Solving Damp Problems due to High Water Table

This is a 250 years old stone farm house with a high water table, where no prior solution seemed to work.

This old stone farm house house had very thick solid stone walls, the wall thickness averaging about 800 mm. When we met the customer we have been told "they have already tried everything": they have injected a chemical damp proof course, laid a French drain with a perforated pipe at the bottom, and they have also redone the pointing. Despite of all these measures the house was still damp with radiating cold.

Old buildings built prior to the 1870s often do not have a damp proof course, as a result rising damp was one of the ongoing problems.

We have identified 5 main factors that should be taken into account when dealing with an old house with no DPC subject to rising damp.

1. Building material/breathability: the less moisture evaporates through the surface the higher damp will rise. The left side (yellow) shows breathable lime, the right side (grey) is non-breathable cement.
2. Water table: the higher the water table the more moisture will want to evaporate through the walls. Very high water table is almost like the wall is standing in water. It will inevitably suck up moisture, just like the wick of a candle.
3. Heating/ventilation: A breathable surface, in the presence of heating and ventilation, will evaporate more moisture and the surface will be dryer. But it affects the surface and the height of damp will lower in thin walls only where the two surfaces are close. As mentioned above, the wall is like the wick of a candle: The more it evaporates, the more it will suck up from the soil.
4. Wall thickness: the ticker the wall the less moisture will evaporate through the surfaces relative to thickness. Damp goes higher in thicker walls than in thin ones.
5. Salinity: The older the building the more salts have accumulated in the building fabric. Salt crystals pass moisture from crystal to crystal due to their hygroscopic nature and damp can rise unchecked up to the level where the salinity ends.

In the case of this farm house two of the five problems listed above were a problem: water table and wall thickness.

When the water table is high the area doesn't drain well. 

In our case the house was built on clay, and the water also pooled under the house from a slightly higher elevation as the house is situated slightly lower than the surrounding fields, causing water to drain into the pool under the house after each rain.

To deal with the problem the customer has installed a french drain with a perforated drain pipe. The purpose of a French drain is to reduce the moisture saturation of the soil under the house by draining the liquid water away from the waterlogged area. This has alleviated the situation to some extent, however due to the amount of water clay contains (about 40% of water), it wasn't enough to make a significant difference.

Due to the age of the building, it did not have a damp proof course and the need for one was very apparent. The thick stone walls were able to absorb a lot of moisture from the ground. At the time of our inspection the rising damp was up to 1-1.5 m height on the walls, which were also radiating cold due to the ongoing evaporation.

Drainage has some effect on rising damp, but it can very rarely solve the problem completely.

There are two ways to tackle the problem of damp proof courses:

1. By inserting a physical moisture barrier at the base of the walls (e.g. injecting a lead sheet between the courses of bricks/stones)
2. By influencing the behaviour of water , decreasing its ability to bond to the masonry (e.g. using a Magnetic DPCs)

In the past, the customer had chemical DPC injected in the stonework. One of the inherent limitations of chemical DPCs is the insufficient spread of chemicals in the masonry. Chemicals often can't saturate a lose stone masonry with gaps and crack in it with rubble infill in the middle. Chemicals simply just pour away in-between stones.

Running out of options, the customer has decided to install a large magnetic DPC system in the farm building, knowing that just few inches under the floor there is liquid water. 

The project turned out the be a success. The walls started to dry out. One year later the radiating cold was gone and the peeling of the paint stopped.

Please see the full interview below: