Damp (structural) - Rising Damp - Treatment of Rising Damp - Replastering

Replastering

Replastering will often be carried out as part of a rising damp treatment. Where plaster has become severely damaged by ground salts there is little argument about the need to replaster. However there is considerable debate about:

1. The extent of replastering required
2. The use of hard sand:cement renders to replaster as part of a rising damp treatment

BS6576:2005 states that "the function of the new plaster is to prevent hygroscopic salts that might be present in the wall from migrating through to its surface, while still allowing the wall to dry." However, writing in the RICS publication "Remedying Damp", Ralph Burkinshaw claims that, "the plaster is really there for two main reasons." He accepts the need for replastering when significant amounts of ground salts have built up in the existing plaster, however he then goes on to say that replastering is often carried out to make up for an unreliable chemical DPC. He also suggests that damp-proofers have an incentive to carry out more replastering than is strictly necessary as it allows them to finish the job without having to wait for walls to dry out, resulting in faster payment.

Although the sand:cement renders typically installed as part of a rising damp treatment are very effective at holding back damp and ground salts, they have a number of disadvantages. These include an incompatibility with the soft bricks and mortars encountered in older buildings and a lack of insulation properties compared with more traditional plasters, resulting in an increased risk of condensation. Replastering is also one of the most expensive parts of a rising damp treatment.

Porous renders to German WTA specification 2-2-91 can be used as an alternative to dense sand-cement renders. These have a minimum porosity of 40% of total volume. Salts crystallise in these pores rather than on the plaster surface, avoiding decorative spoiling. Such plasters offer a better solution than dense sand-cement renders when used on moderately salt-contaminated walls as their porous nature gives them insulation properties, resulting in a warmer surface temperature and making condensation problems less likely to occur. However, when used on heavily salt contaminated walls they may need to be replaced frequently as they lose effectiveness once all the pores have become filled with crystallised salt.

Replastering may not be necessary where salt contamination is not severe. BS6576:2005 states that "Where the plaster appears to be in sound condition, the extent of plaster to be removed may be minimised by delaying any decision to replaster until the drying period is complete." Avoiding the need to replaster in this way can reduce disruption and mess and has the advantage of allowing the original lime or gypsum-based plaster to be maintained. However it should be noted that the deficiencies of any remedial damp-proof course will be more apparent if the wall is not covered with a waterproof render. For this reason it is important to check the BBA certificate of the damp-proofing system to ensure that it is valid for use where replastering is not being carried out.