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Rising Damp

Rising damp is not the most common form of dampness encountered in buildings; this is left to condensation. However, it is very likely that a high proportion of older buildings are affected by rising damp to some degree or another, and it does cause problems with positive identification, appropriate remedial action and ancillary works.

Rising damp in buildings may be defined as the vertical flow of water up through a permeable wall structure, the water being derived from ground water. The water rises through the pores (capillaries) in the masonry by a process loosely termed ‘capillarity’. In other words the masonry acts like a wick. The height to which the water will rise depends on several factors including pore structure and rate of evaporation. Masonry containing a high proportion of fine pores will allow the water to rise higher than a coarse pored material; basically the water is carried up the wall in the finer pores and not those of large diameter. The average size of pores in masonry gives a theoretical rise of around 1.5 meters but where evaporation is severely retarded, for example by the use of impervious membranes, moisture can sometimes rise in excess of 2 metres.

The major paths through which the water rises are the mortar beds; this is illustrated in Figure 1. For water to rise through the bricks then it must cross a mortar bed. In effect the mortar beds form the only continuous pathways for water rising through the wall. If a house is built from impervious bricks then water can still rise through the mortar bed but if an impervious mortar is used then no water will rise even if the bricks are very porous. The mortar beds will form an important part in the chemical treatment for rising dampness.

Ground water also contains small amounts of soluble salts, the most significant of which are chlorides, nitrates and sulphates. These pass with the water in solution up the wall and are left behind when the water evaporates.

Over many years of active rising dampness large quantities of these salts accumulate within the masonry and decorative surface, most becoming concentrated in a general ‘salt band’ towards the maximum height of rise as illustrated in Figure 2. Frequently, the concentrations of these salts are very low towards the base of the wall.

Both chlorides and nitrates are usually hygroscopic, i.e., they can absorb moisture from the surrounding environment and, in general, the greater the amount of salts the greater the absorption of moisture especially under humid conditions. Thus, even though rising dampness may have been controlled by the insertion of a remedial damp-proof course these salts alone can cause the wall and any contaminated decorations to remain damp.

DAMP-PROOFING

Therefore, to provide a ‘dry’ wall and a suitable surface to take new decorations the Damp-proof Course Systems involve two fundamental processes:

1. The insertion of the chemical damp-proof course.

2. Removal of old contaminated plasterwork/decorations and replacing with specialist replastering to prevent the passage of any residual moisture and contaminant salts from passing to the new surfaces from the underlying masonry.

It is essential when investigating the potential for rising dampness to eliminate other sources of water ingress. Care must be taken to eliminate other potential sources of moisture, especially condensation in the colder months, and it is therefore essential to ensure that a full investigation is always undertaken. If any other sources are identified then these must be first eliminated before a proper assessment of any rising dampness can be made as it can be very difficult to distinguish between two or more interfering sources of water ingress. Should it be noted that previous damp-proofing works have been undertaken then it is essential to take great care ensuring that the evidence of dampness is correctly evaluated.

Once any form of dampness has been identified then it is essential that the risk of decay to any timbers is assessed and appropriate remedial measures undertaken. Remember, a combination of dampness and wood leads to potential rot.

It is essential that the survey is carried out thoroughly and that all potential areas of dampness are noted. Special vigilance must be taken where dampness, and hence decay, may be unseen such as beneath timber suspended floors. Wherever timber and dampness coexist the risk of decay should always be reported and the client must be put on notice to that effect.

Where there is more than one source of water ingress then it may be difficult to distinguish between their origins. Generally, the presence of active rising dampness is indicated by excessive moisture at the base of the wall which slowly declines on going up the wall. This moisture gradient is usually observed up to heights of 1.5 metres but, depending on conditions and the structure of the masonry, it may rise to greater heights. Sometimes, a ‘tidemark’ can be observed running almost horizontally along the wall and the area below it being obviously damp.

The contamination of the masonry with a ‘band’ of hygroscopic salts will also confirm the presence of a rising damp but will not differentiate between an active or past complex. The proper use of a surface electrical moisture meter can give a useful indication as to the existence of a rising damp complex but cannot give absolute proof, especially where remedial works have been previously carried out. Generally, with an electrical moisture meter high surface readings are obtained followed by a sudden ‘cut-off’ at the top of the rise of moisture. This pattern of readings is typical of that resulting from active rising dampness.

Our advanced DPC Injection Fluid is based on Polyoxoaluminium Stearate in a Low Odour White Spirit. It is injected into brickwork or masonry under pressure. In the course of a few days the solvent evaporates leaving a gel which crystallises to form a water repellent barrier against capillary moisture, which results in a high performance, low odour injection fluid.

It has a multi-component formulation, which means that it develops initial water repellency quickly, but also allows further diffusion to form a more evenly distributed damp-proof course.

Further advantages include.

• BBA Approved

• Ready for Use - No Mixing Required

• Enhanced Spread Characteristics

• Faster Injection Times Than Conventional DPC Fluids

Carpets and furnishings are removed from the area to be treated. Paths, patios and glass surfaces must be protected from spillages. Remove timber skirting and save for re-fixing where possible. Remove all plasterwork to a minimum height of 1 metre or 500mm above the highest evidence of damp/salt contamination.

A line for the insertion of the damp proof course is chosen not less than 150mm above the external ground level and as close to the internal floor level as possible. Holes should be 10-14mm in diameter, depending on the size of the injector nozzles, and spaced at no more than 170mm centres. Drill either directly into the mortar or down at an angle, through the brick, and terminating in a mortar bed. The precise drilling method should be determined after a trial assessment of a short run of wall.

Insert the injector nozzles into the wall and tighten to seal. Pump the Chemical  into the wall at a pressure between 10 and 80psi. Longer injection times at lower pressures are safer and potentially provide more even distribution of the product through the wall. However, dense, impermeable substrates may require higher pressures.

Any spillage’s onto patios, paths etc. should be removed immediately using water and detergent.

Plug the injection holes with a sand/cement mortar or plastic plugs. Leave wall unplastered for as long as possible to speed up the drying out process.

EARTH RETAINING WALLS

Earth retaining walls can only be treated above external ground level. That area below must be suitably ‘tanked’ to prevent lateral moisture penetration.

In order to complete an effective damp-proofing system it is extremely important that the new plasterwork which replaces the salt contaminated material resists the passage of residual moisture and contaminant salts from passing from the underlying substrate through to the new decorative surface. This function is extremely important because the underlying wall can take many months to dry down following damp-proofing, but more important, the base of the wall may always remain damp due to the inherent limitations of the actual injection damp-proof course.

Finally it is important to understand that chemical dampproofing is a system – (1) the injection of the DPC and (2) the replastering: they are inseparable.

REDECORATION

No decoration should take place for at least 6 weeks after treatment (or until the wall has dried out).

After this time we recommend that only low solids emulsion paints are used (PVA). We do not recommend the use of wall paper or high build paints such as enamels.

OUR GUARANTEE   

We provide a free-of-charge 30 Year Guarantee which covers the services we provide. The Guarantee can be used as proof of works when selling the property also.

Additionally for an extra fee the Guarantee can benefit from Insurance Backing for a period between 20 & 30 years. We are an approved contractor for Sovereign Preservation UK. Please feel free to enquire for further information.

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