Damp diagnosis case studies

Stages 1-4

Property details A single-storey 3-bedroom detached house on the outskirts of a large town in East Sussex. Built in the 1970s Site: a good-sized sloping plot with gardens to front and rear; part of a small residential estate of similarly designed properties in a semi-rural setting Construction: cavity walls, single-storey addition to the rear Roof: conventional pitched, tiled Floors: solid Windows: replacement double-glazed casement sashes Services: combined central heating and hot water system, ground floor WC Survey date: August 2000 – weather: sunny, dry and warm

Aims of the survey

To identify the cause of dampness to the lower sections of the walls at the living room and dining room flank external and internal walls, and to the entrance hall abutting the flank external wall.

Research before the investigation

The present owners had lived at the property since it was built. They first noticed the damp not long after they moved in, and had endured 30 years of being passed from pillar to post by various construction professionals – builders, surveyors, loss adjusters and engineers. They knew the history of the various repairs and attempts to cure the problem.

Investigation

1. Walk-over – stage 1

The surveyor first looked at the owners' large file of documents detailing all the previous investigations and ‘remedies’ before walking around the building.

Sections of the walls within the living room, dining room and entrance hall looked and felt damp. The wallpaper was discoloured and affected by the damp – a moisture meter was not needed.

2. Detailed analysis

The surveyor sketched the floor plan of the property and plotted on it the locations of the damp and all services, drainage and water-carrying pipes. He also looked inside drainage inspection chambers and established the flow direction.

He collected drilled samples of the inner leaf block-work walls to the section of flank wall in both the dining room and living room. The calcium carbide test indicated 8–12% TMC.

The surveyor took scrapings from the walls at the upper margins of the damp zone and tested them for chloride and nitrate ions. The tests were positive for chlorides but negative for nitrates. This was not what he had expected, so he repeated the test with a different sample, but obtained the same results. This meant that the source could be either mains water, or possibly from a leaking heating pipe buried in the floor screed.

There was a radiator adjacent to the area of damp wall in the living room. There was also a vertical soil vent pipe within a timber duct nearby.

The heating and hot water was provided by a fan-assisted combination boiler, and the owners confirmed that they regularly had to re-pressurise the boiler because a drop in pressure had caused the safety mechanism to prevent the boiler from firing. This loss of pressure suggested a leak. There were no leaks elsewhere in the house.

The surveyor asked the owners to commission a plumber and heating engineer to pressure-test the heating system and look for a leak in the soil pipe.

These tests proved negative. They also learned that the mains supply underground travelled in a direct line from the pavement at the front of the property into the kitchen on the opposite side of the house away from the epicentre of the dampness, so a mains leak was not to blame.

3. Second visit

When the surveyor returned, he decided to lift up a section of the timber parquet flooring in the hall to gain access to the cementitious floor screed. He sealed a floor hygrometer in position there and left it for several hours, checking it regularly. It increased rapidly and held steady at saturated, indicating a serious dampness problem in the floor screed as well.

Meanwhile he drilled through the outer leaf brickwork at the junction of the level mortar course and perpends to minimise damage to the face brickwork, then used a borendoscope to look into the cavity of the flank wall. At low level the cavity was full of rubble arising from the original period of construction – in places it piled higher than the horizontal dpc. The rubble was highest within the cavity either side of the flank wall front entrance door – at the centre of the internal signs of dampness.

The presence of chlorides in the internal samples did not in itself confirm that rainwater was the source, but the obstructions were a clear route for rainwater and moisture in the ground to bypass the dpc and the cavity void and penetrate the absorbent inner leaf blockwork.

The surveyor advised the owner to arrange for a local builder to remove some of the face bricks at intervals along the flank external wall and scoop out the material.

4. Third visit

Several months later the dampness had become worse, following a period of sustained heavy rain.

The last resort, and most destructive option, was to remove sections of the solid floor and timber floor covering to inspect what lay beneath. The owners had already been warned that this might be necessary. Even though they had already suffered much disruption, they agreed and the surveyor removed a section of floor in the under-used dining room. He carefully removed the floor screed and slab to uncover:

• a plastic sheet dpm of less than 1,000 gauge (standard required at time of printing would be 1,200 gauge) thickness; and
• the dpm was not in contact with the dpc to the walls, especially to the corner of the room where the internal wall met the external flank wall.

The surveyor removed more of the concrete floor slab, and revealed that the flimsy plastic membrane had been laid directly onto the hardcore and had become perforated under the weight of the concrete slab placed directly onto it. This weight would also have pulled the sheeting out of the wall and from under the dpc.

It got worse as there was some more shoddy workmanship. In one section a large cavity had formed where the hardcore clay substrata had either sunk or had been washed away from a high water table. This meant that the floor slab had become unsupported on a large section of the floor. Generally, the surveyor found that areas of the hardcore and membrane were in poor condition due to lack of adequate compaction of the hardcore into the corners of the room, and puckering of the plastic membrane where the builders had poorly applied the technique of folding the membrane at external and internal right-angled corners.

Figure 1: Floor plan

Diagnosis

The poorly installed dpm was allowing moisture from below the ground to enter the building. Clearly, the original builder had saved a great deal of sand for binding, and this together with the poor technique had led to 30 years of misery for the owners.

Prognosis

The results of the salts tests were unusual. The ‘missing’ nitrate ions could have been partially filtered out through the floor mass, or groundwater from a high water table could have caused calcium chloride (sometimes used as an accelerating agent in concrete) to release chloride ions. A full laboratory analysis might explain the missing nitrates, but that was not important to the owners. The clearly visible evidence established a link between poor workmanship and moisture ingress.

The solid floors would have to be taken up and relaid. Unfortunately for the owners, the builder had ‘gone to ground’ a few years earlier and ceased trading.

Lessons learned

• Text book examples of solid floor construction are often not found in the built environment.
• Even though buildings have been supervised and passed at each stage of construction, this does not mean that major flaws in construction could not be overlooked or, more cynically, stages could be poorly constructed and concealed.
• Sometimes misleading test results can leave diagnosis inconclusive.
• Latent defect liability often only lasts as long as the builder is in business.
• You sometimes have to damage part of a building to find damage.