Damp diagnosis case studies

Stages 1–4 plus monitoring

Property details Early 1900s Edwardian semi-detached house in Deepdene Road, Herne Hill, London Site: level, clay subsoil Orientation: front elevation faces north Construction: load bearing walls of 225mm solid brickwork; non-load bearing walls mainly solid 100mm brickwork Foundations: probably shallow concrete strip Damp course: slate (where visible) External finish: part roughcast, part finish brickwork Internal finish: solid plasterwork to walls Roof: timber/traditional, with plain concrete tiles Floors: Ground, mixture of solid and raised timber flooring; first, traditional timber joist. Windows: timber; single-glazed Services: combined drainage system, full central heating Survey dates: February 2001. Monitoring from March 2001–January 2002

Figure 1: Front elevation

Aims of the survey

The client had lived in the house for nearly 5 years, during which time the property had been gradually repaired and improved. The owner’s attention now focused on modernising the breakfast room and kitchen. Before engaging in redecoration, the client was concerned that re-papering and painting could be spoiled if the damp patch on the breakfast room partition was not remedied. The client now required a second and up-to-date opinion. A previous, detailed survey report by a reputable firm of chartered surveyors was dated August 1996, and it made reference to ‘rising damp’ to this part of the house, as follows:

‘There is rising dampness to the left hand flank wall of the breakfast room within the airing cupboard. We suspect this is more likely to be caused by a long-running discharge from the overflow of the adjacent toilet, which is saturating the brickwork to the base of the wall. We would advise that the ball valve to the toilet cistern be attended to, whereon the brickwork should eventually dry out. If the moisture persists then it may be necessary to insert a new damp-proof course in this short length of wall and one of the chemically injected type should suffice.’

Clearly this report was rather confusing. Rising dampness was inevitably mentioned, but the reporter then went on to discuss an above-ground pipe leak as the likely source of moisture. Even after discussing how repairing the overflow would most likely cure the damp, dpc insertion (to ‘cure rising damp’) again got a plug.

The aim of this survey was to carefully test this part of the house to confirm, if possible, the source of the moisture overload and to then advise on an appropriate remedy.

Investigation – stages 1–3

1. Walk-over

Looking at the floor plan (figure 2) there certainly appeared to be a potential damp zone to the left side of the property. The underground drainage, the water main, a WC, a bathroom at first floor, a solid WC floor to disrupt under-floor ventilation pathways, and a collection of waste and rainwater pipes outside were on this side of the property. In all the surveyor counted 20 possible moisture sources, not including dampness that could be sourced from the property next door.

Figure 2: Floor plan with damp zone epicentre in WC

Figure 3: Just a few moisture sources to consider

Long-term leaks of mains water can cause a build up of limescale. This was evident at this property, with limescale caked around the same overflow (Figure 4) mentioned in the original survey report. Dripping water was soaking into the brickwork and visibly spreading out laterally in the first courses of bricks above the slate dpc (see figure 5 (a) and (b)).

The dampness was almost certainly spreading into the breakfast room partition to produce a triangular brown-stained damp patch (see figure 6). The link of leaking overflow to damp patch could not be more visually obvious – but further testing would probably confirm what could be seen.

Figure 4: The overflow was caked with limescale deposits

Figure 5: Dripping water was soaking into the brickwork and (a) spreading out laterally (b) in the first course of bricks above the dpc

2. Detailed analysis

The brown speckly peppering of salts at the fringes of the breakfast room wall damp patch looked very like nitrates. This is a potential source of misunderstanding because, according to some textbooks and leaflets from instrument makers or remedial treatment organisations, such salts are particularly associated with ‘rising damp’. Salts scraped from the surface of wall lining paper showed positive for nitrates and chlorides. Similar tests to lining paper, skim plaster, base plaster and brickwork were negative, with the exception of a positive chloride result in the skim plaster.

If the surveyor's assumptions were correct, the moisture causing the problem was originating from the cistern. A sample of water from the cistern was collected, and this too registered a positive result for nitrate and chloride (the Drinking Water Inspectorate sets a permissible level for nitrates in tap water of 50mg/l).

Diagnosis

The salts found at the wall surface could either have originated from nitrates in the mains water from the WC cistern, or they could have been ground salts (from a historic damp problem) leaching through the brickwork by the moisture fed into it from the dripping overflow.

Prognosis

The sensible strategy here was to eliminate the likely moisture source, then monitor the wall to see if the problem continued. If the dampness persisted even after the dripping overflow was repaired, The surveyor would have to look elsewhere (probably underground) for the source.

In the meantime, it would not be sensible to decorate until the source of moisture causing the damp patch had been cut off. The option of ‘damming the damp’ by replastering with a waterproof plaster system would not be an appropriate remedy in this case.

Monitoring

Having decided on a strategy, the surveyor then needed to assess the extent of the problem as it stood so that he could monitor the wall’s performance.

The damp patch seemed to become damper towards its right side. Electrical resistance moisture meter readings to the skirting board near the damp wall patch ranged from 10.8% to 60.9% at the right section of skirting. The wall plaster was 8.2 R/R well away from damp patch but was 100R/R where the plasters were salt affected. The presence of salts meant that these readings were higher than one might expect, but carbide tests would confirm whether the wall was damp within its thickness.

Two test stations were drilled (see figure 7). The carbide test of brick dust from the drilling showed 12.8% mc at Station A and 7.8% mc at Station B. Both results confirm walls to be wet at depth. Other test results are summarised in the table below.

Figure  6: Signs of dampness to the breakfast room wall. The fringes of the stained area were peppered with salt depositions, the moisture appeared to have soaked from the wet flank wall

Test results from the initial visit form the baseline data for the monitoring exercise

Test	Results
Moisture meter	10.8 to 60.9% at skirting board near damp wall patch (measured left to right) 8.2 R/R at wall plaster in dry zone 100 R/R common to affected plaster
Salts tests	Surface deposits on wall lining paper – very positive for nitrates and chlorides Lining paper, base plaster and brickwork – negative Skim plaster – positive for chlorides Water in WC cistern – positive for chlorides and nitrates (25-50 mg/l)
Carbide tests	Station A – Flank wall of WC = 12.8% mc Station B – Breakfast room (see *) = 7.8% mc
ARH of drilled holes	Station A – 83% ERH Station B – 80% ERH
Deep probes	No need to test, given other results

It would take many months to confirm that the wall had dried out. The client would need to be patient until redecoration could be safely carried out.

The surveyor made a regular visual inspection of the wall’s condition. However, because of the presence of salts, he ecided not to rely on my moisture meter to check whether the wall was drying out. Instead, he used the moisture content of the skirting board – a more useful indicator because a drier wall meant a drier skirting board. He also took drillings from deep in the wall and used the carbide meter to test whether the wall was drying at depth.

But most of his attention focused on the salt deposits on the wall. Over a period of 334 days, he painstakingly scraped salts from the lining surface.

He used extremely sensitive calibrated scales in South Bank University’s physics laboratory to weigh the very small quantities of salts (the scales could measure down to 0.001g). For most field testing, scales that measure to 0.1g would suffice.

Even before weighing the samples it was clear that fewer and fewer salts were forming at the wall surface. At the end of the monitoring phase it was difficult to actually find any salt deposition to scrape. The table below shows the weight of salts collected and these results plotted as a graph.

From the graph you can see that the salts deposits were reducing over time

Date collected	Day no.	Sample weight (g)	Days since last collection	Deposition per day (g)
18/2/01	0	0	0	0
8/3/01	18	0.8657	18	0.0481
19/3/01	29	0.8275	11	0.0752
4/4/01	45	0.576	16	0.036
2/5/01	73	0.5104	28	0.0182
23/5/01	94	0.372	21	0.0177
26/6/01	128	spoilt	34
11/7/01	143	0.1333	15	0.0088
21/9/01	215	0.161	72	0.0022
24/10/01	248	0.1027	33	0.0031
16/11/01	271	0.1128	23	0.0049
18/1/02	334	0.031	63	0.00049

Decreasing salt deposits over time (note: graph scale is mg)

At the end of the monitoring period it was safe to say that the salts had ceased forming. Monitoring by moisture meter and carbide test also showed the skirtings and wall to have dried out considerably since the ball valve repair. Outside the wet flank wall had dried out noticeably.

Could the client now redecorate? Tests showed that there were still nitrates and chlorides in the wall plasters, which could make it difficult for the wall finish to completely dry out because the salts are hygroscopic (attracting moisture from the air). However, as the damp patch was restricted to quite a small area of wall plaster, it seemed sensible to hack off the offending plaster (a lime plaster reinforced with hair) and re-plaster with similar materials.

A time-served traditional plasterer was employed to plaster the wall using lime-based materials. (Cement-based render is not always advisable for traditional buildings.) Six months after this, the wall remained acceptably dry and stain-free. One year on and the lime plaster replacement was still acceptably dry and free of hygroscopic salts.

If funds had been available, it would also have been advisable to grub up the solid WC floor and replace it with suspended timber to help the base of walls breathe at this part of the house, and also improve through ventilation to adjacent timber flooring.

Figure 7: Breakfast room side of WC partition, sampling sites and test stations. This side of the partition was traditionally finished using lime plasters, whereas the WC side was dense cement rendered

Lessons learned

• It is often worth giving buildings time. Once certain of the origin of the damp, building finishes and decoration were put in hand, safe in the knowledge that this was an end to the problem.
• Survey reports should include summaries that highlight important remedial work. A tiny drip from a leaking ball valve eventually caused quite a costly plaster replacement.
• Confusion over what ‘rising damp’ is can result in misdiagnosis. According to our definition, dampness sourced from above ground (e.g. from an overflow drip) is ‘penetrating dampness’. Labelling the cause as rising damp could have led to an unnecessary dpc injection.
• Monitoring by weighing salt depositions would rarely be commercially feasible. However, the reduction in salt formation was also obvious visually.