In simple terms rising damp occurs when ground water travels upwards through porous building materials such as brick, sandstone, or mortar, much in the same way that oil travels upwards through the wick of a lamp.

Rising damp can be identified by a characteristic “tide mark” on the lower section of affected walls. This tide mark is caused by soluble salts (particularly nitrates and chlorides) contained in the groundwater. Due to the effects of evaporation these salts accumulate at the “peak” of the rising damp.  Due to rising damp often being caused by moisture from wet ground, it is not common to find rising damp on floors above ground level.

Having thus taken care that the air and moisture shall have no chance of rising into the house from the ground beneath the floor, we must now turn our attention to the walls, which is equally necessary to protect from rising damp. If you plant a brick or stone wall on ground which is capable of retaining moisture, it will inevitably happen that unless you take means to stop its progress, the moisture will climb up the walls in obedience to the law of capillary attraction. The way to prevent this is to insert above the ground level but below the floor level, either a course of vitrified stoneware made on purpose, or two layers of slates laid in cement, or some equally effective impervious material, the intervention of which between two courses of brickwork bars will prevent the further upward progress of the damp.

As to the damp-proof course, however, it is possible by knowing what to look for, and where to look, to find out for a certainty whether there is or is not such a thing. Examine carefully the joints of brickwork between the ground and the level of the lower floor. A vitrified stoneware damp-course will be conspicuous from its perforations, and the difference in colour between it and the bricks. Asphalt or slates or cement alone will all appear, the two latter like mortar joints about three or four times the usual thickness. A favourite material  is tarred or asphalted felt, the presence of which can generally be detected by portions of it projecting from the wall. 

Diagnosis of rising damp

The first step in assessing damp is to check for standing water. Removing water with good drainage will remove any form of dampness. Once done, and dampness remains, the next step is to look for the presence of a damp-proof course. If a damp-proof course is present, it is likely to be functioning, as the materials from which damp proof courses are manufactured tend to have a long lifespan. However, it should be acknowledged that there are cases where existing damp proof courses fail for one reason or another.

One indicator that is often used to determine if the source of dampness is rising damp (rather than other forms of dampness) is to look for the presence of salts – in particular a tell tale “salt band” or “tide mark” at the peak of the damp’s rise. This is not a reliable method as salts and dampness can enter the fabric of the wall in other ways – e.g. unwashed sea sand or gravel used in the construction of the wall.

If there is no damp-proof course and rising damp is suspected (tide mark, moisture confined to lower section of wall etc. …) then a number of diagnostic techniques can be used to determine the source of dampness. Obtaining samples of mortar in the affected wall using a drill and then analysing these samples to determine their moisture and salt content to assist in providing appropriate remedial building solutions. The fact that this technique is destructive to the wall finish often makes it unacceptable to homeowners. It is for this reason that electrical moisture meters are often used when surveying for rising damp. These instruments are unable to accurately measure the moisture content of masonry, as they were developed for use on timber, but the reading patterns that are achieved can provide useful indicators of the source of dampness.

Rising damp treatment

In many cases, damp is caused by “bridging” of a damp-proof course (DPC) that is otherwise working effectively. For example, a flower bed next to an affected wall might result in soil being piled up against the wall above the level of the DPC. In this example, moisture from the ground would be able to ingress through the wall from the soil. Such a damp problem could be rectified by simply lowering the flower bed to below DPC level.

Where a rising damp problem is caused by a lack of a damp-proof course (common in buildings over approximately 100 years old) or by a failed damp-proof course (comparatively rare) 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 (DPC Injection)
• Damp-proofing rods
• Porous tubes / other evaporative
• Land drainage
• Electrical-osmotic systems

Replacement physical damp proof course

A physical damp proof course made from plastic can be installed into an existing building by cutting into short sections of the mortar course, and installing short sections of the damp proof course material. This method can provide an extremely effective barrier to rising damp, but is not widely used as it requires experienced contractors to carry out if structural movement is to be avoided and takes considerably longer to install than other types of rising damp treatment. The cost is also several times higher than for other types of rising damp treatment.

Injection of a liquid or cream chemical damp proof course (DPC Injection)

Injection of a liquid or cream into bricks or mortar is the most common method of treating rising damp. Liquid-injection products were introduced in the 1950s and were typically installed using funnels (gravity feed method) or pressured injection pumps. The effectiveness of liquid injection damp proofing products is dependent on the type of formulation and the skill of the installer. In practice injection times tend to be lower than those required to provide a damp proof course of optimum effectiveness.

Damp-Proofing Creams

Damp-proofing cream leaking from injection holes. This can make it difficult to ascertain whether sufficient cream has remained in the holes for treatment to be successful.

Since the early 2000s, damp-proofing creams have taken over from liquid products due to improved ease of application. As with liquid products these are based on silane/siloxane active ingredients which line the pores of the mortar to repel damp.

The effectiveness of liquid and cream based rising damp treatments varies considerably between products due to variations in product formulations. As with liquid injection systems, cream based treatments rely on the competence of the installer for treatment to be successful. Injection holes need to be fully cleared of drill dust and debris before the cream is injected, and it is often difficult to know if each injection hole has been completely filled with cream. Furthermore, damp-proofing cream can sometimes drip out of the injection holes after treatment, reducing the effectiveness of the damp-proofing treatment.