Solving Damp Problems Due To High Water Table

Solving Damp Problems due to High Water Table

NO DAMP PROOF COURSE + HIGH WATER TABLE = PROBLEM

This is a case study of a 250 years old stone farm house, with a high water table, where nothing seemed to work... 

Old buildings built prior to the 1870s often do not have a damp proof course. 

The fact that there is no damp proof course, whose purpose would be to prevent damp from rising, does open the door to potential damp problems. But how much damp is actually getting up into the walls does depend on several factors. In actual fact one can see old buildings ranging from literally no damp to really damp walls.

Water table, as illustrated by the following case study, is one of those factors that can make life difficult.

A Brief History of the Project

The property is a 250 years old long stone house with 600-800 mm thick stone walls. When we met the customer we have been told "they have already tried everything.": they have injected a chemical damp proof course, did french drain with a perforated pipe at the bottom, redone pointing, you name it. The house was still damp with radiating cold.

We have a reputation of being able to solve difficult damp cases, so we have taken on the challenge. 

Why Was this Rising Damp Case Difficult?

To be able to understand the difficulty here one has to have a clear idea of what are those factors that influence rising damp in an old building. 

We have researched and identified five main factors one should be looking for when analysing a no-DPC rising damp case:

Factors Influencing Rising Damp

Factors Influencing 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.

This farm house was suffering from two of the five main problems above: Water table and wall thickness.

High Water Table

When the water table is high it also tells us that the area doesn't drain well. 

In our case the house was built on clay, and even worse, water came together in a pool under the house from slightly higher elevations. The house is located just a little bit lower than the surrounding fields, which is barely visible, but it causes water to drain into the pool under the house for weeks after the rain.

Water pouring under the extension from the field

In this part of the country it is the norm to have rain for weeks - subsequently the water table is always high.

High Water Table

French Drains

To prevent water coming together under the house the customer has installed a french drain with a perforated drain pipe. The drainage has helped somewhat, and one can see the amount of water discharging from the pipe. 

This is one step forward.

Please note that the purpose of a french drain is to reduce the moisture saturation of the soil under the house by draining a waterlogged area. This has an effect on rising damp and walls often become dryer, but it very rarely solves rising damp completely.

Due to the amount of water and how clay behaves under the house (contains 40% of water and doesn't dry easily) there was more to do.

Damp Proof Course

Thick stone walls were still wicking quite a lot of moisture from the ground. Damp was rising up to 1 - 1.5 m high and walls were radiating cold.

The masonry was built without a damp proof course, and the need for one was very apparent. 

The customer attempted to inject a chemical DPC into the stonework. It is one of the technical limitations of injected DPCs that chemicals can not saturate a lose stone masonry. It has gaps and crack in it, with a rubble infill in the middle. Chemicals simply pour away between stones. 

Nevertheless, the idea of a damp proof course was the right way to go.  But with the water table only a few inches away the retrofit damp proof course HAD TO WORK!  

There are two kinds of damp proof courses:

  1. Inserting a physical moisture barrier (such as injecting a DPC or inserting a lead sheet between the courses of bricks/stones) 

  2. Influencing the behaviour of water making it not want to rise (such as electro-osmotic DPCs or Magnetic DPCs)

It is difficult to insert a consistent physical moisture barrier into a thick stone wall, but the second category above does work and can turn a thick stone wall dry and warm.

We have installed our  largest Magnetic DPC system, knowing that just a few inches below the floor there is liquid water.

It turned out to be a success, 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: 

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