Damp diagnosis case studies

Stages 1, 2 and 3

Property details Late 19th century terraced house in Dover Site: sunny, open aspect Orientation: front elevation approx. south/southeast Construction: load bearing walls of 225mm solid brick Damp course: probably slate External finish: brickwork painted to front elevation, rendered to rear elevations Roof: traditional, with interlocking concrete tiles Floors: timber suspended, except to rear addition (concrete) and front porch extension (concrete) Windows: replacement PVC-U double-glazed Survey date: May 2001 – weather: sunny, early afternoon

Aim of the survey To verify whether dampness diagnosed as ‘rising damp’ at the front of the property required a retrofit chemical injection dpc. Investigation 1. Walk-over The surveyor could feel the damp felt as soon as he entered this property. The small cottage was filled to the brim with furniture, cluttered with personal effects, chairs, clothes racks, and radiators draped with wet laundry. There was a dark brown shag-piled carpet to boot, and he saw that the lightweight plastic curtain railing was stressed and bowed by the increasingly heavy burden of damp drapes. At the rear of the house he noted mould growth on the walls – the type commonly associated with condensation. He immediately thought back to the ‘dampness report’, which referred to a problem at the front, not the rear of the property. Moisture was certainly being generated by the clothes drying, and no doubt cooking, breathing, washing and so on.

Figure 1: Outside the Dover cottage

The high moisture load in the air is not necessarily dumped within the room where it is generated. Often the whole house can become heavy with vapour if the moisture is not extracted efficiently near its source by opening windows or by mechanical extraction.

In this case, the rooms in the rear addition to the house seemed to be suffering the worst mould development because:

• these rooms on the ground and first floor were the less heated ‘non-day’ living rooms;
• they all had outer walls, suffering the most heat loss of all walls in the building; and
  
• the rear addition walls faced north and were shaded by surrounding buildings.

It looked as if the outer walls in the rear damp zone were acting as crude dehumidifiers – with moisture from the damp house being dumped on the cold wall surfaces. (Single-glazed windows often perform this function, but the glazing and window sill are often wiped clean to reduce mould spotting.) To make matters worse, building ‘improvements’ over the years had compromised ventilation. For example, draughts from once lively and cheery coal fires were just a nostalgic memory – the dead flues were blocked up, scarcely vented – and the replacement PVC-U windows had large top-hung sashes offering the all-or-nothing ventilation option, with not even a trickle vent. Thus in cold weather the cottage was probably virtually sealed.

After just 5 minutes inside the property the surveyor had already created a mental shopping list of measures that might help reduce the visible condensation problems the property and occupants were suffering.

2. Detailed analysis

The most serious mould development was in the first floor rear addition bedroom, where classic crescent-shaped mould patterns at high and low level were visible, particularly to the left room corner. The walls externally were in good condition, with no cracks or flaws to cause a damp penetration problem, and without using any instruments except his own senses the surveyor could form the sound professional opinion that the mould growth was due to a fairly persistent condensation problem.

Interestingly, the dampness report produced by a remedial treatment firm made no mention of this very obvious problem. (The cynic in you might think that this was perhaps because that company did not install extraction fans.)

With this aspect relatively clear, the surveyor was free to spend the rest of the time investigating the reported ‘rising damp’ problem at the front of the property.

Looking again at the report by the remedial treatment firm, the surveyor had to wonder at the aims of the company where the opening paragraph of the report began:

‘As per your instructions, we have carried out an inspection to identify the areas of rising dampness and timber infestation … ’

This opening sentence makes it appear that the remedial treatment company has already made conclusions concerning the likely source of a dampness problem prior to inspection.

Figure 2: Investigating the front of the house (a) inside and (b) outside. A line of concrete blocks have created a raised flower bed right up against the base of the front elevation

Further into the report there was first a mention of ‘high and patchy readings of dampness’ followed immediately by a quotation for chemical dpc injection. Such a huge leap from finding high moisture meter readings to specifying a chemical injected dpc flies in the face of all good practice guidance (e.g. BS 6576). The company recommended injection to the front wall, together with associated re-plastering, replacing of skirting boards, etc. on the basis of electrical resistance moisture meter readings (and their experience of this type of house, site conditions prevailing, etc.).

The surveyor noticed a raised flowerbed against the front wall – a dry-laid line of concrete blocks currently filled with large pebbles. Could they once have contained garden soil? Figure 3 is a scaled drawing that clearly shows how vulnerable the ends of joists were to wet masonry. Although there was quite a deep subfloor void, there appeared to be very limited subfloor ventilation: just 1 paint-encrusted 225mm × 75mm air vent for the whole front elevation run.

Figure 3: This sketch clearly shows how the wet masonry could damage the ends of the joists

It is quite simple to calculate the total area of ventilation opening, and then compare this to the requirement of current Building Regulations – which gives an indication of how well ventilated a timber floor is. There may well have been another floor vent further to the right, but the new porch extension with its solid floor had probably blocked off the original ventilation pathway. This practice is extremely common even though building inspectors often insist on ventilation ducts being built into a new concrete floor to maintain a good airflow to the existing timber floor. There is insufficient consideration given to the effect a new concrete solid floor will have on ventilation around the base of the abutting wall.

Inside, the surveyor took moisture meter readings around all the ground floor walls, and apart from the condensation damp to the rear addition, the front wall was the zone where high readings were obtained. Having identified another damp zone, he spent time attempting to pin-point the dampness, using the methodical approach described in Invisible damp. The results were worth the effort.

The surveyor crouched on the damp and tacky deep-pile carpet, on top of the timber floor, looking towards the front window. He took resistance readings to skirtings every 150mm starting by the cupboard at the left end of the front wall and moving towards the right hand end. Readings peaked to the right side of the window, with readings up to 100 R/R in plasters, and 24% in the skirting board. (Skirting boards are liable to rot if moisture content exceeds 20%.) Readings of 100 R/R in plasters means that they are either significantly damp or that they are subject to salts deposition – or both. It certainly looked as if there was a damp problem at the base of the wall – but was it ‘rising damp’?

The surveyor took a scraping of plaster near the high readings, where there appeared to be salts at the surface, and the nitrates test was positive.

At this stage he was confident that there was no rising damp problem, and suspected that the nitrates could have originated from garden fertilisers used for the front planting bed.

There was no obvious localised penetrating damp source at higher level. The front elevation was in reasonable condition and the guttering looked in good condition – certainly no obvious leaks or over-spillage stains. There were 1 or 2 small sections of wall pointing missing, but they were not likely to be causing the kind of dampness problems identified by the moisture meter. There was a low garden wall abutting the front elevation to the left boundary – but not likely to be causing damp problems where readings were so high internally.

The surveyor thought it would be worth looking under the timber floor, so he teased up a corner of the carpet from the left front corner of the room and pulled it back, removing a piece of stapled underlay which he set aside without any noticeable damage. This exposed a small floorboard section – luckily a very wide board.

Using torch and mirror (note that an extension light is even more useful) he could see there was no sleeper wall. He noted that the joists were built into the front wall and under the joist ends was what used to be a sound square-shaped wall plate. It now looked misshapen – noticeably rotting, as were the ends of the joists, which were also dotted with woodworm flight holes. Cobwebs under the holes were filled with frass – the attack may well have been live.

The surveyor took a moisture meter reading in the joist at the seat of the woodworm outbreak: 20.7% was recorded. This is an example of not so wonderful Victorian construction. He gave the occupier the chance to take a look.

Having discovered the timber problem (the surveyor could easily push a screwdriver into a woodworm-infested joist end) he took vertical measurements at each side of the opened front window so that he could understand the relative heights of external ground, wall and internal floor level. He did this by placing a spirit level over the top of a window and measuring down either side of the external wall with a 5m tape. As figure 3 shows, any rainsplash at the base of the front elevation would soak brickwork within only about 120mm of the vulnerable built-in structural floor timber, and some rainwater could also be trickling through into the wall’s thickness through the air vent. (One could only guess at the footing depth, but the floor details are all as measured on site.)

The surveyor also checked the joists that were built into the rear elevation wall by direct inspection; that is, he lightly jumped on all corners and they appeared to be at least relatively sound. (Joists can also be hit with a hammer as a basic first test of soundness. A firm resounding hard vibration usually means all is well.) Unusually the front left corner found to be rotting had not given up its rotten secret from the ‘jump test’.

Diagnosis

All does not appear to be ‘above board’. The ‘specialist’ had clearly specified a remedy without the necessary tests and investigations to verify causation. The original builders can be blamed for poor construction methods. They built timbers into the base of the wall, making them vulnerable to dampness. Some quite unsatisfactory changes at the front of the property had compromised ventilation to the timber floor.

Prognosis

It would not be particularly costly to cut back the living room floor joists and re-support them on joist hangers, thus dispensing with the vulnerable built-in wall plate, which could be removed and bricked up. One or 2 joists would need to be replaced back to the first sleeper wall. The occupier had seen the floor defect at first hand, and could be given details of the extent of repair and basic specification details – so hopefully a builder would be less likely to over-quote for the work.

The ground outside should be lowered back to its original level, perhaps by creating a drained channel, possibly filled with coarse shingle, and more subfloor ventilation should be installed. It is highly likely that such builder’s work would cure the dampness problem once and for all, without the need for an injected dpc. There was probably an effective slate dpc lying ignored and waiting to be used and appreciated just below the raised yard level. Hacking off some of the salt-damaged plaster may help the drying out process.

In an ideal world, you should return to such a site a few months after the remedial work to assess how well the wall and associated timber, skirting, etc. were drying out.

As for the condensation problems, the remedies were obvious: clothes washing and particularly drying needed to be rethought (a tumble drier with an extraction hose would be advisable); and humidistat-controlled extraction fans in bathroom and kitchen would remove moisture-laden air from the space in which it was generated. Rear rooms should not be allowed to become too cold, and background heating would be essential in the back addition rooms to keep surface temperatures above dew point. It is even possible that keeping central heating on for much longer periods during the winter could actually reduce heating bills. These simple and relatively economic measures may have been all that was needed to limit mould development to the rear of the house.

Lessons learned

• Having a preconceived notion prior to an inspection on the cause of a likely dampness problem can introduce bias into an otherwise objective survey.
• Strike while the iron is hot: lifting a floorboard can save much guesswork and speculation on building condition. If that board had not been lifted, there would have been a nagging doubt in my mind and a growing desire to revisit – which would cost much more time – to ease my guilty conscience.
• When reading any report by anybody on anything, bear in mind the aims of the author. And be downright sceptical of any diagnosis of ‘rising damp’ based upon sole reliance of the moisture meter.
• It is important always to let the occupier know how long your survey is likely to take so that they are aware that you might be there for a long time and do not get annoyed by it. 

Additional comments It is worth noting that the original report by the ‘specialist’ investigators said that: ‘the inspection was carried out in accordance with BS 6576 using a moisture meter’. In fact, the inspector in question contravened the requirements of BS 6576 because no other means of testing were used. The occupier observed the brief survey and acknowledged that only the electrical resistance meter was used. BS 6576 clearly states that ‘surface measurements alone cannot give proof of rising damp, so further evidence should be obtained by measurements taken within the depth of the wall.’