Damp diagnosis case studies

Stages 1 and 2, plus monitoring

Property details An early 1900s Edwardian semi-detached house in East Dulwich, London Site: very sloping, clay subsoil, large trees to rear Orientation: front elevation faces east Construction: Load bearing walls of 225mm solid brickwork; non-load bearing walls mainly solid 100mm brickwork Foundation: shallow concrete strip Damp course: probably bituminous felt External finish: part roughcast, part finish brickwork Internal finish: solid plasterwork to walls Roof: timber/traditional covered with plain concrete tiles Floors: traditional suspended timber Windows: some double and some single-glazing Services: combined drainage system, ground floor heated by gas fire and portable gas (bottle) heater and electric panel heaters. Survey dates: July 2001, December 2001, March 2002

Figure 1: There was wet staining to the front elevation near the right end of the lower guttering run

Aims of the survey

The surveyor was called out to look at a dampness problem in the bedroom of this Edwardian property. The occupier had already been advised in good faith by 2 surveyors that the mould development in his front bedroom was caused by unsatisfactory heating, poor ventilation and inadequate insulation of the roof, walls, floor and windows.

Before visiting the property the surveyor was able to see a photograph of the frontage (figure 1) which showed what looked like wet staining to the front elevation near the right end of the lower guttering run. A second photo seemed to show surface mould internally in the front left hand bedroom (figure 2), seemingly linked to the external dark staining. The armchair diagnosis was that penetrating dampness had resulted from a gutter leak, leading to damp masonry and mould development on internal wallpapers. Would this initial hunch prove accurate?

Investigation – stages 1 and 2

1. Walk-over

The street names leading up towards the property were Underhill Rd, Langton Rise, Wood Vale and finally Westwood Park; perhaps there was some subsidence or landslip damage up here?

There seemed to be cracks peeping out of cracks on the road, as all the front boundary walls seemed to be falling towards the pavements. It was impossible to park due to the large yellow skip outside the subject property (and several others) with the usual trappings of sand bags, timber boarding, ballast, etc. Figure 1 shows the front elevation.

The photos of the house front certainly gave the impression that there could be a gutter leak to the right side of the house, because darker colouration of finishes often means ‘damp’ (see figure 2). If the front solid wall was damp there could be a cold bridge caused by wet masonry resulting in colder internal plaster surfaces, creating conditions conducive to condensation-led mould development.

Figure 2: Dark colouration to front elevation

There appeared to be symmetrical dark vertical patches each side of the right hand bay. Getting right up close to the pebbledash, the surveyor could see what looked like new work to each side of the window. On a map of the location there was a disused railway cutting at the bottom of the garden; could the dark areas have resulted from train engine smoke?

The rainwater pipe looked in satisfactory condition, well clipped and quite new, but some tiling was a little disturbed on the right hand verge of the concrete-tiled roof. The surveyor also noted that the right hand plinth appeared half-submerged by tarmac, and it was clear that the drive resurfacing some years ago had caused this. The drive sloped down to the bay, and when it rained there would be quite a flow of water cascading down the driveway to soak the base of the bay wall.

The owner took the surveyor straight to the front bedroom. The floor plan (figure 4) and photograph (figure 3) show that the mould damage was very focused to the left side of the front bedroom window-opening. The room itself seemed more like a storeroom, with ironing stacked up on an electric panel radiator (portable). The floor had no ‘give’, which seemed odd for such a house. The owner said:

‘This used to be a garage, before I moved in with my younger sister in 1992. I’m on my own now.’

What does this tell us?

• He had only 1 income.
• He had lived there long enough to know the property’s repair and maintenance history.
• The garage conversion could have compromised dpc detailing (when an opening is bricked up it is difficult to marry up new to existing dpcs).
• The concrete floor used to have cars on it. The concrete would also make it more difficult for walls to breathe at their base. There could also be a detailing problem at the floor edges, where floor damp membranes may well not be linked to wall dpcs, or there may not be a dpm under the solid floor to link to the horizontal dpc to the walls. Poor detailing of physical dpm’s at internal and external corners of rooms are often the cause of dampness problems at the abutment of solid floors to walls. Often garage floors are constructed at a lower level to adjoining finished floor levels within the main dwelling. Therefore the height of the garage floor may have been made up.
• The darker pebble-dash must be the original – darker not from damp but smoke.

Figure 3: Surface mould in the front left-hand bedroom

Figure 4: Floor plan – why mould in just one place?

The mould patch had already been looked at briefly by a local authority surveyor, who could be forgiven for saying that the problem was easy to solve by upgrading heating, ventilation and insulation. The telltale signs were all present: the owner said that the room was his bedroom and that he did not heat it at all; he said the heating was too expensive, and he only heated the living room. This probably also meant that he kept the living room door shut so the other rooms coming off it could be cold. Rooms such as the front bedroom would be regularly very cold, with some surfaces at or below dew point.

The front bedroom would not only be cold but badly ventilated because the front window was probably rarely opened as it would create a draught and be a security risk. There was also an unsanitary smell from the rubbish bin.

The scenario was reminiscent of a case study in the BRE Report Tackling Condensation. In ‘The Unventilated Bedroom’ bad mould growth was experienced on the external walls of a small bedroom. The door and window were kept shut all the time. Advice from the BRE was to ‘open the top-light window of the bed-sit room a little overnight, and air the room properly in the morning to avoid any build up of moisture in the room fabric’. Such advice could almost certainly help reduce mould development in the front bedroom at Westwood Park.

The surveyor looked around the rest of the flat. There were the usual minor mould developments in the kitchen and bathroom (which regular surface cleaning would eliminate) and a noticeable brown stain above the living room picture rail. This had been caused by leaks from the bathroom above.

Three diagnostic questions remained:

• Why was there mould growth in this front bedroom rather than the other one? (The BRE case study seemed to go some way to answering this particular question.)
• Why was the mould development restricted to an area of walling to the left of the window opening rather than to any other wall in the bedroom?
• Was the solid floor actually damp? Was there a damp membrane properly detailed to the dpc in the wall? The floor would require further testing and possible excavation of part of the floor at the abutment with the 2 external walls.

2. Detailed analysis

The surveyor began to take resistance meter readings in the affected area (see figure 5). Sure enough the readings focused him towards the bottom left corner of the room. A reading of 26 R/R is not particularly high, but it is well into the WET zone on the moisture meter’s progression scale, and therefore most probably indicative of significant dampness – and of course this was confirmed by the mould development.

Figure 5: Resistance meter readings focused on the bottom left-hand corner of the bedroom

Figure 6: The surveyor made full use of the moisture meter’s various facilities. A deep probe reading of 100 probably shows the wall to be significantly damp within its thickness. Objections to drilling are unlikely where you can see that the finishes are already defective

Using the electronic thermo-hygrometer in conjunction with taking surface temperature readings it appeared that, at least at the time of survey, condensation was not a risk at the wall surface. But the surveyor was still uncertain. Why this corner? The other bedroom corners were mould-free, in spite of also being poorly heated and festooned with drying clothes. He went back outside, where it had started to rain.

Not a drip from a rainwater pipe joint or gutter. The downpipe had not really started to get going, as initial rain had mainly been absorbed by the tiles.

The surveyor went back inside and took a plaster scraping where damp readings were highest, and the scrapings were duly tipped into a camera film case to test later. Testing post-survey showed the plaster scrapings to be negative for both nitrate and chloride, meaning dampness would probably not be sourced from the ground.

As the rain had become heavier he took another look outside. There was still no sign of any faulty rainwater goods. Looking carefully at the gutter, it appeared to be leak-free. However, it was becoming increasingly wet from the rain so it would now not be so easy to actually see a leaking joint until after the rain had stopped – and the gutter might drip for some time afterwards. Then the surveyor looked at the rainwater shoe.

Diagnosis

The rainwater gully was filling with water (see figure 8). Not only was the obviously blocked gully full of water, but also the water was very deep. This reservoir of collected water was soaking back into the masonry. This just goes to show how important it is for a surveyor to be there and looking when it rains. (The gully could, of course, have been checked by pouring a bucket of water down it, and a blockage was on the cards due to the dirty grating.) What was revealing was the sheer amount of water in contact with the walling because the raised tarmac around the gully grating had in effect made a deep pond around it. Wonderful stuff: another problem caused by raised ground levels.

Figure 7: Salts tests were run on the plaster scrapings; negative (yellow) for nitrates, negative (brown) for chlorides. Ideally salts tests should be conducted on site – to drive the investigation
Figure 8: The gully was beginning to fill with rainwater	Figure 9: Carbide test results during the initial inspection

Prognosis

The surveyor asked the owner for a soup ladle. (Stainless steel catering soup ladles are pretty good for pulling gunge out of gullies.) A screwdriver helped lift off the grating and a 65mm bolster helped scrape the mass of black and grey gunge from the gully sides. It was certainly not a recent build-up of builder’s cement mortar; this was a long-term compacted build-up of dark gritty dirt, possibly including grit washed down the rainwater pipes from the tile surfaces. The black ‘gunge’ did not appear typical of the kind of grey ‘sludge’ created by builders pouring cement slurry into gullies at the end of a day’s work.

Cleaning out the gully was unpleasant, but eventually the gully’s slot was exposed and facing towards the right property, giving an indication of the drain run direction.

Having cleaned the gully, the surveyor advised the occupier that, although a drain can be cleared quickly, a wall takes a long time to recover. Most authorities consider it takes, on average, 1 month for every inch of masonry to dry out. The surveyor explained how to prevent the front bedroom from becoming too cold and unventilated in the winter, and gave some practical advice on redecorating. The surveyor persuaded the occupier allow a revisit a few months after to check how the wall was drying out. If you are enthusiastic the occupier becomes enthusiastic (and cooperative) too.

Lessons learned

• Surveyors need to check gullies.
• Being at a property before, during or after rain offered a useful opportunity to help the diagnosis.
• After unblocking a gully, make sure you check to see that the gully actually holds water. After filling the gully, mark the water level with a black felt tip marker. If the water level appreciably drops over the next few minutes the gully could be holed or cracked.

Monitoring

After the initial survey, the surveyor returned in December 2001 and March 2002. The aim of these ‘monitoring visits’ was to find out if unblocking the gully was enabling the front corner of the building to dry out.

When monitoring, you need at least 3 results to demonstrate a trend (see table below). By the third visit the moisture content of the brickwork near the front bedroom corner just above the skirting height had reduced from 9.6% to 5% to 2.6%, so the wall was almost certainly drying out. At 2.6% mc the brickwork was still significantly damp – and would probably dry out still further down to a moisture content of around 1%. The deep probe result of 36.6 was indicative of masonry that was still significantly damp.

Results obtained during monitoring

Date	Deep probe test	Carbide test (% mc in terms of wet weight)	Comments
1/7/01 (initial visit)	100	9.6	Definitely significant dampness (9.6% mc is significant for ANY brick type)
19/12/01	100	5	Very noticeable reduction in moisture content, but the deep probes would not have flagged this up
14/3/02	36.6	2.6	The third carbide reading confirms a reduction in moisture content, as does the deep probe result

Additional comments Figure 10 shows a gully at another property. In this case the blocked gully was actually causing rising damp. Water was seeping into masonry adjacent to the defective gully, and as this was happening just below ground level that damp problem falls into the ‘rising damp’ category. The solution would require a gully replacement – NOT a chemical dpc insertion. Figure 10: Note the cracking that extends below the gully waterline