Dampness in building elements

The words 'damp' and 'basement' seem to be inexorably linked. From the first day the basement is built or waterproofed, forces of nature try to fill it up with water until it reaches a height equal to the surrounding water table. In fact British Standard 8102:2009 suggests that you should assume that the surrounding water table to the below ground structure will reach ¾’s of the height of the structure at some stage during the life of the basement. The distance between the wet subsoil and the internal plaster finish or the floor of the basement is often no more than the length of 1 brick in many existing older buildings (Pre-1910) – an easy and short journey for moisture.

Basements may be subject to any of the 5 key moisture sources:

• air moisture condensation;
• penetrating dampness;
• internal plumbing leaks;
• below-ground moisture; or
• site/building specific sources.

Dampness problems may arise from a combination of any of the factors already discussed in previous pages. For that reason, the general surveying of basements merits a dedicated section.

First, we need to tackle some terminology: there is a difference between a 'cellar' and a 'basement'. The Oxford Dictionary says a cellar is

'a room below ground level in a house, used for storage, esp. of wine or coal'

whereas a basement is

'the lowest floor of a building usually at least partly below ground level'.

The Penguin Dictionary of Building mentions the use of cellars for locating a central heating boiler. It follows that a cellar originally used for storage in old property will probably become a basement if it is changed or improved to be used generally as habitable space. A 'sub-basement' is a further storey below a basement. Semi-basements are partly underground.

A cellar in an old building may be nothing more than a deep void under the ground floor, often with limited headroom, with unfinished or merely limewashed masonry walls, and some form of 'solid' floor (usually without a dpm).

Ventilation may be provided by an air vent below the front entrance door threshold, coal chutes, trap doors, windows to walls above ground level, entry doors, and stairways from the ground floor. Any or all of these may well produce an internal environment in which materials perform satisfactorily for many years, and particularly so if the site is not unduly wet.

In the context of a pre-purchase survey, you need to assess whether significant dampness exists, what the cause and source of the dampness might be, the implications of the dampness should it persist, and what may be done to solve any problems associated with significant dampness if it is found.

Dampness may threaten the construction or stored materials, or it could make conditions in the basement unsatisfactory for living in. Either of these 2 scenarios could reduce the potential value of the space, or limit its potential use and enjoyment. However, the basement may be in a condition fit for certain usage such as general goods storage, although unsuitable for use as a habitable space.

Equally, you must not lose sight of the other very important aspects of basement surveying: you must assess the basement in terms of means of escape in case of fire, fire separation, rules and regulations concerning use of habitable space, structural issues such as basement walls acting as load bearing and retaining walls, or where basement floors have been lowered with a resultant effect on foundations. Town Planning and Building Regulation requirements must be considered carefully too.

Above all, the client needs to be realistic: creating a good standard of habitable space could involve completely stripping out the space and installing a patent tanking system, which would be expensive.

Dampness investigations of basements are difficult for various reasons:

• There is often a change of ground levels around the basement so that some wall areas are below ground and some areas above ground.
• It is necessary to establish the height of the local water table.
• There is a lack of access to the structural walls and floors, with internal surfaces often lined or covered with a variety of materials and fixtures, e.g. gas meter.
• Where a wall changes from below ground to above ground there will usually be 2 kinds of damp-proofing required – vertical damp-proofing and a horizontal dpc at least 150mm above finished ground level – and careful linkage of the damp-proofing is needed.
• Other methods could include building an inner wall and cavity, or a 'bund' wall.

Just as damp-proofing companies make their living by damp-proofing, so firms specialising in basement waterproofing earn their crust by installing waterproofing systems. Advice from any specialist must be viewed with the aims of the specialist company in mind. If an existing basement suffers a dampness problem, the full range of remedies must be considered before expensive remedial solutions are entertained.

However, times are changing and a new wave of basement extensions have sprung up in many large cities, mainly starting in central London, where even older properties without a basement have been extended downwards and in a growing number of cases by 2 storeys below the original ground floor of a property. Often referred to as iceberg’s these basement extensions can sometimes be deeper underground than the building is tall above the ground. These have attracted a great deal of adverse comment from neighbouring property owners who have encountered disruption from heavy plant and machinery as well as large ready mixed concrete trucks and other site vehicles halting local traffic. There have also been disturbances to adjoining buildings and Party Wall Awards being made.

The results of resident lobbying and complaints have resulted in some central Local Authorities banning iceberg basement extensions, meaning that anything over one storey below ground would not be allowed.

The effects of adding new basements wholly below ground (technically a cellar extension) creates difficulties in investigating when dampness appears internally, as many of these types of extensions use a ‘wet’ underfloor heating system through a network of heat resistant plastic pipes connected to manifolds before the boiler. Some have been found installed without drainage sumps, pumps connected to alarms and with battery back-up in case of power failure.

The effect on the local water table is not fully understood and more eimportantly, given the changes in rainfall intensity, often blamed on climate change.

Some basements to new constructions have used continuous piles and waterproof concrete and some prefabricated interlocking concrete panels. Problems have been discovered where ground water penetration has been found due to inadequacy of the water proofing additives into the concrete. This causes excessive cracking due to missing water bars at the critical floor slab to wall junctures.

There is a lack of adequate external additional attenuation schemes to manage the water table in accordance with the principles contained in the SUDS guidance and British Standard of site condition geotechnical surveys. Often the water table is not monitored for a full 12 months due to pressure on time and costs. This can result in a lack of understanding in sub ground profiles and ground water conditions.

Typical ground condition surveys (BS: 5930, BS1377 and Eurocode 7) would involve bore holes being put down to carry out the following tests:

• Piezometer tubes to determine the height and fluctuation of the water table at depths typically up to 10 meter depths.
• In situ soil shear strength –using shear vane technique.
• Log soil samples and soil sampling.
• Make-up of the geology of the site and permeability or impermeability of the sub strata, including PH and sulphate levels.
• Radon gas analysis.
• Soil thermal resistivity (STR).
• Atterburg Limits - liquid limit, plastic limit and plasticity of soils.

From the above tests and monitoring, the design of the basement structure can be drawn to including the type of water proofing and attenuation scheme required to manage the ground water conditions at or near the site.

Preparation

Before visiting the site you should make a number of enquiries:

• Research the local geology, soil conditions, and water courses.
• Ask the local building inspector about basement tanking systems used locally and their performance.
• Find out if there have been any building repairs or improvements that might affect basement moisture conditions.

Also obtain details of any tanking or structural waterproofing specification and a copy of the guarantee. Guarantees for basement waterproofing are usually of limited duration, perhaps only 5 or 10 years, and many contractors do not offer guarantees for such works so the client would have to rely on the protection of common or statute law in the event of any failure. (As for all other guarantees for building works, it is important to find out whether the guarantee runs with the property or just the individuals that are party to it. Also find out if there are any unreasonable exclusion clauses.)

External conditions

Once on site, your first task is to sketch the basement plan. Establish which walls are structural or load bearing, and which walls are party walls or internal partitions. You need to know which walls are earth retaining, as these will be subjected to lateral water penetration from the ground. You need to know the thickness of the walls underground. This can be difficult to determine, although checking the wall thickness at openings above ground level or where otherwise accessible will help. In old buildings walls usually thicken by half a brick from ground to basement storey, often easily seen by a ledge at a stairway side wall.

Also plot the position of underground drainage or water supplies in relation to the basement, and note the position of any ponds, streams or other sources of water. Also note the positions of potential dampness threats above ground – leaking rainwater goods, poor detailing and so on – and whether the original construction has been compromised or modified.

All or part of any basement will be underground, so the characteristics of the site and subsoil will have a major influence on the condition of it. Clearly, it is important to establish the relationship between external ground and internal floor levels. The site may be sloping or flat. A sloping site could mean a semi-basement exists, for which a cut-off drain may be provided to help reduce the pressure of groundwater. Without a cut-off drain the basement wall on the higher side will be subjected to a considerable water load (it will be acting as a dam). Flat sites also create their own problems because they are likely to be more difficult to drain effectively, particularly if the water table is high and the subsoil impermeable.

Local knowledge of soils help you understand whether the basement is likely to be under a considerable dampness threat. Free-draining coarse soils generally present less of a problem for basements than finer impermeable clays. High water tables mean basement walls may be subjected to considerable wetting and, if the basement is deep, to considerable hydrostatic pressure. This may put pressure on waterproofing to the inside faces of walls, which could then be under threat of failure through detachment if the bond strength or mechanical connection between substrate and lining is insufficient. There is also the possibility of 'flotation', which can lead to more serious and sometimes spectacular failure.

In addition to the implications for subsoil drainage of different soil types, there is also the possibility that the subsoil could contain harmful contaminants or gases. These could damage tanking or other parts of the construction. Sulphates are a well-known example. And surveyors have long been aware of the risks of high radon levels and the protection measures that are needed.

British Standard BS 5930 contained general advice on site and soil investigation that may assist surveyors in assessing ground conditions around the basement.

Other clues that will help you to understand the site: Foundations or soakaways may have been recently excavated on the site or nearby which allow you to see the subsoil type and condition. There may be a report on subsoil conditions, perhaps prepared in connection with a subsidence investigation. (If subsidence is ongoing to the property it is likely that any existing rigid tanking system may have been damaged and any new tanking system must be designed to withstand the likely movement. Cementitious tanking systems usually incorporate detailing at floor or wall junctions to withstand stresses inherent at that position, but no amount of strengthening at the junction would be able to withstand the kind of forces produced by major structural movement.) Other external changes can affect moisture conditions for basements. Tree removal, for example, may cause formerly desiccated subsoil to hydrate. Site flooding may mean there is a high or 'perched' water table. (Improvements in subsoil drainage may help to lower the water table locally and reduce the amount of lateral damp penetration through basement walls.)

If the property is part of a terrace, or if the buildings adjacent are of similar design and construction, it may be helpful to make contact with neighbours who can provide useful insight into local conditions and how basements are performing, and whether flooding is a problem. The frequency and severity of flooding must be ascertained because some occupiers may tolerate infrequent flooding if the basement space is only to be used for occasional storage.

If there are not enough external clues about the property and its conditions, you may decide to excavate a trial hole or holes to establish subsoil type and water table. You also need to establish direction of movement of subsoil water, as this will have important implications for the basement in terms of lateral water penetration and will influence your decisions on site drainage.

Internal conditions

The site investigation for a basement often relies more on the assessment of site conditions from visual inspection than on other techniques. Salts testing, for example, will be of less use in a basement because you already know that dampness is likely to be sourced below ground.

It may be obvious that a basement suffers dampness, but it is sometimes extremely difficult to establish why the walls or floor of a basement are significantly damp. Even if you cannot always fully understand below-ground moisture conditions, you may still be able to recommend a waterproofing solution that would produce a dry habitable space for the client, if that is the aim.

As we saw in Damp management and remediation, in some basements you see direct and indisputable evidence of damp penetration: pooling of water can sometimes reach such a height that entry into the basement would be inadvisable on grounds of health and safety. The water could have come from a defective foul drain (confirmed by the pervading odour). A sample from pooled water could be taken for chemical analysis if necessary, although sometimes the colour of the collected water can indicate its source – certain rocks or soils underground produce a characteristic stain. Water can often simply flood into a basement via leaky trap doors, access steps and doors, or may well up through the floor. (Tanking can, of course, hold water within it just as effectively as it may resist water from without.)

The basement will either have no tanking system, a partial tanking system (more correctly termed 'structural waterproofing') or a complete tanking system to walls and floors below ground level. There may have been attempts to waterproof the basement using unsuitable materials or systems. Materials used range from asphalts and slate to bitumen-impregnated corrugated sheets, polymeric sheets and waterproof renders. A waterproof render system is probably the most common tanking system used today for existing house basements, although the cavity drain system is rapidly growing in popularity, and is a lower risk system.

Of course, the condition and integrity of the tanking system, if it exists at all, will vary, and the greater the water penetration the basement is subjected to, the better the specification and condition of the tanking must be to resist it. The potential pressure of water penetration into basements can be calculated by reference to the head of water above the water table (hydrostatic pressure); and in deep basements water can literally spurt out of weaknesses in walls or tanking.

Tanking needs to be continuous, with good linkage between the floor and wall tanking systems, and you need to consider carefully how abutting walls or columns of masonry have been detailed. In any event, it is unlikely that you will find traditional masonry, stone walls or floors underground to be 'dry', unless the basement has been properly tanked. And even if a perfect tanking system has been installed, there may still be the 'enemy within' – moisture generated within the habitable space may condense on cold surfaces to produce dampness problems, if the space is not carefully heated and ventilated.

As well as tanking, there are other techniques for reducing the amount of soil in contact with the basement walls and for promoting dry, habitable conditions internally. For example, it is quite common to find the ground excavated along at least one wall of a basement (perhaps where there are access stairs). Ernest Blake in Damp Walls (1938) terms such a zone a 'dry area' – basically a deep trench around the outside of the building excavated down to a short distance below the horizontal dpc level. He also described 'closed areas' that comprise a trench, a retaining wall to hold back the bank formed by the excavation, and a closing of the space between the main house wall and retaining wall, typically by a stone slab.

If significant dampness is focused on certain key areas, you may be able to identify a link between the symptom and the cause or source. For example, faulty rainwater goods or drains may cause a very obvious damp patch internally, or rainsplash might cause a local damp patch where part of the basement is above ground. Find out if the dampness or pooling occurs at particular times, intervals or seasons.

As with any internal inspection, you should look at the condition and nature of finishes, check for signs of movement (particularly at wall or floor intersections), and look for signs of mould growth or timber decay. Where there is no obvious pooling of water you can test walls and floors to assess their moisture content (but see Moisture meters – a warning, below).

Additional tests may also be necessary:

• Drains tests can eliminate or confirm a water leak as a contributory factor, as can tests for leakage to internal services and water mains.
• Data logging can be used to monitor a condensation threat.

Remember that if the basement has been partially waterproofed moisture may track around the waterproofing to cause problems nearby – laterally, below or above.

Solutions

Just as dampness at the base of walls above ground is often 'managed' rather than 'cured' (i.e. the symptoms of dampness are masked) there are cases where basement moisture is managed rather than cured. For example, you may see dehumidifiers in basements collecting vast quantities of water each day, where work to solve the root cause of the moisture generation may be too costly for the property owner or occupier to contemplate.

For an existing building tanking internally is often the most viable solution. Any specification would aim to achieve a '5-sided box' where walls and floor are made waterproof.

If management is not an alternative, but the cure is too expensive, you could also consider advising infilling the basement to eliminate that space as a dampness threat (and the associated cost implication). But you must fully consider all the implications of such a change on building layout, legal considerations, building value and condition.

Example: a damp basement What was originally a cellar was upgraded 30 years ago to create a basement in a large property in Highgate, North London. There was evidence of an attempt to damp-proof the walls by electro-osmosis, probably installed at least 20 years previously. (Such systems have been criticised and were largely withdrawn from the market, although some electro-osmosis systems are available today with an improved specification. However, these systems attract controversy and are not considered to be effective by the BRE (BRE 466). Their ability to resist lateral damp penetration under hydrostatic pressure is still open to question.) The electro-osmosis system in this case did not work. The installer had revisited the property, but refused to accept responsibility for the ongoing dampness problems because the guarantee referred only to 'rising damp' rather than 'lateral damp penetration'.   The basement is split into 3 areas: bathroom, study area and utility room. Walls were lined with pegboard or dry lining on internal faces. On inspection, there did not appear to be any mould growth problem from surface condensation, although the occupier said surfaces were cleaned regularly. This would remove visible evidence of condensation problems. There was an independently controlled through-wall extraction fan in the bathroom, hit-and-miss vents to dry lining to ventilate the void behind and a washing machine and drying machine in the utility room. There was obviously a high moisture generation in this space, hence the need for a dehumidifier to help collect moisture from the air.   The survey was prompted by the client, who had realised that the amount of water collected from the dehumidifier had been on the increase, although there were no obvious changes to basement usage.   There was no obvious rot to any timbers within the basement, and it appeared that dampness had been successfully 'managed' in the past. We could not use the pin probes of the moisture meter over nicely decorated walls, so the capacitance function was used. A check over the rear wall linings, which appeared to be subject to the most obvious dampness problems, revealed that dampness focused around 2 patches – with readings up to 657 recorded, compared to readings of around 150 to drier parts of the wall linings. This wall is below ground up to 900mm from floor height. An external inspection showed that the damp patch to the left of the window was probably due to a plinth that was coming away from the brickwork, allowing a build-up of moisture behind. There was also a more serious damp patch to the right corner likely to be due to leaking waste branch connections and a poorly installed rainwater pipe. The whole waste stack system and associated branches needed to be renewed and the rainwater pipe needed renovation.   In this case, the wall condition was monitored after the external repairs were completed, and the client was asked to monitor regularly the amount of water collected in the dehumidifier. Ways to improve mechanical ventilation were researched.   Interestingly, the house next door, which was of identical construction and in the same ownership, incorporated a cellar of similar proportions. That cellar was used for general storage and was where the central heating boiler was installed. Its walls were left as exposed brickwork internally (possibly limewashed in the past), and there had been no attempts to modify construction, which appeared as original. A large coal chute to the rear wall was a considerable ventilation opportunity. There was no obvious musty smell, and no sign of a condensation problem, although localised condensation formed on coldwater pipework when conditions allowed. Any leakage from pipework would simply find its way by gravity to a drain gully located in the centre of the cellar floor. This building appeared to be coping with walls that were certainly damp but did not appear physically damaged in any way after well over a century.   Please also refer to Damp management and remediation.

Moisture meters – a warning Be very careful when using a moisture meter in a basement. If the basement has been tanked internally, the meter's pin probes could damage the tanking and render any guarantees invalid. Capacitance meters can help you to identify a plaster system that is itself 'significantly damp' or high concentrations of moisture behind a protective waterproof plaster system. Not only that, but readings may be misleading. A BWPDA Code of Practice (now PCA) says: 'Moisture meters in a basement situation should be used with great caution. Due to environmental conditions a small degree of dampness will usually be present in basements and show on a moisture meter. The meter should be used for comparative readings, and then only by an experienced person'. The code continues: 'Under no circumstances should a wall be tested for moisture in depth. Results will be meaningless as there will usually be significant dampness immediately behind a tanking system'. You may face similar problems to those commonly encountered to walls above ground, where you will need to establish whether high moisture meter readings at wall surfaces are indicative of condensation or of other causes. For this, you need to use moisture meter accessories, temperature and humidity sensors and general observation in an attempt to confirm causation. Where walls are tanked internally, drilling for sampling or use of deep insulated probes would not only be likely to damage the tanking, but would be of less use than when used with above-ground walls, as the below-ground walls would be in contact with the subsoil and would be likely to be wet or very damp. If basement walls are tanked to the outside face such intrusive survey methods may be applicable, if permission to drill is given.