Waterproofing for High-performance Basements and Foundations

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Salts from dampness in the structure is one of the problems to be faced in converting a basement to living accommodation

Dampness and mould growth are often indications of condensation but can also be a result of penetration of moisture from the ground. The diagnosis of the cause of dampness in basements tends to be complex; however, dampness in unoccupied existing buildings is rarely due to condensation. This --- deals with penetrating moisture from the ground.

Condensation, which is linked to ventilation and heating regimes.

Penetration of groundwater may be due to the water table being high or the excavation for the basement was in impermeable subsoil. Any history of flooding in the basement, and whether the water table level ever exceeds that of the basement floor level is useful information in these circumstances.

Flooding can lead to lifting of waterproofing treatments if they are inadequately loaded. A detailed investigation of the ground may be warranted in order to establish the water table level and the soil type. In some situations it’s possible to lower the water table by improving the drainage system around the outside of the building.

If there is an external light well, it will be important to check that it’s not bridged by debris, that drainage is still functioning and not silted up, and that air bricks are not obstructed by soil or vegetation. The floor will need to be examined for signs of moisture penetration. The survey should include any constructions such as garden walls and arches under steps which abut the main structure as these are a potential source of dampness. Plumbing leaks may also be in evidence.

Floors and walls of basements are often impregnated with hygroscopic salts, particularly if the basements have been used to store solid fuel. If lightweight plaster has been applied to walls, this will accentuate any dampness problems.

Remedial methods for dampness occurring in basements. In respect of walls and ground floors, dampproofing arrangements were also discussed in Walls, windows and doors, Floors and flooring.

Since it’s the cellar or basement which bears the brunt of any inundation of flood water.

Flooding in a basement. The lower courses of brickwork are also saturated.

Characteristic details | Basic description

Cellars are often damp, suffering from moisture passing through poorly constructed walls and floors. Where dampness problems are not too severe, simply increasing the level of ventilation in the cellar can help to dry them out. Excessive moisture should be avoided because it could lead to rot in the timber floor above the cellar.

Local Code provides guidance on the level of protection against water from the ground required for new construction. This may be adapted for use in improving the resistance of existing construction.

For new construction it’s important that the client has a clear idea of what present and future use will be made of the basement. Exclusion or control of moisture and, in some circumstances, water vapor is the chief consideration gives four grades of basement usage; these are listed together with acceptable forms of construction. The grades can also be used for assessing existing properties when refurbishment work is being considered. The previous use is likely to be for some other purpose than that being proposed and dampness was probably then of less concern.

Existing tanking systems are usually based on asphalt and have been in general use for many years.

Since asphalt is capable of accommodating a degree of movement in the structure, and provided the workmanship is satisfactory, it’s comparatively rare to encounter a failure.

Materials used in liquid applied membranes for applying dampproofing fall into three categories:

- bitumen emulsions and solutions, some containing rubber latex

- polyurethane compounds

- epoxy resins.

All these materials can provide protection, although they depend on good quality workmanship for complete integrity.

--- Poorly constructed piled walls may not be waterproof: Partial contact only - not watertight

- Table of Level of protection to suit basement use:

Grade Basement use; Performance level; Construction Comment; Grade 1 2 3 4

Basement use

Car parking lots, plant S rooms etc d (not electrical)

As a for Grade 1 N above but need for p drier environment v (e.g. retail storage); Housing, offices, D restaurants, r leisure centers etc.; Archives and T controlled; e, environment; a, areas; Performance level; Some seepage and R damp patches tolerable; No water T penetration but of vapor penetration; tolerable; Dry environment T required; Totally dry, T; environment, p; Construction Reinforced concrete; Tanked or as above C or reinforced concrete. Tanked or as above A or drained cavity a and DPMs Tanked or as above A plus vapor control a or ventilated wall f cavity with vapor i control and floor cavity and DPM; Comment Groundwater check for chemicals; Careful supervision.

Membranes well lapped. Check for chemicals in groundwater



Water-stops are needed in in-situ reinforced concrete construction where day joints are to be formed during the casting process, and where the structure is intended to be resistant to water penetration. Several types are available and they prevent the ingress of water in different ways.

--- Dovetail-section rubber or plastics strips (water stops) cast into the shuttered face of the pour: External water bar; Daywork or construction joints; External water bar

--- Dumb-bell section rubber or plastics strip cast into the open face of the shuttering: Internal water bars;

--- Perforated tube cast into the open face of the pour for later injection of resins.

Rubber or flexible waterstops.

The most common forms include extruded sections designed to provide a continuous barrier to the ingress of water through joints in the concrete structure. Strips of rubber or plastics, dovetailed on one side, are fixed to the face of the shuttering and cast into the wet concrete.

These are external to the structure. Where used horizontally they must be cleared of debris before placing the concrete.

They will only resist the passage of water from the face on which they are fixed.

Waterstops designed to function in the middle of the wall are difficult to install as successful placing of concrete cannot be guaranteed. Strips of rubber or plastics of dumb-bell shape are cast into the open face of the pour. These are internal to the structure.

Water-swelling waterstops:

These function by the sealing pressure being developed when the hydrophilic material absorbs water. In strip form they are placed against the concrete joint before the next pour. They can be attached to rubber or PVC waterstops to provide a combined system.

Cementitious crystallization waterstopping

The product comprises cement, fillers and chemicals mixed on site as a slurry and applied to the face of the concrete before the next pour. Salt crystallization within the pores and capillaries of the concrete provides the waterstopping.

Post-injected waterstopping

A perforated or permeable tube is fixed to the first pour of concrete in the joint, leaving the open ends accessible. The second pour is made and when the concrete is hardened a polyurethane or proprietary fluid is injected into the tube to seal any cracks or fissures in the construction joint.

--- A basement on a sloping site which is to be converted to a living area must incorporate the necessary components and features that will provide effective dampproofing: Inner ventilated lining on effective DPM Dampproofing on part it ions or separating walls; Provision for rainwater run-off; New floor screed to protect DPM; DPM; Concrete subfloor to provide a firm base for dampproofing treatment; Provision for rainwater run-off

Main performance requirements and defects


During housing rehabilitation, basements which have been in use only as utility rooms are often considered for conversion to living areas. This change of use requires very careful assessment. If basements are to be converted to provide living space they must be protected from penetrating dampness, associated mould growth and moisture related deterioration of components (-16). The practicability and cost of conversion should be evaluated carefully, particularly bearing in mind structural implications, risk of flooding and the presence of positive water pressure in the ground related to a high water table. Also, basements are difficult to ventilate and an assessment of condensation risk in the finished basement is advisable before work starts.

There is always a risk that alteration works will exacerbate problems of dampness. E.g., lowering floors to improve headroom, and extensions to increase plan area, present considerable structural and dampproofing problems.

Investigations should be made of groundwater levels and the causes of any previous dampness be established.

Few masonry basement structures would have comprehensive external tanking incorporated when built and retrospective installation is often discounted on cost grounds. Designers have to decide whether asphalt tanking is required or whether one of the cheaper proprietary waterproofing systems would be satisfactory.

Proprietary systems available include waterproof renders containing special additives, paint-on high-build coatings and moisture resistant lathing materials that allow plastering of damp walls. When considering the use of any of these products, it should be borne in mind that discontinuities in the waterproofing are difficult to avoid and are a common cause of dampness later.

An alternative approach, which is more of a palliative than a solution, is to improve drainage to lower the local water table around the structure. This technique is only practicable in some soils.

Penetration of moisture from the ground or flooding commonly results in surface dampness, salting and timber decay. Structural movement can cause failure of any damp-proof membrane (DPM) applied internally or externally. Holes for pipes or removal of internal waterproofing for electrical fixings may cause localized leaks. There may be difficulty in providing adequate integrity at junctions between internal and external walls and in achieving correct overlapping between vertical and horizontal DPM materials. Internal treatments may lift if not restrained.

High water table and high wall salt content will increase the difficulties of providing waterproofing.

Obvious signs of a dampness problem are visible salting or tide marks on walls or floors and decay of timber components. Estimates of the moisture content of masonry is obtained using a calcium carbide meter or by weighing and drying samples of wall material. Moisture content of timber can be checked with an electrical resistance meter, but readings will be inaccurate if salt content is high. To assist in the specification of suitable remedial measures, a detailed ground investigation may be needed to determine the soil type and the position of the water table.



Results of a comprehensive survey of moisture content of basement walls covers the estuaries of various rivers and had one of the highest tidal ranges in the world of 14m. Construction of a barrage across the mouth of a bay and has created a large area of freshwater. Locks and sluices are incorporated into the barrage to allow small craft to pass through and to release the flows from the two rivers.

Many of the properties in the city near to the impounded water have basements which range in size from those under the whole area of the building to a small coal storage bunker usually located under the front entrance. Some basements have already been converted to habitable accommodation incorporating dampproofing, some are used only for storage, and a few have insufficient headroom to be of any practical use.

With dampness already being visible in the walls of many of the properties, concern was expressed that the effect of impounding a large area of water, even though the resulting water level was well below that experienced during high tides, would be to increase the levels of dampness in the walls. Accordingly a Development Corporation wished to establish and record moisture levels in the walls both before and after impoundment.

Govt. Agency was contacted to prepare a methodology for sampling the moisture in the walls and to undertake the sampling. The need to measure moisture in masonry accurately had arisen on many occasions at Govt. Agency and for this reason the Gravimetric method was developed. (A full description of the method is contained in Govt. Agency). Briefly, samples are drilled from the mortar or masonry and weighed as found (i.e. damp). The sample is then oven dried, re-weighed and the percentage moisture content is calculated.

A further test is also carried out to check for the influence of salts in the sample which establishes the hygroscopic moisture content.

A method statement was prepared covering procedures for locating suitable walls for sampling, carrying out the laboratory tests and presenting the values obtained. The preferred locations for sampling for each property were

- a party wall

- an external earth retaining wall

- a freestanding internal wall

Heights of samples above floor level were to be 300 mm, 900 mm and 1.5 m, and measurements were to be taken to locate where the sampling was actually carried out. Where necessary, holes were filled after sampling. In total, nine drillings were taken in most of the 94 properties examined, though in a few of them some of the locations were inaccessible. Results were presented as a single sheet for each property giving a plan of the basement including the location of the lines of drillings with dimensions from fixed points. Moisture and hygroscopic moisture content values were recorded for each drilling together with a note of the material sample -- brick, stone or mortar.

Results given in the Figures alongside are only the first round of measurements reported for a height of 300 mm above floor levels.

They show a fairly wide range of values, probably typical of moisture content levels in basements in older properties in tidal and riverine areas generally. Three pie charts present the measurements recorded at each of the three locations. The values obtained for party walls, for earth retaining walls, and this for internal walls.

The second round of sampling will be taken as close to the first set as possible and should enable any differences caused by the impounding to be apparent.


Remedial work to cure dampness problems

All methods for waterproofing existing buildings aim to provide a moisture resistant envelope to walls and floors. There are several types of envelope system in common use, ranging from asphalt tanking to ventilate dry lining. In rehab work, a more traditional alternative is to provide a drained cavity by building a new inner leaf.

In the treatments that follow, several result in loss of space because of the additional wall thicknesses required. In each case, there must be a sound concrete subfloor, minimum 150 mm thick, as a stable base.

Drained cavity This is a tried-and-tested solution, but there is a space penalty, and it relies on effective gravity drainage or installation of a sump and pump. The method is not suitable in conditions of high water pressure or high water tables. It can be completed by non-specialist contractors.

--- Dampproofing a basement by means of interconnecting cavities on walls and floors. This method can only be used where drainage is possible. The drained cavity wall and floor construction provides a high level of safeguard.

Providing a ventilated cavity and horizontal DPM prevents moisture ingress: Sheet DPM One course engineering bricks laid with open joints; 50 mm cavity; Drainage to sump


EXAMPLE: A defective basement DPM

The Govt. Agency Advisory Service was asked to investigate the causes of puddles of water in the basement areas of a 10 year old house. The external walls of the basement had been built directly off a concrete floor slab and footings. An external land drain had been provided. The 150 mm thick slab had a layer of thermal insulation, covered by a polyethylene DPM and a 63 mm screed. PVC tiles or textile carpets were then laid on the screed.

Early dampness problems attributed to condensation and insufficient time being allowed for the screed to dry before PVC tiles were laid, had all been rectified before Govt. Agency were called in.

Substantial puddles of water were seen on two occasions during the eighth year of occupation.

The first occasion coincided with a period of heavy rain, while the second did not. Just prior to the Govt. Agency examination, there had been a period of exceptionally heavy rain, although only a small area of dampness was seen.

Observations of the internal walls of the basement showed that many of these had tide marks of salts to a height of 200 mm or so. Removing some of the plaster showed that these walls had been built off the slab with a DPC laid generally, but not exclusively, on the top of the first block course.

The external walls appeared to have a brushed-on DPM on the surface of the inner brick leaf, and then covered with render and plaster. There appeared to be little attempt to provide continuity between the vertical and horizontal membranes. However, measurements with an electrical resistance meter showed readings which could be considered satisfactory.

The possibility that moisture could result from condensation was investigated by measuring the surface temperature of the screed and adjoining walls with a surface thermometer, and the air temperature, relative humidity, and dewpoint temperature with an aspirated hygrometer.

Surface temperatures were measured at 16-18 °C; with a dewpoint temperature of 10 °C, condensation was not occurring.

Although construction water had most probably been the cause of earlier problems, this had now dried out. No leakage from the heating system was apparent.

So far as ingress of water from the exterior is concerned, there appeared to be a weakness at the junction of the wall and slab DPCs. For a tanking to be successful, the horizontal portion needs to be laid beneath the slab, and well lapped with the vertical. Although there was now no direct evidence to be seen on site, it was concluded by the Govt. Agency Advisory Officer that it was possible that some movement in the building had occurred which opened up a crack between slab and wall, and which subsequently closed. Otherwise, water penetration during recent wet weather could have been expected.

It was pointed out to the building owner that painting a coat of bitumen onto brickwork is not normally a reliable method of preventing water from outside from penetrating into a basement.

However, as no water appeared to be entering the basement at the time of the inspection (the already mentioned small patch of damp was drying out), perhaps all that would be needed would be to make good the existing screeds and plasterwork and redecorate. A more robust solution would be that afforded by a cementitious tanking system, with a proprietary render on the inside leaf of the external walls at the junction with the floor. The owner and his agent were recommended to read Govt. Agency Safe Building Guide.


EXAMPLE: Failure of newly constructed tanking

The waterproofing membrane for a basement was a bituminous treatment applied in liquid form to the concrete walls and floor. The membrane was then supported and protected by a blockwork inner leaf with mortar filling the narrow cavity. When the basement was inspected, water was lying on the floor and bitumen was running out from mortar bed joints. Removing areas of blockwork revealed that the bitumen had not set.

Site records showed that application of the bitumen was during cold weather and had been quickly followed by the blockwork and mortar infill, preventing evaporation of the solvent and curing of the bitumen. As the prime requirement was rapid completion of the project, the recommended solution was to remove the existing blockwork, mortar and uncured bitumen, and rebuild using the drained cavity system.

---- Bitumen running from bed joints indicated that the waterproof membrane had not set. This was confirmed by removal of the blockwork


--- Damp-proofing a basement by mastic asphalt tanking. The structure itself does not prevent water ingress, and protection therefore depends on a total barrier system applied externally or internally. The system may also need a vapor control layer, depending on the use to which the basement is put. Two coats mastic asphalt to walls and three coats to floor Loading coat cement: sand mix; Reinforced concrete loading slab; Original concrete slab.

--- Damp-proofing a basement by cementitious render coats: Cementitious render or cementitious compound.

Wall and floor finishes, together with existing flooring materials and screed need to be removed. A course of engineering bricks is then laid on the slab, creating a 50 mm cavity with the existing wall and a drainage channel, and leaving perpends open at intervals. Above the brick course a physical DPC is then laid and a new blockwork wall constructed, tied into the original wall with stainless steel wall ties at standard spacings. A drain or sump is installed, a self-draining underfloor layer of triangular drainage tiles and a sheet DPM (which is lapped up the side of the engineering bricks) are then laid. As an alternative to tiles, a heavy-grade plastics dimpled sheet, which will also act as a DPM, may be used. Finally, a new screed, at least 50 mm thick, is laid and the walls replastered.

Mastic asphalt tanking

This is a costly but durable solution. There is a significant space penalty, in terms of area and usually height as well, and the system needs to be installed by specialist contractors.

The existing flooring, back to subfloor slab, is first removed ensuring that all surfaces are left with an adequate key. Horizontal brickwork joints are then raked out to a depth of 25 mm, and coated with proprietary high-bond primer. Glazed brickwork needs to be hacked or bush hammered.

On smooth concrete, a wire brushing with either the addition of a proprietary cement: sand slurry with plasticizer or a light application of a proprietary high-bond primer may be necessary. External angles of masonry and concrete must be rounded.

A two-coat asphalt angle fillet is then built up at wall-to-floor and wall to-wall junctions, and three coats of asphalt to a total of 30 mm on floor slab and 20 mm on walls are then completed. Joints between successive coats should be staggered by at least 150 mm on floors and 75 mm on walls. Finally, at least 50 mm protective cement: sand floor screed is laid and a brick or blockwork lining wall is built, backfilling the cavity progressively against the asphalt with a cement: sand mix.

Cementitious render or compound

Correctly mixed render or compound, properly applied to a stable background, should last for many years. Specific allowance must be made for services if these need to be run in the wall. The cementitious layer is vulnerable to accidental puncturing unless special wall fixings are used, or an inner blockwork wall is added. The method can be used in areas having high water tables, provided particular care is taken, and in this respect a specialist contractor is more likely to achieve a satisfactory solution.

First, a cement corner fillet is built at wall-to-floor and wall-to-wall junctions. Then, three coats of proprietary mix (thinned with clean water) are applied by trowel.

Successive coats must be lapped in strict accordance with the render manufacturer's instructions, and curing completed. If using a cementitious compound, the substrate must be dampened and two coats applied to manufacturer's recommendations, followed by a loading coat and floor screed. Walls may be skim coated.

Self-adhesive membrane

This provides a durable solution and a suitable surface for services and fixings, but with a space penalty. The method can be used in areas having high water tables, but specialist contractors are not usually necessary.

First, the brickwork is cleaned, and all flooring removed down to the subfloor slab. All brick surfaces are flush pointed or rendered if the masonry is uneven (see Walls, windows and doors for suitable mixes), and the concrete slab cleaned and dried. Wall-to-floor and wall-to wall fillets are then constructed, and the membrane applied to dry wall and floor surfaces following the manufacturer's instructions, allowing at least 150 mm overlap at the joints.

The floor membrane is then protected and a blockwork lining wall built, progressively backfilling with cement: sand mortar. Finally, a new floor screed of at least 50 mm thickness is added and the walls re-plastered. Some contractors have suggested that when applied internally, water pressure can cause detachment. Careful workmanship and attention to detail will result in a satisfactory job.

Liquid applied membrane

This too provides a durable solution and a suitable surface for services and fixings, but with a space penalty. The method can be used in areas having high water tables, and it’s not usually necessary to employ specialist contractors.

All flooring is first removed down to the subfloor slab. The brickwork is then cleaned and flush pointed, and a final clean given to all surfaces to be coated.

All products should be used strictly in accordance with the manufacturer's instructions, particularly any ventilation requirements during and after application. The procedure is usually as follows:

- wall-to-floor and wall-to-wall fillets are constructed

- one or more liquid coats are applied and allowed to cure

- a new floor screed is laid

- a new inner leaf is constructed (normally having the cavity backfilled with cement: sand mortar) with the floor membrane being protected from damage during building operations

Ventilated dry lining

This has the advantage of only marginally decreasing space in a room. However, it’s only suitable if the dampness is slight, and it does not protect against groundwater under pressure. The dimpled plastics sheeting is vulnerable to accidental puncturing unless special fixings are used. Specialist contractors are not usually necessary, though. The linings have a life of 20 or more years.

First a high density dimpled polyethylene sheet is laid on the floor, turning up the wall by at least 150 mm and overlapping to manufacturer's recommendations. Next, a new floor screed at least 50mm thick is laid.

Proprietary dimpled plastics sheeting is fixed to the wall surface by nailing, screwing or special plastics plugs, leaving a gap top and bottom for ventilation. Finally the wall surface is plastered, or covered with plasterboard, while retaining the ventilation gaps.

Partition walls

Partitions must be dampproofed, with careful attention to detail. There are three approaches.

- The partition is completely removed, dampproofing and screed are installed by one of the methods already described, and the partition then rebuilt. This is normally only advisable for partitions which are not load-bearing and not contributing to the stability of the building

- A DPC is inserted at the base of the partition, overlapped with the floor and wall dampproofing. This is usually only appropriate when the partition is not connected to an external wall

- The external wall dampproofing is continued along the partition. This does not prevent moisture entering the wall masonry but stops dampness penetrating to wall finishes. Full protection may be needed, or a ventilated dry lining may be adequate. Timber door frames in partitions must be protected by continuous dampproofing around the sides and base of the opening. This may be the only solution for situations where the partition is connected to an external wall and where the partition cannot be removed because it has a structural role.

--- Dampproofing a basement by liquid applied membrane: Cement: sand loading coat into cavity as blockwork is built; Liquid applied membrane

--- Dampproofing a basement by dimpled sheet dry linings: Proprietary dimpled sheet

Existing DPCs at ground level in external walls

If an existing DPC in an external wall is defective, it must be repaired or replaced before a new dampproofing system is installed in a basement.

There should be effective lapping between the horizontal DPC and the vertical dampproofing.

Installing lining walls and new ceilings to basements may reduce ventilation around ground floor joists.

It may be necessary to improve ventilation to joists, and to isolate joist ends from walls, where possible, with DPC material or by using joist hangers.

Door and window frames in basements

The dampproofing layer in a basement must be taken into the reveal to abut the frame. Lining to walls is usually stopped at the edge of the reveal and plaster, or adhesive fixed plasterboard taken round to complete the reveal.

Interior sills, of durable or preservative treated timber, should be fixed to the wall linings.

Door and window frames of durable or preservative treated wood can be retained in position if there is no indication of rot and if they are likely to remain dry. If there is a risk of wetting, frames should be isolated from damp masonry by a physical DPC which laps the wall dampproofing.

Replacement timber frames should be durable or treated as specified in Federal Building Code, and isolated from damp masonry. If excessive wetting is likely, aluminum or PVC-U frames may be fitted.

Fixing services:

If services are run behind ventilated dry linings, moisture resistant fittings and a waterproof seal at outlets should be provided. With cementitious dampproofing, services should be run in recesses in the walls. Services can be run on dry internal partitions or inside hollow skirting systems.

Basement floors cannot normally accommodate heating and water pipes.

Chimney breasts:

If a fireplace is to remain in use, dampproofing can be taken to the chimney breast reveal and the heating relied on to maintain dryness round the breast and hearth; otherwise an envelope treatment will be required around the breast. Alternatively, the chimney breast can be removed, new support provided for the stack and the whole basement wall dampproofed.

Built-in timbers

Built-in, non-structural timbers which would be vulnerable to rot if sealed behind dampproofing should be removed. With embedded structural timbers, the safest solution is to replace them with materials less sensitive to moisture. Structural timbers can only be retained if they are sound and of durable species, and if dampness can be minimized in the supporting structure. If there is any timber rot, the cause should be investigated and preventative measures taken.


In basements, because of the greater risk of contact with penetrating moisture or exposure to high humidity levels, materials and components are more prone to premature failure.

Special care should be taken to isolate timber from dampness.

Few old masonry basements have a fully effective tanking system and some form of dampproofing will be needed to protect against groundwater.

Adequate access must be allowed for cleaning underground drains and drainage gullies, sunken areas and light wells adjacent to basements.

Drainage should be adequate to eliminate risk of flooding from water run-off in periods of very intense rainfall.

Timber decay caused by wet or dry rot, and salting and spalling of plasterwork, caused by migration of salts, are common problems. The lack of effective tanking can lead to dampness and even flooding.

Corrosion of metal components such as plaster angle beads, and electrical boxes and conduit, may be hastened in a basement environment. French drains and similar drainage structures may silt up over time. Mechanical equipment such as drainage pumps may become obsolete and uneconomic to repair.

Obvious indications of long term problems are decaying timbers, spalling brickwork, salting, crumbling plaster, mould and dampness.

Drainage systems under and around basements should be carefully examined and replaced if suspect.

Where drains are to be covered over by new construction they should not only be checked for efficient discharge but tested by air or water methods to detect any leakage.

Investigations will need to be carried out to discover whether sulfate or frost damage to mortar has weakened walls.


Most membranes will have been placed beneath loading coats, so will be inaccessible. Of great importance is that they are not punctured by subsequent alterations. Cementitious systems within a basement may crack if there is any movement in the structure, but it will be obvious where repairs are needed.

Specific advice may need to be given to occupiers on how to attach shelves and other fixings to basement walls.

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EXAMPLE: Dampness in a hotel basement

Dampness and efflorescence was showing on plaster at the base of the walls in a hotel and the Govt. Agency was called in to advise. It was thought that water was leaking inwards through the kicker/wall joint and it was recommended that the joint be sealed with a resin. Dampness resulting from a flood in a basement during construction of the hotel may have masked the wall joint leakage. These separate incidents emphasize the importance, when dealing with any problem, of obtaining holistic appreciation of the planning, design, construction and subsequent performance of a building if the causes of problems are to be correctly identified and remedies found.

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Work on site

Treatment of areas which have been flooded Immediate action after flood waters have subsided is important in reducing reoccupation times and in minimizing repairs and replacement. Immediate action in a building should include:

  • checking for structural damage
  • switching off electricity supplies
  • shutting off the gas supply
  • checking drainage systems are clear
  • removing soft furnishings that are wet
  • draining structural cavities
  • beginning the drying process

Close attention should be paid to personal hygiene in the cleaning process because of risk of contamination of flood waters by sewage.

Structural damage

Houses that have been buffeted severely by flood waters, floating baulks of timber or other debris may suffer structural damage by:

  • undermining foundations on sandy sub-soils
  • compaction of certain soils not previously flooded
  • in clay areas, ground heave and, later, shrinkage as the subterranean water dries out, leading to cracking of masonry
  • floors weakened to the extent that they affect the stability of adjacent walls
  • Cracks exceeding 5 mm in width, or several narrower cracks occurring together will need further investigation.


The building must be drained thoroughly. Although most of the flood waters may have subsided or been pumped away, some might remain:

- under basement floors. Where the level of the bottom of the floor -- this is usually in concrete but may be earth in older buildings -- is below the levels of the adjoining drains, water is likely to be trapped…

- within sumps, pits or access to drains. These must be drained and cleared

- radon ventilation systems

- between the outer and inner leaves of basement walls.

Holes may need to be drilled in the vertical brickwork joints in the inner leaf between every fourth brick to allow water to drain away to the interior of the basement.


Mud and silt must be cleared scrupulously and the areas sprayed with disinfectant. Local authority environmental health officers may be able to give advice.

The building should be inspected for signs of trapped mud, particularly:

- inside wall cavities. Bricks should be removed at intervals, carefully, and the mud raked or flushed away.

If this requires the removal of insulation layers, these should be reinstated

- in air bricks and vents which provide essential air flow to boilers.

Balanced flues of boilers and gas water heaters should also be checked


Drying out may have to continue for many months before the building can be reinstated completely. The walls - both bricks and plaster - absorb large quantities of water during flooding. A solid wall, one brick thick, E.g., may absorb as much as 55 liters/m^2 and then take over a year to dry to its former state. As the walls dry, efflorescence may appear on the surface. Efflorescence should be removed by brushing when the wall has dried completely, taking proper precautions against inhaling dust. In some cases, however, salts may have been introduced into the walls by sea water; over the years, the walls can attract more moisture and give similar symptoms to those of rising damp. In these cases plaster must be removed and be replaced with new plaster capable of withstanding the actions of the salts. See also Walls, windows and doors.

Thin walls dry more quickly than thick ones; other things being equal, a thick wall of twice the thickness of the same material as a thin one will take four times as long to dry.

Stone walls are likely to dry out more rapidly than those of brick, since they are often less porous. However, if the walls have rubble cores these may need to be drained just like cavities.

The following points should be observed:

- warm air should be kept flowing through the building by both heating and ventilation

- windows and doors kept open to give good ventilation, even when the heating is on. Measures might have to be taken to prevent housebreaking

- loose floor coverings and carpets should be removed for disposal

- impervious wall coverings stripped to help the walls dry out

- pictures and furniture removed or kept away from damp walls and other areas of dampness

- cupboard doors kept open

If the heating system is operable, it should be run with the thermostat set to 22 °C or above and as much ventilation as practicable.


During cold weather, wet walls may be damaged by frost causing the surface of the brickwork to crack and powder away. Some walls may expand because of the dampness and contract on drying, producing fine cracks which usually can be dealt with simply by repainting or skimming over with a plaster filler. Some types of wall plaster soften readily when wet and crumble when they dry out again.

Others may expand and contract to such a degree that replacement is needed.

Probably the greatest danger caused by flood waters, even months after inundation, is rot in timber. The longer timbers remain wet, the more likely outbreaks of rot will occur. Splitting in timber can be minimized by ensuring the timbers dry on both faces, which is helped by removing paneling and skirtings.

All timbers, including door frames and skirting boards, attached to or embedded in damp walls are vulnerable and should be moved away or cut back from the walls.

Salt contamination from sea water or other sources will affect the readings obtained when using an electrical moisture meter. Where it’s possible to accurately measure moisture content of timber components, target values should be 24% or lower when measured during the months of October to May and below 22% for the remainder of the year. It’s recommended that timbers are inspected six months after they appear to have completely dried out and again after another 12months. There are several types of rot likely to affect the timbers; rots will show up as brown or white strands, small orange or white blotches, cracking or splitting of the wood, soft areas which offer no resistance to a sharp instrument or penknife, and, in extreme cases, fungal growth. See Recognizing wood rot and insect damage in buildings and Remedial treatment of wood rot and insect attack in buildings.

Wood blocks and other coverings like vinyl or linoleum which have been stuck to concrete screeds may have lifted because the adhesive has weakened on wetting and require relaying. Some materials, particularly wood block and strip, may have swollen and become damaged.

Impervious floor coverings should not be laid until drying out is acceptably complete. Testing should be carried out using a hygrometer, and readings should be in the range 75-80%. Valuable timber paneling must be dried thoroughly and not be replaced until the backing walls are completely dry. In timber framed walls (e.g. partitions) some plasterboard panels should be taken away to expose the timber framework, any insulation removed as necessary, and the wall linings not replaced until drying is complete.

Paneled doors are unlikely to be affected seriously unless the panels are made of the type of plywood which expands because the adhesive is sensitive to water. Modern flush doors are often more severely damaged and require replacement. Other doors and windows may stick but should not be eased by planing the edges until drying (and associated shrinkage) is complete.

Metals are likely to have escaped serious damage unless flooding is by sea water. However, steel reinforcement embedded in concrete may corrode and expand causing long term damage. In other cases the drying process should leave the metals unscathed, although locks and hinges should be oiled to prevent rusting and seizing up.

Redecoration should be delayed until walls have dried thoroughly, and new coverings should be confined to porous coatings like emulsion paint rather than wallpaper. Walls should be treated with a fungicide if there are signs of mould growth.

It’s essential that all electrical installations and appliances that have been immersed in water or found to be in a damp condition are disconnected, examined, thoroughly dried out and tested. Particular care must be taken with any electrical equipment. Cables in good condition should not be affected by immersion but junction boxes etc will.

Water may be trapped in ducts or conduits containing cables and should be opened up to assist drainage. Once the cleaning and drying is completed, the installation should be tested for earth continuity and insulation resistance as laid down in the most up to-date IEE Regulations, and an inspection certificate issued. The electricity installation should be inspected every month for the first six months after the initial test, and at least twice again in the following six months.

-- EXAMPLE: A flooded basement

The basement of a health building was flooded to a depth of 450-600 mm. A specialist firm of contractors were commissioned to insert a chemical injection damp-proof course and to re-plaster affected walls. Subsequently problems arose with the plaster bulging and becoming detached.

Drilled samples were taken from the affected walls by the Govt. Agency Advisory Service and, after laboratory testing, moisture contents in excess of 24.5 % wet weight were established for the walls. Further tests carried out on render samples demonstrated that sulfate attack on the render had occurred and this was the reason for the bulging of the plaster. Remedial measures were discussed with the building owners: the Advisory Officer considered that the walls were probably too wet for a chemical system to be successfully injected, but insertion of a physical damp-proof course might prove successful although there were practical limitations on the use of this method. Other approaches for consideration included a proprietary render system and a dry lining based on dimpled plastics sheeting.



The water table may have to be lowered by pumping while work on basements is carried out.

--- Inspection

The problems to look for in damp basements are:

  • - ineffective existing tanking
  • - water table higher than basement floors
  • - external wall cavities bridged by debris
  • - blocked land drains
  • - blocked air bricks
  • - plumbing leaks
  • - hygroscopic salts in walls (from old solid fuel storage)
  • - former use of lightweight plasters

The problems to look for after flooding are:

  • - defective electrical wiring and apparatus
  • - defective gas supplies
  • - contaminated water supplies
  • - drains blocked with silt



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Updated: Sunday, May 12, 2013 8:52