Damp diagnosis case studies

Stages 1 and 2, plus monitoring

Property details A semi-detached 'sunshine' house in Denmark Hill, London, built in 1939 (also known as a 'Wates’ semi') Site: sloping, with clay subsoil Orientation: front elevation faces east Construction: load bearing walls of 225mm thick solid brickwork; non-load bearing walls, mainly solid 100mm brickwork Foundation:concrete strip Damp course: bituminous felt External finish: part roughcast, part finish brickwork Internal finish: solid plasterwork to walls Roof: timber/traditional, with plain concrete tiles Floors: traditional suspended timber (ground and first floor) Windows: single-glazed, metal frames Services: combined drainage system; full central heating Survey dates: February 2001 – initial survey June 2001 – monitoring visit January 2002 – monitoring visit

Figure 1: Front elevation

Aims of the survey

Feeling walls by hand can be misleading: coldness does not necessarily mean dampness. In the case Fryer v Bunney [1982] 2 EGLR 130, a surveyor was admonished by the judge for not using the moisture meter carefully enough to detect dampness that was not visible to the naked eye.

This survey was undertaken using both a resistance meter (with pin probes) and a capacitance meter (in search mode). The aim was to demonstrate the performance of a modern moisture meter in pin-probe and search modes, and to find out how useful a methodical ‘pinpointing’ exercise might be. The results were surprising.

This also demonstrates a simple monitoring exercise.

Investigation

1. Checking out the ‘dry zone’

Using the pin probes first, the surveyor checked wall finishes and skirtings in a part of the house likely to be ‘dry’ – in this case, the first floor landing.

• Softwood skirtings showed 8–10% moisture content; the wall finishes registered 8–12 R/R.
• Using search mode (scale: 0–1000), readings at the same positions were 85–90 for skirtings and 105–140 at wall finishes.

2. Checking the ground floor

The partition with the living room appeared quite healthy, with readings as per the dry zone, but on the flank wall opposite skirting boards recorded readings of up to 13.3%. This was not in the ‘AT RISK’ zone, which is pre-defined on the meter, but was certainly not as dry as it should have been. Readings of the wall finishes were up to 20.3 R/R, and therefore in the AT RISK zone.

This was a surprise. What was intended as a trial run with the meter then turned into a damp hunt. There were no stains or signs of deterioration on the surface: was the meter not working correctly? However, resisting this conclusion, a methodical survey of the suspect area was embarked on.

3. Detailed analysis

The surveyor moved a table back from the wall, laid a tape measure along the floor, and at right angles to this placed a light, wooden batten that was marked at intervals. This set-up created a grid against which meter readings could be plotted.

After about 20 minutes the surveyor had prepared 2 grids of readings, one using pin-probes and the other in search mode. (Time could have been saved time by just logging, say, half a dozen readings near the epicentre of the dampness.)

Readings from both meter modes showed a peak midway along the skirting run. The readings were significantly higher than dry zone control readings taken around the first floor hallway. From the surveyor's knowledge of the building he was confident that salts or carbonaceous materials were unlikely to be present, so he was able to take the readings at face value. (A salts detector later confirmed the lack of salts which, if present, could have swayed moisture meter readings.)

Having found what may be a damp area from the internal moisture meter survey, pinpointed to 1,800mm from the front left hand corner of the house, the next step was an external inspection of the wall. The flank of this house, as of many houses, is the 'forgotten elevation'. The surveyor instantly spotted an obvious area of green staining, and this lined up with the damp zone on the internal side of the wall in the hallway.

Figure 2: Data gathered on 7/2/01 using the moisture meter in resistance mode and capacitance mode, making the connection between internal and external conditions

The highest reading lines up with leaking gutter joint.

Figure 3: The view from outside

Figure 4: Moisture meter readings on 19/6/01

Figure 5: Moisture meter readings on 9/1/02

Diagnosis

Just as the green stain was added to the surveyor's site notes he heard a splash of water. Looking upwards to the cast-iron guttering another drip was ready to fall from the defective gutter joint. It had rained a few hours before the surveying exercise. The joint was located 6 feet from the building's front corner – the length of a cast-iron gutter section. Even though the flank faced south, it was heavily shaded by the property next door. The evidence was clear: the gutter joint leak, the green plinth staining and internal dampness ‘epicentre’ all pointed to a case of penetrating dampness.

Prognosis

If the leak was not attended to, the wall would become progressively wetter through its thickness, causing internal plasters to become damper, and leading to stained and damaged wall coverings. In time, the skirting board would rot. In short, a stitch in time saves 9, and modest outlay on repairs at this stage would save much greater repair bills in the future.

Suggested report clause (Stage 2)

‘On the flank side of the hallway moisture meter readings obtained indicate a potential (and very localised) dampness problem focused at low level approx. 1,400mm from the coat cupboard door frame. No visual symptoms of dampness to the wall were noted internally, but externally the base of the wall is stained green and subject to splashing from a gutter leak. In our opinion, repairing the defective cast iron gutter joint should help this area of walling to return to an acceptable moisture content; and some time after it is repaired we would advise a re-inspection to find out whether the wall is drying out.’

Monitoring

The cast-iron gutter joint was taken apart, sealed and re-bolted, and 4 months later the exercise was repeated. Again, a grid of resistance and capacitance readings was prepared. From figure 3 it is clear that readings taken in June were generally lower, which strongly suggests that brickwork, finishes and skirtings were drying out. There was further evidence of the drying out process: white salt staining (effluorescence) appeared on the external masonry surface, caused by evaporation of moisture as the wall dried out. This staining would eventually be washed away by the rain.

A third inspection in January 2002 confirmed that the wall was continuing to recover. Skirting boards and plasters had returned to normal – below 17% for skirtings and R/R 17 for wall plasters (see figure 4).

Comments

In the context of a homebuyer survey, you would not normally be expected to prepare a grid of moisture meter readings. The damp zone survey described here took around 45 minutes. However, you would certainly be expected to determine the epicentre or focus of any dampness, and ought then to make a careful visual examination of the wall internally and externally in order to make the link between high readings internally and any possible defects or detailing problems externally. If your report merely states ‘high moisture meter readings to the flank wall of the hallway were noted’ and you do not follow the trail of evidence personally you will probably not have surveyed that part of the property thoroughly enough to meet professional survey guidelines or the wider legal expectations.

Lessons learned

• In both resistance and capacitance mode, the moisture meter flagged up a potential dampness problem before any visible symptoms were evident internally.
• Sharp pin probes did not noticeably damage the blue patterned wallpapers. Had the walls been lined and painted, the pin marks would have been much more obvious.
• Taking control readings in a dry zone helped me to appreciate the significance of readings obtained in a damp zone.

Additional comments It is often well worth taking cast-iron gutter joints apart, resealing and bolting, etc. because in many cases a repaired cast-iron gutter would outlive the alternative plastic replacement and be generally more robust, serviceable and in keeping with the property.