Rising Damp

Rising Damp

What is Rising Damp?

Rising damp is a form of structural dampness, the unwanted moisture inside the fabric of a building, it is the movement of water or moisture originating from the ground rising up through masonry by capillary action. This moisture dissolves soluble salts on its journey through the building materials. The penetrating moisture can also carry other soluble compounds from its source. As the moisture evaporates from the permeable surface the salts concentrate and bloom giving rise the characteristic ‘tide mark’. The height of this rise is rarely above 1.5m. When evaporation occurs within the building material, salts can be deposited and can crystallise in the pores and small cavities resulting in fractures forming in the building materials. Damage of this type can be seen in porous masonry and brickwork.

Rising damp has been a widely observed phenomenon for at least two hundred years. Rising damp is deemed controversial because it is often misdiagnosed in buildings, many misdiagnose a wall stain as rising damp instance due to misinterpreting the visual evidence of the wall and the readings of moisture meters.

Further reading – The Truth about Rising Damp

Further reading – A Review of Rising Damp in Masonry Buildings

Diagnosis of rising damp

In assessing damp first check for standing water, removing water with good drainage will remove any form of dampness. Once done, if dampness remains, next look for the presence of a damp-proof course. If a damp-proof course exists, it is likely to be functioning as the materials from which damp proof courses are manufactured tend to have a long lifespan. However, it is possible in some cases the existing damp proof courses can fail, for one reason or another.

One method that is often employed to determine if the source of dampness is rising damp, rather than an other form of structural dampness. Is to look for the characteristic tide mark’ at the top of the damp’s peak. This is a useful indicator, but it is not completely reliable as salts may been present in the material used in construction.

If no damp-proof course is present and rising damp is suspected and confirmed by test above. Then, there are a number of diagnostic techniques to determine the source of dampness. The most efficient method is to take motor samples by drilling in the the affected areas to determine their moisture and salt content. This method is destructive to the wall and decorative finishes can make it unacceptable to home owners. For this reason, electrical moisture meters are often used when surveying for rising damp. The results these instruments generate are not accurate in measuring the moisture content of masonry, however the reading patterns that are produced can provide useful information to the source of dampness.

Further reading “Rising damp in walls: diagnosis and treatment” – Building Research Establishment Digest 245

Treatment of rising damp

In many cases, damp is caused by the bridging of a damp-proof course (DPC)  that is functioning correctly. As an example, a flower bed next to an affected wall might result in soil being piled up against the wall above the level of the damp-proof course. Moisture from the ground would be able to migrate through the wall from the soil. This damp problem could be corrected by lowering the flower bed to below damp-proof course level.

Where a rising damp problem is caused by a lack of a damp-proof course this is common in buildings over 100 years old, or in some rare case by a failed damp-proof course, then there are a wide range of possible solutions available. These include:

  • Replacement physical damp proof course
  • Injection of a liquid or cream chemical damp proof course
  • Porous tubes
  • Electrical-osmotic systems
  • Land drainage