Damp diagnosis case studies

Stage 1, 2 and 3

Damp zone survey duration: 90 minutes.

Property details Part of a staggered terrace of cottages built in the early 19th century in Hornsea, East Yorkshire Orientation: front elevation faces west Construction: the brick-built structure incorporates some 18th century stone walling to the rear (some very thick walls) Damp course: probably no original damp course (in view of the age of the property), but some walls have been given a chemical injected dpc Roof: timber framed with old-style clay pantiles Floors: a mixture of suspended timber and replacement ground supported concrete Windows: most are double-glazed Survey date: December 2000

Aims of the survey

The property had been acquired for refurbishment. A mortgage valuation report referred to quite wide-ranging dampness problems in the property, which would be expensive to remedy. A more thorough survey was essential because money for repairs had to be allocated carefully due to a limited budget.

There was already a comprehensive shopping list of refurbishment works – from roof overhaul to a new hot water and central heating system. The owner was conscious that poorly thought-out remedial work could damage the building, and wanted any remedies to work over the longer term and be sympathetic to the age and character of the property.

The mortgage valuation report included a somewhat alarming clause:

‘Widespread high moisture meter readings were recorded, more particularly towards the front parts at ground and first floor, and we therefore recommend that a specialist damp-proofing contractor should undertake an inspection to verify the cause of the high readings and specify the remedial works required.’

Figure 1: Terraced cottage in Hornsea, Yorkshire

Investigation – stages 1 and 2

1. Walk-over

The owner and the surveyor spent about 15 minutes walking around the cottage to get the feel of it and identify any signs of defects, noting likely shaded parts, site conditions and so on. They noted that new air vents had been installed for the suspended timber floor and also saw the telltale sign of ‘damp-course machine-gunning’ – a row of small plastic plugs to seal off injection drill holes – which probably stemmed from the previous mortgage valuation report. The surveyor noticed discolouration and a green stain on the rear elevation where rainwater gutters appeared to have become blocked and had been causing rainwater to cascade over the brickwork at a high level.

Inside it was soon apparent that the rear part of the property had damp problems. Stains were evident in quite a few locations (for example, at a high level to the rear wall corresponding to eaves height, and to the rear chimney breast). They also noted that the rear half of the cottage had undergone floor replacement works – from suspended timber to a new solid concrete floor. There appeared to be salts deposition to the party wall above the skirting, but only to the wall run abutted by the new concrete floor. The surveyor's opinion here was that this was a clear example of a solid floor replacement causing lack of ventilation and ‘breathing’ of a wall base. The original ventilated timber floor would have helped dissipate moisture in the wall base below the floor; this moisture was now forced to find a way out at a higher level and the resultant salts depositions from evaporation were now damaging plasters and attracting their own moisture. In effect this meant that the remedial works had resulted in ‘rising damp’ at the floor perimeter. If the remedial treatment company had not found true rising damp in their original survey, they had most probably now created it.

However, the surveyor could not understand why the mortgage valuation report had made such damning comment about dampness at the front. The wall linings appeared to be free from any staining or blemishes, and the wallpaper was coated with cream emulsion paint. Surely a damp problem would have caused light-coloured papers to obviously discolour? The decorating did not appear very recent – the previous owner had mentioned a complete internal redecoration 5 years previously when the ground floors were upgraded. It was a mystery.

2. Detailed analysis – upstairs

On the first floor the surveyor used the pin probes of an electrical moisture meter (resistance mode) to check for potential dampness.

There was certainly a possible cause for concern – readings often moved to 100 R/R when the pins were pressed in firmly. High readings were recorded under the window sills and to walling near window reveals (see figure 2). These were the parts of the wall a surveyor would naturally check first.

Figure 2: Readings were taken all around the suspect area – maximum reading of 100 from pin probes over the whole front wall … why?

Immediately questions sprang to mind. Could there be a shortcoming in the sill detailing? Or was some pointing missing at the abutment of brick reveal to window frame? Windows were replacement double-glazed units – aluminium frames with hardwood surrounds. Perhaps the mastic was in a poor state and in need of re-running?

However, the owner mentioned that some receipts were on file, and the replacement windows were only 5 years old, so seals should still have been intact. Next the surveyor wanted to see how exposed the front elevation might be to driving rain as it was east facing.

He drew back the curtains fully and the sun’s rays spread across the front bedroom wall to give improved lighting and definition to the internal finishes … and to a small area of glinting near the detached lining paper in the left corner which caught his eye. This shiny foil had not been so easy to see at first because of the lack of light. Gently he peeled the wallpaper back just a little further – and the metal foil behind the lining paper was pulled out into view. The game was up!

Knowing how foil linings affect meter readings there was now good reason to check the wall not just where we might expect a damp problem but over the whole wall. Readings regularly hit the 100 R/R spot across the front wall of the bedroom, but there was a sudden change to lower readings (around 20) to the side walls.

The surveyor switched the meter to capacitance (search) mode. It was easy to see what the mortgage valuer might have experienced with a similar instrument. Readings repeatedly reached into the 400s over the whole area of the front wall. Capacitance readings taken on other walls reached 200 only, except at high level to the chimney, which appeared to be subject to a penetrating dampness problem. (The surveyor later climbed up to the roof and found the chimney back gutter – a hidden and forgotten place on so many roofs – completely blocked with dirt and vegetation.) He also noted some mould growth to the underside of the wall lining. This may have meant that there had been a condensation problem in this part of the property, although no mould was evident to the room face of the linings.

3. Detailed analysis – downstairs

Downstairs the surveyor did a careful and systematic moisture meter survey to establish whether any pattern of readings might help pinpoint a potential moisture source. In the ground floor front room he plotted a vertical profile of moisture meter readings in the area where a cursory gliding over of the capacitance meter had already flagged up a potential damp zone. (This detailed analysis would often be beyond the scope of a stage 2 investigation because of the additional time required.)

The surveyor noticed a gradual increasing of moisture meter reading from around 13% to 15.5% on the front wall skirting board, with readings increasing towards the right side of the wall. High readings were also recorded at wall finishes towards the right hand room corner. Luckily a small section of floorboard was almost asking to be lifted adjacent to the lobby partition. Clinging underneath the board was a small sheet of mycellium. The reason for this was clear: there had been a build-up of builders’ rubble – no doubt the floorboard section had also been handy for the last builder to use as a rubbish chute. When the surveyor looked outside there was also an air vent blocked off by a flower planter to the left of the living room window (see figure 1).

The surveyor also noted high readings to front wall plasters, and particularly below the ground floor front window sill. Pouring a kettle of water showed just how water could track around the poor sub-sill detailing to soak masonry below.

Diagnosis

When uniformly high readings are obtained – particularly up to 100 R/R over a whole wall – the surveyor should take the investigation further there and then by closer visual observation inside and outside the property. If high readings are obtained to particular positions, perhaps under a window sill or to the sides of a window opening, more readings ought to be taken to the remainder of the wall. This puts the high readings in perspective.

Apart from the chimney area and possible condensation, the front rooms upstairs were free of damp.

Downstairs, the floor’s ventilation pathways were blocked by rubble (internally) and a planter (externally). The blockages allowed timbers to become damp. Figure 3 shows the floor construction, which is perfectly satisfactory but for the loss of ventilated void in places due to rubble build-up. Indeed, a pleasing aspect of the construction is the way the timber floor joists stop short rather than enter the front walling – thereby being less likely to rot.

Figure 3: Base of wall subject to rainsplash. Floor construction satisfactory. Foam-backed carpet can hold moisture and reduce ventilation through any gaps in flooring

Prognosis

• When the ventilation pathways under the floor have been cleared (a basic maintenance job) the timbers will dry out; this area should be monitored.
• If the sill detail is improved there will be less damp penetration to the wall, and monitoring should reveal that the wall recovers.
• Clearing gutters at the rear of the property and clearing the chimney back gutter should have a similar beneficial effect on the staining observed internally (e.g. at eaves height).
• The improvements to central heating planned by the owners should contribute to a reduction in the condensation that probably caused the slight mould growth to the foiled areas upstairs.

Suggested report clause (Stage 2)

‘High moisture meter readings of 100 on the electrical moisture meter relative scale were recorded using pin probes to the front walls of the property at first floor level. We noted the presence of metal foil underneath the emulsioned linings. This may well have caused the meter to record high electrical conductivity rather than high moisture content where probes were pressed into the wall surface. Some relatively minor mould development was noted behind the lining papers, possibly caused by dampness from condensation. This should not be a problem when the whole house is brought up to more satisfactory air and surface temperatures by the proposed gas central heating system. There are no signs of a penetrating dampness problem externally to the front parts of the building. Roof coverings and guttering above appear in good order, although some dampness penetration was noted from what appears to be a blocked chimney back gutter to the rear roof slope. Further investigation is recommended to confirm whether the front wall is significantly damp within its thickness.’

Investigation – stage 3

Having solved a key mystery, the investigation progressed to stage 3 so that the surveyor could give the client a more informed ‘professional opinion’ on the likelihood of dampness problems upstairs.

1. First floor walls

Pin probes (see figure 4) had registered readings of up to 22 R/R to the skim plaster behind the aluminium foil. The surveyor then used deep insulated probes to check the potential moisture content of the first floor walls at greater depth.

Figure 4: Where the foil lining is peeled back, the pin probe reading shows wall plaster to be only slightly damp

The surveyor drilled 2 × 6mm holes to 75mm depth. Even before inserting the probes, the sight of light pinkish brick dust trickling out from the holes during drilling was a sure visual sign of the dry inner wall. Deep insulated probes pressed firmly to the backs of the holes gave a moisture meter reading of 16.1 R/R (figure 5). In masonry a reading of 16 or less indicates a pretty dry inner zone.

The deep probe test was repeated in another position and produced a similar result.

Figure 5: At the back of the drilled hole, deep probes measured 16.1 R/R, indicating a reasonably dry wall

Diagnosis

The results obtained using the deep probes, considered in the light of the site conditions, had eliminated the likelihood of a penetrating dampness source at the front walling. This put the owner’s mind at rest.

Comments

It appears that the mortgage valuer had not noticed the aluminium foil vapour barrier behind the wall linings. Nevertheless, the valuer should have been suspicious of uniformly high readings, and clearly did not follow the trail of evidence.

Lifting the carpet in the room corner downstairs may have been outside the scope of the mortgage valuation, but an inspection of the building externally would have shown that a flower planter blocked the air vent.

The deep probe test gave the surveyor an insight into moisture conditions deeper into the wall, so that he could eliminate penetrating dampness from the diagnostic equation. Even if, strictly speaking, this was outside the scope of the valuer’s survey, the valuer could certainly have recommended such further investigation, given the results observed.

Lessons learned

• Beware: ‘dampus metallicus’ is lurking behind a wall covering somewhere waiting to trap you. High meter readings cannot be considered in isolation and must be analysed in the light of readings to other parts of the wall, and to control readings in similar materials in drier parts of the house.

Additional comments There were high moisture meter readings to the ground floor front wall, in spite of the presence of what looked like dense cement render re-plastering to a height of around 1,200mm (probably carried out in connection with chemical dpc insertion by the remedial treatment specialist). However, looking outside the surveyor could see that the base of the wall had been subjected to considerable wetting by rainsplash. This was not particularly surprising: the property was set at the edge of a roadway with tarmac right up to the front elevation wall base. Many houses suffer this problem, and it is common to see the wetted masonry suffering the effects of frost damage in such a very wet zone. It is likely that the root cause of dampness to this wall always was rainsplash – the new damp coursing and re-plastering do not appear to have solved the problem. A concrete or tarmac surface causes rain to bounce high up onto a wall. Building Regulations recommend that the dpc is a minimum of 150mm higher than the ground level, but this would offer scant protection at the base of a house like this. In an ideal world we should have to step up into a house like this one, with the house provided with a porch at this point to reduce the risk of rainsplash bouncing off the top step and soaking into the masonry.