofing. An existing Grade 2 basement could be upgraded to Grade 3 by additional ventilation, heating or insulation – or by dehumidification.
What constitutes a 'dry' environment?
What exactly is a 'dry environment'? CIRIA Report 139 offered useful guidance on how to interpret Table 1 of BS 8102. In section 2.2.3, 'Quantifying the required internal environment', each grade of basement was considered with respect to 'environmental parameters'. Table 2.2 of the CIRIA Report extended the limited information contained in Table 1 of BS 8102.
For example, for a Grade 3 basement (habitable space) we should expect RH 40–60% when the air temperature is at 18–22°C. Such conditions could be achieved from appropriate waterproofing, heating and ventilation. Regarding dampness, CIRIA recommends that materials at the surface of the completed interior should be air dry – and in CIRIA's terms this would mean an environment of RH less than around 70%.
It is clear from BS 8102 that the required grade of basement is achieved by a combination of waterproofing and heating and ventilation measures. Waterproofing contractors should themselves be aware of the need for correct heating, insulation and ventilation to achieve the required internal environment (i.e. to satisfy the standard's clause 8.4 'Control of Condensation'). You cannot just waterproof a basement and expect a dry interior. You may well consider monitoring the basement space after refurbishment, so a check can be made in the months that follow of internal air temperature, humidity and surface temperature. At least then if problems arise you are better able to advise your client on causation.
It is common for waterproofing contractors to be called back to a property suffering dampness once occupied and used. Condensation could be the cause, rather than a failure of waterproofing.
Waterproofing: damp control principles to apply
The author strongly supports the rationale adopted by Peter Bannister, explaining how you can stop or control moisture in 3 ways, and possibly 4:
- by barriers
- using drainage; and
- from evaporation.
'There is a case for adding "sheltering" – perhaps more relevant though to above ground damp management. Just adding a canopy over a doorway can eliminate rain penetration through defective door seals or threshold detailing. Older buildings seem to make more use of sheltering, a "support measure" that can reduce rain load on vulnerable external detailing…'
If you look closely at all the available methods on the market to waterproof a new or existing basement, you will see one or more of the above clearly evident. To achieve reliable waterproofing or damp-proofing it seems sensible for more than one of the above strategies to be used, so creating a second line of defence.
Types of waterproofing (as per BS 8102)
The standard classified waterproofing protection is plit into 3 distinct structural types:
- Type A – structures requiring protection against damp penetration
Such as concrete or masonry. These structures are not in themselves able to prevent damp penetration (of either water or water vapour) below ground, but need to be waterproofed by application of waterproofing materials internally or externally. Such structures, if in reinforced concrete, meet the requirements of BS 8102. Waterproofing would be by barrier materials, e.g. mastic asphalt, waterproof sheet, waterproof render or polyurethane resin tanking. - Type B – structures in watertight construction
Reinforced concrete designed and built to withstand damp penetration as well as to be structurally sound. - Type C – structures with internal drainage
Usually by means of wall and/or floor cavities. The drained cavity in the basement collects groundwater that has entered through the building fabric from outside in the ground. This water is usually drained to a sump and then removed by pump.
Waterproofing to existing basements commonly falls under category A – where, for example, concrete or masonry elements below ground are waterproofed directly internally by applied finishes or membranes – or category C – when a cavity drain waterproofing system is applied inside the structural walls. Either of these waterproofing methods can achieve basement waterproofing to Grade 3 or 4 standard.