What is Rising Damp?

The Mechanism of Rising Damp

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What is Rising Damp - A Simple, Non-Technical Explanation

Rising damp is generally known to be caused by capillary action: the walls acting as a sponge wick up liquid moisture from the ground making the walls wet. Although this is a good analogy to illustrate the concept of capillarity, real walls behave differently.

To understand what is rising damp and how water rises in masonry, we need to understand what drives moisture in Nature. The video below explains the whole concept of rising damp - feel free to watch it or keep reading.

The Water Cycle in Nature

Rising damp can be best understood by looking at the movement of water and moisture in nature, described by the Water Cycle. Liquid water falls as rainwater, accumulates in the ground, then evaporates and rises upwards as vapours. So moisture is constantly evaporating upwards from the damp soil into the air - the planetwide Water Cycle being driven by the Sun's energy and to some extent by Earth's underground heat.

what is rising damp and the water cycle — The water cycle in nature

Rising Vapours

When a wall is built onto the soil – blocking the free evaporation of moisture – the moisture from under the wall will evaporate INTO the capillaries of the porous wall fabric. The extremely small volume of the capillaries get instantly saturated by the rising vapours, and due to the ongoing vapour influx (ongoing vapour pressure) liquid water will form inside the capillaries.

soil evaporation 3 — Rising damp is driven by the ongoing evaporation from the soil

Thus, VAPOR MOVEMENT from the evaporating damp soil plays an essential role in the development of rising damp. Through a number of stages, soil evaporation can lead indirectly to capillary action and the development of rising damp over time, as shown below.

capillary 6 stages sm — Rising damp and its development stages - accumulating vapors gradually develop into capillary flow

What Can Cause Rising Damp?

Rising damp is a complex phenomenon, involving multiple moisture transport mechanisms where vapour movement plays an essential role.

Technical research has shown that there are two different mechanisms that can cause rising damp: liquid water or water vapours. Depending on the height of the water table and the amount of moisture available in the soil one or the other mechanism dominates:

At normal water table [most common scenario]: rising damp can be caused by indirect capillary action as a result of excess vapour accumulation.
Commonly, the water table is situated much lower than the base of the walls. This makes direct capillary action or suction impossible due to the absence of liquid water. Above the water table we have a layer of damp but drained soil (depicted in orange on the sketch below). which evaporates its moisture content slowly upwards in form of vapours. The moisture content of the soil, even if well drained, is always vapour-saturated, being at 100% RH. This makes the wall capillaries near ground also vapour saturated, leading to moisture accumulation.
At high water table [in special cases]: rising damp is caused by direct capillary action or capillary suction, the known "wicking effect". This only occurs during flooding or very high water table scenarios, when the water table is touching the base of the masonry, the wall being in contact with the water table. Although this can happen, it is a relatively rare occurrence, and should be regarded as a special-case scenario rather than being the everyday norm.

soil-zones-2a-core-conservation — Indirect capillary action, caused by vapour deposition and accumulation in the masonry

Water vapours evaporate into the wall fabric, where they start accumulating layer by layer, leading through a number of mechanisms to non-saturated or saturated capillary flow. This indeed checks out, as in in real life, there is very often not enough liquid moisture in a drained topsoil to sustain a capillary flow, so capillary action in the walls can only occur by vapour accumulation through a liquid (water table) -> vapour (soil) -> liquid (masonry) transformation.

What Makes Rising Damp Different?

To complicate matters even further, there is more to rising damp than just water. Despite its name stressing "dampness", rising damp is far from being "just" a dampness problem. The water brings up salts from the ground and deposits them into the masonry, salts making rising damp a completely different proposition from rainwater wetting.

the effect of salts — Water breaks down the salt lattice into salt ions and transports them into the masonry

In dry state, salts are hard, solid minerals organized in a fixed 3D-lattice, held together by intermolecular electrical attraction forces.

In the presence of water, however things change. Salts get dissolved and broken down by water into their constituent parts - positive and negative salt ions. These travel independently with the liquid water from the soil into the masonry where they cause a number of irreversible changes:

Salts act as a moisture collector attracting and trapping large clusters of water molecules electrochemically (hygroscopic action), acting as a wetting mechanism.
Salts reduce the breathability of the masonry, salty masonries drying much slower than non-salty masonries.
Drying and crystallizing salts expand in volume, causing crumbling, flaking, spalling, powdering and other similar manifestations leading to permanent damages of old masonries.

The presence of salts explains why the masonry tends to be damper near ground level, in the lower 1 metre also known as the "salt band", and why most crumbling also occurs near ground level.

At higher elevations, outside of the salt band, the masonry is primarily affected by rainwater that contains no salts. As a result the fabric can dry out much faster, while also not being subject to crumbling due to the lack of salts.

Thus, rising damp is a dual problem of water and salts - the salts being the main source of problems.

Here are some other related pages that you might want to read to broaden your knowledge in this field.

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Solutions

Here are some practical solutions related to this topic:

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Solving Rising Damp

Building-Friendly Solutions

Completed Projects

Here are some of our projects where we have dealt with some of the issues discussed on this page:

Solving Damp Problems due to High Water Table

Old Farm House Ongoing Dampness Problems

Residential

Photo Galleries

Here are some photos demonstrating these concepts. Click on any image to open the photo gallery.

Victorian House - Full Replastering after only 6 months. What went wrong?

This 150 year-old Victorian house has been replastered with modern building materials (cement and gypsum plasters). 3 months after the replastering the paint started to go. Another 3 months later the walls had to be fully replastered due to salt related damages.

Broken Down Slate DPCs

Here are some photos of disintegrated, broken down original slate DPC which have been frequently installed in old buildings in the Victorian period – a means to prevent and combat rising damp.