Damp diagnosis case studies

Stages 1, 2, 3 and 4

Property details A Victorian semi-detached property that had been converted into flats Site: sloping, with the raised ground to rear supported by a retaining wall Orientation: front elevation faces north; flank wall faces east; shaded Construction: load bearing walls of 225mm solid brickwork Foundation: probably spread footings to main building, and strip concrete to side extension Damp course: 2-course slate External finish: face brickwork Internal finish: solid plasterwork to walls Roof: timber/traditional, with slate tiles Floors: first and upper floors, traditional timber; ground floor originally timber, probably replaced during the 1970s by a concrete ground supported floor Windows: single-glazed Services: combined drainage system; full central heating Survey date: May 2001 – weather: very light rain (pooled surface water to rear yard was evidence of earlier rainfall)

Aims of the survey

The former resident of this property had suffered dampness problems.

This investigation shows how a relatively brief survey can encompass elements of all the 4 levels of investigation in a single compact and efficient visit. It also illustrates how there can often be a fluid transition from one item of equipment or test to another as you follow the natural progression of inquiry.

The surveyor could spend only 30–45 minutes inside the property. Due to the time constraint, this was not a complete property survey. The surveyor restricted his work to surveying the damp zone, while of course bearing in mind the fact that the damp zone needed to be understood in the context of the building as a whole.

Investigation – stages 1 and 2

1. Walk-over

The front elevation faced roughly north, and the former occupier said that during the course of the day the sun arced around the rear of the property but offered little direct sunlight to what is quite a shaded flank. The close proximity of neighbouring buildings, together with the slope of the rear garden and tree cover, all added to a feeling of Dickensian dankness. On entering the flat, the air felt very damp.

Clearly the house had been closely watched by surveyors: there were monitoring studs on the front elevation that were being used to measure crack movement. The tree cover not only alerted the surveyor to potential dampness, but also to potential movement.

With a retrofit concrete floor in an old Victorian house there may well have been problems in the linkage between any floor dpm and the wall dpc (if one existed). Unfortunately the surveyor did not have time to check the dryness of the floor by using a humidity box because it would have taken too long to reach a reliable equilibrium.

2. Checking out the ‘dry zone’

The surveyor used the resistance meter pin probes to check out typical readings to the inner and drier part of the flat around the central hallway. Readings were in the region of 12% to skirting boards and 9–10 R/R to wall plasters (see figure 1).

Figure 1: Floor plan with ‘dry zone’ and ‘damp zone’ shown

3. Detailed analysis

Back in the living room on the south-east corner of the building it was evident that both flank and rear walls were significantly damp. Wallpapers were stained and dark mould growth present, and the surveyor could see a concentrated black mould at the flank rear corner, with staining and mould present in the characteristic crescent shape typical of condensation-led damp mould damage (see figure 2 (a), (b) and (c)).

The surveyor made a sketch of the room corner so that he had a good record of the pattern of dampness, both in terms of its visual manifestation of mould development and the moisture reading pattern. The sketch included an exact pattern of staining and the moisture meter readings recorded on a grid (see figure 2(a)).

The moisture meter readings suggested that the wall plasters were probably significantly damp along much of the flank and a short way along the rear return – to a height up to around 500mm. In the rear corner the mould development and dampness rose to a greater height (800mm). This pattern is typical of air moisture condensation, and is illustrated in more detail in Energy efficient building: a design guide (Hancock M. and Roaf, S. Oxford, 1992). At around 600mm the moisture meter readings generally fell to 12 R/R, indicating that the wall finish was ‘air dry’.

Figure 2: The surveyor made a detailed sketch of the room (a) and on it marked the damp patterns observed – see (b) and (c). High pin probe readings were found towards the corner at a low level. Dark mould patches were also found there. Staining was to a maximum height of 500mm

Clearly the walls were damp enough to encourage mould development, and the skirting boards along the flank wall and the return were at risk of degradation, with readings often above 16% and 20%.

At this stage the surveyor could only make an educated guess concerning the internal air humidity conditions when the flat was occupied. However, as there were no signs of mould development elsewhere in the flat it seemed likely that excess humidity was not the source of the problem, and he moved outside to view the wall externally (see Figure 3 (a), (b) and (c)).

It had rained earlier that morning, and there were pools of water on the flank concrete pathway, and particularly towards its rear section. He also saw that water was still dripping onto the pathway from several of the plastic soil-pipe branches. The surveyor used the binoculars and saw that a section of guttering several storeys up was overflowing, probably due to accumulated debris. The sheer weight of debris and dammed rainwater looked to have put the guttering under considerable stress judging by the sag, and this was not helped by a lack of gutter fascia brackets.

There was a distinct pattern of staining to the flank: green staining along the base of the wall, then above this there was a band of quite dry brickwork, and then above this a wide band of wet masonry. The latter seemed to become wetter towards the soil stack branches.

It was difficult to judge whether it was the rainsplash at the wall base or the saturation of masonry from splashing off the soil branches higher up that was causing the most distress to the building. But visually it was clear that both of these moisture sources should be eliminated to help bring this part of the property back to a safe moisture equilibrium. To complicate matters it was also likely that some moisture was finding its way downwards from about first floor level via the chimney flues (unvented), possibly causing the dampness that the moisture meter had picked up to the right side of the chimney breast.

Suggested report clause (stage 2)

‘We noted a serious mould development along the rear flank (living room) wall and some way into the rear. Moisture meter readings suggest that both walls and skirtings appear to be potentially significantly damp. In view of the green staining externally at the base of the flank wall, saturated brickwork at higher level and pooled water to the flank pathway, we can offer the strong professional opinion that there is a problem of penetrating dampness to the wall base on this elevation. The source of the moisture ingress appears to be a defective section of main roof guttering at high level above, which was seen to overflow demonstrably during our inspection. We cannot preclude the presence of an additional or related condensation problem internally due to the presence of moulds typically associated with condensation at the wall surface.

We recommend prompt repair of the defective guttering and some modification to the flank pathway and wall base to reduce the likelihood of future rainsplash and pooling problems. Internally we would recommend firstly stripping wallpapers to the flank and rear walls, cleaning away the mould growth and monitoring the moisture content of both wall and plasters to ensure that the walls have dried out sufficiently before redecoration is considered. Damp skirtings should be removed and set aside to assist their drying out.’

Figure 3: Outside, the surveyor made a sketch of the evidence (a). (b) Did the damp come from up there? (c) Green and white staining at the wall base indicative of a long standing damp problem.

Investigation – stage 3

Back inside the property, the surveyor drilled two 6mm holes about 50mm apart, at a height of 300mm. The holes were at least 200mm from the rear corner of the room. He inserted 2 insulated deep probes and the moisture meter readings were 100 R/R, meaning that within its thickness (i.e. 75mm into the wall) the wall was potentially significantly damp.

This could indicate:

• a penetrating dampness problem;
• a problem of below-ground moisture;
• an internal plumbing leak;
• a combination of any of the above; or
  
• a condensation problem.

The fact that there is black mould present – typical of a condensation problem – does not prove that this is condensation-led mould development. Dampness must, however, have been present persistently to produce such heavy black mould contamination.

Interim diagnosis

It is most likely that the wall became wet through its thickness from rainsplash at its base and higher too, and that internal plasters became wet enough to support mould growth. Condensation may also have taken place where the wetted wall became less thermally efficient – cold surface temperatures (often below the dew point) would promote mould growth.

To confirm this theory the surveyor carried out a salts analysis:

• Plaster scrapings showed negative for chloride and very low in nitrate (0–10mg/litre). Such a low nitrates result would probably make a below-ground source unlikely.
• Deeper into the masonry both tests were positive. Such a result does not help to narrow down the range of potential sources, as the 'Interpreting salts test results' table in Surveying equipment and tests lists several potential moisture sources from a positive nitrate and chloride result.

The surveyor drilled out 1 of the deep probe holes large enough to insert a hygrometer and left it for 30 minutes to equilibrate. ERH was recorded at 82.4% – again suggesting that the wall was significantly damp within.

Suggested report clause – stage 3

In addition to previous comments:

‘We have carried out a salts analysis of a plaster skim scraping and found hygroscopic salts to be present in low concentrations. We were not able to rule out any moisture sources by that test. Moisture meter readings using deep insulated probes at the wall base of 100 R/R, together with an equilibrium relative humidity of 82% within the wall suggests that the wall may well be significantly damp within its thickness. Thus the moulds noted to the wall surface cannot be attributable to a condensation problem alone. This means that measures designed purely to combat a condensation problem might not solve the root cause of the dampness, or even any important contributory cause.’

Investigation – stage 4

The surveyor drilled a 75mm hole and collected the brick dust for a carbide test. The carbide meter reading was 4.9% mc, confirming that the wall was significantly damp at depth. There no time to drill elsewhere for a control reading, but the surveyor was familiar with the bricks used in the property and knew them to be air dry at less than 1% mc, so brickwork at 4.9% mc was, without doubt, quite damp.

Diagnosis

From sketches and observations, there were clear patterns of dampness to the flank wall (rear section) both internally and externally. Deep insulated probes read 100 R/R, suggesting potential significant dampness within the wall. The carbide test confirmed dampness within the wall’s thickness.

The lead source of moisture appeared to be overspill from a defective gutter, linked to a possible condensation problem internally.

This was a good example of ‘falling damp’ – in fact the dampness was falling, bouncing and splashing.

Prognosis

Mould is unsightly, demoralising and can cause health problems. Tenants living in such an environment may well request a visit from the Environmental Health Officer, and if the landlord has not adhered to leasehold obligations, legal proceedings could result. It would not be practical to redecorate this room before remedies had taken effect, because decorations would soon spoil. The skirtings were damp enough to rot.

The problem would continue unless the faulty guttering was repaired. This work would take no more than 20 minutes once a safe access had been erected. (It was rather high for safe working from a ladder.) Once this had been done moisture conditions within the wall could be monitored to confirm that the wall had dried out. Redecoration could then be considered.

In addition, various modifications to the pathway at the wall base could reduce the risk of rainsplash problems in the future. For example, a cement plinth could be installed to protect the wall base, and a drained gravel-filled channel along the flank would help to reduce direct wetting of the wall base.

Summary of tests

Test	Results	Conclusions
Resistance (pin probe) readings	Up to 23.8% in skirtings Up to 26.7% R/R in the plaster surface	The timber is rotting or is likely to rot Wall finishes are very likely to be significantly damp
Deep probe readings	100 R/R	The wall is probably significantly damp at depth
Salts analysis of plaster scrapings*	Chloride test negative Nitrates result low – 10mg/l	Moisture origin could be from organic material or possibly the ground
Carbide test	Brickwork drillings register 4.9% mc	The wall is certainly very damp at depth

Footnote: *This may need to be double-checked, because such a result is unusual.

Lessons learned

• Drilling just a single hole for carbide testing a brick dust sample is useful. Having obtained a carbide reading of moisture content on a particular date and at a particular position, you could return in the future to test again, and find out whether the wall appears to be drying out at depth. Drilling at least 1 more hole would help to clarify the monitoring result.
• Horizontal ‘tide marks’ such as those exhibited in this property can be due to causes other than ‘rising damp’.
• Deep probes were invaluable as they confirmed that the wall was significantly damp at depth.

Additional comments Mould is typically associated with condensation problems. Some reference books suggest that condensation-led moulds do not readily develop when salt contamination is produced at the wall surface from a source of moisture carrying nitrate and chloride salts. However, when the inner thickness of a wall appears significantly damp there may well be more than 1 mechanism responsible, as this case study shows.