Case study
Ashfield: Water Table Under the House
The Property
A builder renovating an older house in Ashfield had the suspended timber floor out and was digging new pad footings under the existing brick footings. The excavations were about 600mm below the subfloor soil.
They were dry in the morning. By the time I arrived they had 50 to 80mm of water in them, and it had not rained.
What I Found
That water was the local water table. Water soaks down through the soil until it hits a harder layer of clay or rock, then runs downhill across the top of it, and in this street it was running under the house. There was no service or easement under the building to explain it.
The brick piers were heavily salted, and the deposits had colour to them, which told me the ground under there stays wet. The builder assumed the salt came from the old lime mortar. It does not. It comes in with the water.
Why more drying is not always better
You cannot stop a water table. You manage it. Subfloor fans help with timber, mould and smell, but run too hard over salted, wet brickwork they speed up the damage. Every time water evaporates from the wall, more comes up behind it and more salt is left in the mortar. Dry it faster and the salt eats the mortar faster.
The approach:
- Injection above external ground level on the front wall, not below it
- Inject above the highest point of bridging
- Keep new timber out of contact with the masonry
- Ventilation sized for air quality, not maximum drying
- Injection before the new floor closes the subfloor in
The lesson
Water carries the salt, and the salt does the damage. Some sources can be stopped and some can only be managed. See subfloor ventilation risks and solutions.