Case study
Mortdale: Five Years After Treatment
The Property
I treated part of this Mortdale house five years ago. The owners called me back to look at more of the house, which gave me a chance to check how the original work had held up.
The rear studio used to flood regularly. In five years since the treatment there had been one small event, about half a litre of water.
What I Found This Time
Under the floor, the subfloor walls had been encased in a thin concrete jacket, about 10mm thick, probably a DIY job from the 1940s or 50s. You could still see the formwork board marks.
The damp was showing up at 1.2 to 1.6 metres, which is unusually high. The brick piers were clean, but the perimeter walls were heavily salted. The inside of the house had been waterproofed at some point, so the salt was building up in the external render instead.
What was causing it
The concrete jacket stopped the walls drying out at the bottom. When a wall cannot dry where it normally would, the water keeps climbing until it finds somewhere it can. That pushed the wet band, and the salt damage, much higher up the wall than usual.
Options I quoted:
- Injection to the front room subfloor walls
- The same, plus render removal
- Whole-house injection, including a two-line option for the worst walls
- Keeping the outside face breathable so the wall can still dry
- Advice for the owners on the parts they wanted to do themselves
The lesson
Sealing the face of a damp wall does not stop the water. It moves the damage somewhere else, usually higher. The fix is a damp course at the base and letting the wall breathe. See rising damp myths.