Damp diagnosis case studies

Stages 1–4

Property details Victorian (c1900) mid-terrace in a residential area, formerly a single dwelling house with back addition, now converted into 2 self-contained flats, close to Catford town centre, southeast London Site: on a busy main road near to the main arterial road A205 (South Circular); internet research highlighted the risk of flooding and subsidence in the postcode sector where the property is located Orientation: south-facing rear elevation Construction: solid brick walls Damp course: slate Roof: pitch-tiled roof Floors: internal timber suspended floors throughout the ground floor except the kitchen, which was solid Windows: mainly single-glazed, timber-framed, vertical sliding, sash windows Services: gas-fired central heating Survey date: October 2000 – weather conditions: cool and dry

Aims of the survey

To identify the cause of dampness at the chimney breast in the dining room and to the rear addition, extending along the party wall into the hall within the ground floor flat, and to recommend appropriate remedial action. The client believed that the problem was caused by rising damp due to a failure of the existing horizontal dpc.

Research before the inspection

Apart from gathering information from the internet, the surveyor also spoke to the occupier and other sources. This is a summary of the findings:

• Dampness to the party wall had become more apparent over the preceding year, but was not more pronounced either during or following periods of rainfall.
• A musty smell (which the clients associated with damp) had also become more pronounced over the preceding year or so.
• The damp was first noticed to the face of the chimney breast at low level and gradually spread up the face of the chimney breast extending into the returns and along the party wall at low level.
• The original fire hearth was still in place, but the fireplace opening had been boarded over.
• The present owners had occupied the property for over 15 years and no external or internal works had been undertaken during the client’s (lease-hold) occupation other than routine external painting by the freeholder.
• The property had been converted into 2 separate self-contained flats approximately 25 years before the current occupancy.
• The occupiers of the adjoining property were not experiencing any dampness problems to the party wall.

Investigation – stage 1

1. Walk-over

The surveyor began with the wall in question. The wallpaper to the face of the chimney breast in the rear addition dining room was almost wet to touch. Timber skirtings to the left side of the chimney breast return and a section of the party wall adjacent to the kitchen partition wall were rotting. The electrical resistance moisture meter indicated dampness to the sections of party wall to the left and right of the fireplace, but further along the hall the party wall towards the front entrance door was dry.

The surveyor checked the kitchen plumbing for leaks by turning on taps and looking closely at, and running dry fingers over, the plumbing pipes below the sink unit and around the washing machine. No leaks were observed or felt.

Outside the rear back addition flank and rear walls were checked for signs of any defects. At the flank wall, the abutting courtyard area was partially obscuring the 2 air vents which should have been supplying ventilation to the timber-suspended floor of the rear dining room. The surveyor could not see the physical horizontal dpc line, indicating that the ground levels were too high against the wall.

High-level gutters and vertical rainwater pipes appeared to be in good condition, and the client confirmed that there was no leakage or spillage of water during rainfall.

The surveyor removed the cover to the drainage inspection chamber and checked for blockage by discharging water from the kitchen sink. The vitrified clay drainage pipe appeared to be in good condition within the inspection chamber and the gully free of blockages.

The external projecting window cills appeared to be in good condition, as did the fenestration detail in general.

2. Detailed analysis (stages 2–4)

The surveyor sketched a plan layout and made an isometric drawing of the back addition of the property and marked relevant information, e.g. location of service pipes (see figure 1).

Figure 1: The floor plan (a) shows the site of the source of moisture, while (b) shows the resultant damp problem (timber floors and sleeper walls not shown)

Figures 1(b) and 2 indicate the pattern of dampness around the walls. The highest point the dampness reached was approximately 800mm across the face of the chimney breast.

Figure 2: Stains clearly seen on the wall

This looked like a textbook case of ‘rising damp’ caused by failure of the dpc (BRE Digest 245). However, the surveyor wanted to identify and eliminate the lead source, as well as to recommend remedies. The brief look around the property suggested that there were several potential sources of moisture:

• Inadequate airflow to the subfloor void area below the dining room caused by partial obstruction of the through-wall air vents, due to the high abutting ground levels.
• Linked to the lack of adequate airflow could be the additional problem of a high water table causing excessive hydration of the oversite.
• The age of the property would suggest that the original water main pipe would be made of lead and prone to leakage due to age.
• Leakage through the party wall from the adjoining property.

Moisture from any of these sources could also cause condensation. In particular, given the location of the external through-wall air vents, rainwater could easily enter the subfloor void creating additional and unwanted hydration internally. If this was the case, as the air temperature fell the temperature of the masonry and floor timbers would cool below the dew point temperature of the air. The air would then give up its moisture to form water droplets on the cool wall surfaces between wall and timber skirting board, and even on the timbers themselves. This excess moisture could result in rot, or it could be transmitted through the absorbent plaster and decorative surface.

The surveyor took scrapings of deposits from the stained wallpapers, and found that both chlorides and nitrates were present (60mg/l and in excess of 75mg/l respectively). This suggested that the moisture could have travelled via soil from below the foundations. However, from previous experience of the locality, the surveyor knew that the local tap water contained chlorides and nitrates – although the concentration of nitrates was usually lower than the nitrate concentrations found at the site. He took and tested a sample of the tap water to confirm this.

The next objective was to understand how the moisture was moving through the wall’s thickness by drilling for samples and testing them using the calcium carbide method. He removed an area of the wall plaster from the face of the chimney breast where the moisture meter had identified the highest level of dampness. This revealed the bricks beneath. He then used a tungsten carbide drill to make a hole (diameter approx. 12mm) in the wall to a depth of approximately 70mm, taking care not to allow the drill bit to heat. The sample was weighed, then placed inside a pressure vessel together with a measured amount of calcium carbide (CaC2) reagent. (The CaC2 extracts every molecule of moisture from the brick dust sample, producing acetylene gas. The gas produced increases the pressure in the vessel, which incorporates a pressure gauge. The pressure increase is directly proportional to the amount of moisture extracted from the sample, and thus the total moisture contact (TMC) of the sample.)

The surveyor repeated this exercise at 2 more stations, and at a ‘control’ station at some distance from the staining. The control station sample registered 0.4% mc – the normal hygroscopic moisture content (HMC) for the material under examination. The capillary moisture content (the amount of moisture present due to capillary action) is the difference between TMC and HMC. Thus for the highest reading (obtained to the face of the chimney breast) where TMC 12% mc, the CMC was 11.6%.

Where the CMC is greater than the HMC then the source of the moisture has probably entered either by penetration or by rising damp in the wall. Condensation would not, on its own, have resulted in the dampness levels found, but it could not be ruled out as a contributing factor.

When compared to the advice in BRE Digest 245 the results of the salts test, the moisture levels deep into the brickwork, the pattern of damp and even the location would confirm without a doubt that rising damp was the problem. Even the most experienced surveyors could be forgiven for thinking they were witnessing rising damp due to a failure of the existing dpc.

The surveyor needed to understand what was going on below the floorboards. He drilled a small hole (approx. 15mm) in a board and inserted a hygrometer (thermister). The high humidity (87% RH) in the subfloor area clearly established that condensation was active – an ideal environment for the dry rot fungus (Serpula lacrymans) to flourish.

Next he inserted an illuminated borendoscope into the drilled hole, and was able to view part of the oversite, which was visibly wet. He could also see the fire hearth wall below the floor and the slate dpc in the party wall, which appeared to be intact.

Most fire hearths are constructed of loose rubble or aggregate enclosed by a perimeter brick wall, are covered at the upper surface by a concrete or cementitious upper layer and are often tiled. There is not usually a dpm within the hearth construction. Where fire hearths are found in older properties with timber suspended floors, the hearth itself usually bridges the physical horizontal dpc to the chimney breast, so that if there is a high moisture presence within the subfloor area and ‘oversite’, this is where it can get through. Unfortunately in this case the surveyor could not see what was going on in the chimney breast area.

At this point the surveyor had not identified the mechanism by which the moisture was moving, nor had he identified the lead source of moisture. However, he knew that the tap water contained both chlorides and nitrates, and the owners had no knowledge of whether the original potable water mains supply pipe had ever been renewed or repaired. In a property of this age, the surveyor was fairly confident that the original water pipe would be lead pipe and at this age it could be prone to leakage. In addition, he knew that the majority of Victorian fire hearths do not contain a dpc and have been found to be a vehicle for the transportation of moisture from below the floor into the habitable space. So a water leak was still suspected.

The surveyor asked the client to prepare for the second visit by:

• contacting the relevant water utilities company to report a suspected waste of water from the mains water pipe below the suspended floor;
• clearing the room so that the surveyor could make a more thorough examination of the subfloor area including the condition of the hearth; and
• removing the boarding concealing the fireplace opening and checking for rainwater ingress.

The surveyor explained how to check for rainwater ingress by installing a simple rig constructed of cardboard or stiff paper placed into the fireplace near to the top of the opening and supported by a small waste paper bin or similar container. The occupiers needed to observe and check the card or paper during and following periods of rainfall.

About 10 days later the client telephoned to say that the water inspector had carried out a sounding and siphon test to confirm that there was a waste of water problem at the property. (House surveyors should consider investing in a ‘listening stick’ – the more serious leaks are not that difficult for even the untrained ear to detect by sound.) A number of the floorboards in the dining room had been removed, and they found that the original lead water main pipe was perforated causing a water leak under the floor. The client had arranged for a plumber to repair the pipe.

The client also told me that they had decided to brick up the fireplace opening and remove the hearth. This would save space in the room. It would also greatly assist the drying process and prevent any future dampness problems.

The surveyor gave instructions for the removal of the hearth construction, and recommended that the timber joists be extended by adding new sections of timber bolted to the existing joists. Excavating a narrow trench into the oversite, laying a concrete strip foundation and building a new ‘sleeper’ support wall incorporating a horizontal dpc would support the new extended joists. (An alternative method would be to install galvanized steel joist hangers by bricking up the fire opening from the oversite laid on a strip foundation and incorporating a horizontal dpc level linked under the existing and adjoining dpc.)

3. Second visit

Heavy rainfall during the intervening period had demonstrated that the chimney was not being affected by rainwater ingress: the cardboard ‘rig’ was totally dry.

The hearth had been completely removed and the surveyor could see the physical horizontal dpc to the chimney breast, which was slate and in good condition.

Diagnosis

Water from the leaking mains pipe (and possibly also rainwater entering via the air vents) had caused excessive moisture levels in the subfloor void. Capillary action was enabling the moisture to travel via the porous hearth construction and wicking or soaking through the solid materials; condensation was exacerbating the problem by forming on the wall surfaces behind skirtings and wicking through the timber into the wall plasters.

A historical perspective The question could be asked ‘why was it that physical dpc’s were introduced as far back as 1877?’ A brief review of some key historical details illustrates that dpcs may have been introduced as a less costly alternative to dealing with the more fundamental causes of damp. Edwin Chadwick (secretary to the Poor Law Commissioners) prepared a report commissioned by Lord Russell in 1839. Chadwick’s report contained a number of useful and relevant proposals for improving the infrastructure of towns. He also worked on the General Building Bill towards the end of his term of office. Among Chadwick’s recommendations were the following: the construction of land drains to lower the water table of a building site by 3 feet; water closets to have siphons (although first siphons did not appear until 1870); floors to be constructed of impermeable materials to exclude damp; provision of new road sewerage; and space about buildings to ensure a free circulation of air for the ventilation of buildings. Section 6 of the Liverpool Building Act 1842 first introduced the requirement for the level of ground floors to be 6 inches (150mm) above adjoining footways. And Pridgin-Teale (not a surveyor but an excellent illustrator and pragmatist) found many faults with the construction of rural cottages: damp sites; porous ground floors; leaking roofs; lack of rainwater, foul and waste water systems; and incorrectly assembled rainwater, foul and waste water systems. He also found builders in towns deceiving the authorities by not connecting drainage to the sewerage system.

Prognosis

True, the damp was ‘rising’, but the dpc was not at fault.

Many owner-occupiers are impatient and have been partly educated to the quick fix ‘get rid of the damp quickly’ syndrome that can put practitioners under pressure to recommend a chemical dpc and waterproof render to the walls. This would not have solved the problem. Indeed, if the leaking water main had remained undetected and the fire hearth had remained in its original state, the moisture in the wall would have been pushed higher than the extent of waterproof render and damp patches would start to appear above the height of the new render.

In addition, if the walls had been affected by heavy concentrations of crystallised salts and had not been allowed time to dry out fully, any new plaster would become deliquescent or hygroscopic as the salt concentrations attracted atmospheric moisture from the air. In extreme cases the wall plaster would have to be removed and replaced with new plaster. (It should be noted that lightweight Gypsum plaster should be avoided for all ground floor walls.)

Instead, the surveyor recommended that the internal wall plaster affected by the damp should be removed and the wall left to dry out, and he suggested that he should monitor the drying out process. He also suggested that:

• The client should consider renewing the entire lead water main from the isolation valve located under the pavement outside the front of the property through to the main stop valve in the kitchen. Part of the original mains supply pipe had been laid under the solid floor in the kitchen. The new section of pipe should be surface-routed around the perimeter walls to the kitchen sink.
• External ground levels abutting the flank wall of the back addition should be lowered to a height of 150mm below the physical horizontal dpc and the through-wall vents cleaned and cleared of any detritus.

The client wanted to totally remove the timber suspended floor and lay a solid floor in place, but the surveyor advised that this would not be wise. The original timber floor was in good condition generally and he explained the difficulties often associated with linking a solid floor dpm with the dpc to the walls. Pouring a solid floor into what was previously a ventilated subfloor space can serve to displace any ground moisture present in the oversite towards the perimeter walls and, in cases such as this, toward the hearth, which is vulnerable to the transference of moisture from the ground.

The surveyor's advice was to maintain a suspended floor with an adequately vented subfloor void.

Monitoring

Six months later the surveyor returned and tested the moisture content of the walls, as described above. The results are shown in figure 4.

Figure 4: Calcium carbide testing 6 months after the water main pipe was repaired gave a clear indication that the walls were drying out

Dampness levels had greatly reduced to the chimney breast and the remaining sections of the party wall. The surveyor suggested that the client should wait a further 3 to 4 months to ensure that the wall was sufficiently dry for render, plaster and decoration.

Lessons learned

• It is important to talk to the client, who can often provide valuable information about the problem and the condition and history of the building. It is also helpful to involve them with the survey (where it is safe to do so), providing explanations of what test is being conducted and why. This often prompts a response from the client and further information flows from them.
• Wherever possible take the ‘laboratory’ to site rather than taking bits of the property to the laboratory. It greatly aids the investigation process at the site and enables you to follow leads to prove or disprove a particular line of investigation at the property.
• A holistic survey should be completely independent of any commercial pressure to sell products and services, and earn commission, as these pressures alone can lead to misdiagnosis.