Stage 3 of your damp cure - tracking down the real causes of the damp

Once the severity and full extent of the problems is known, the REAL problems can be hunted down.

The checks in the previous stages have all been on the symptoms - the causes are the true problems. Again, the knowledge already gained of the construction is critical. When this is combined with the patterns, severity, and extent of the symptoms, the true causes either start to become apparent, or contributory causes and pointers to further underlying causes show up. It is only this final detective work that will reveal what needs doing to resolve the damp problems bothering you. Tracing the causes of damp by following the pointers can lead to some surprising discoveries about the underlying defects - and where they are.

A real investigation…

An investigation into an area of damp at the bottom of an internal wall in an 1850s Victorian house revealed the following important facts:

  • the wall was solid brick;
  • the floors on each side were timber floorboards laid over joists with a space underneath;
  • the space under the floors was ventilated with airbricks through the outside walls;
  • the skirting board on the hall side of the wall was timber and about 10" (250mm) tall;
  • the skirting board on the dining room side of the wall was a thick strip of painted concrete about 12" (300mm) tall.

Checking the nature and extent of the problem

A thermal imaging camera revealed the concrete skirting in the dining room was cooler than the wall above, suggesting moisture was trapped behind/in it. A moisture meter gave high salt readings on the concrete skirting but low readings on the wall above – backing up the theory of moisture being trapped by the concrete.

In the hall, the thermal imaging camera shew the top moulded part of the skirting as a cool horizontal strip and a second cool horizontal strip 2” (50mm) higher – where the top of the concrete skirting was on the other side! Close examination of the skirting board revealed it had warped slightly, so was curved away from the wall at the bottom but was held in contact with the wall by its fixings at the top where the moulding was. The moulding was not only very damp when tested, it was starting to rot in places. The rest of the skirting though was dry and rot free because it wasn't touching the wall. Using the moisture meter on the wall above the skirting board, salts were found in the vicinity of the horizontal strip that had been seen with the thermal imaging camera, but the strip wasn't very distinct. This confirmed that moisture was being trapped in the wall, and the concrete skirting was the culprit. The strip above the hall skirting board was being caused by the moisture evaporating where the concrete stopped on the other side. This explained the symptoms but not the cause, as there should not have been any moisture to get trapped by the concrete skirting.

The cause is revealed

The checks above showed the moisture was not coming down the wall or along the wall, so that only left the floor. In the hall, there were two loose floorboards beside the wall that could easily be lifted. These revealed that polythene sheeting had been laid over the top of the ground and turned up the sides of the brickwork under the floor. In places where the sheeting didn't properly cover the ground, the soil was dry and dusty, but under the sheeting, it was very damp. This sheeting was the cause of the problem.

The sheeting was stopping the moisture from evaporating and escaping through the air vents so it was being forced into the brickwork. The floor joists that were touching the brickwork were also very damp but hadn't yet started to rot. Because the polythene sheeting was sitting against the sides of the brickwork, the moisture was stopped from evaporating out of the brickwork under the floor, forcing it up above the floor – where it found the concrete skirting board on one side of the wall!

A simple cure

The cure was to remove the polythene sheeting from under the floor. A further optional improvement to help dry the wall (and improve the look of the room) was to hack off the concrete skirting and fit a timber one.

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