Damp Proof Plaster

When replastering after remedial damp proofing it is essential to use the correct products to prevent future damage. It is equally important that the damp problem is accurately identified in order to remove the source of moisture, and to justify whether or not walls need to be replastered with damp proof plaster.   

Rising damp, penetrating damp, condensation and mould growth are common problems that affect properties across the country and they can have a significant impact on the appearance and condition of plasterwork. But this does not always mean that walls must be replastered.

Damp proof plaster is typically used for a confirmed rising damp problem, with a CSRT or CSTDB qualified surveyor finding evidence of the upward movement of moisture into masonry through capillary action. Once a new damp proof course has been installed, damp proof plaster can be used to prevent migration of hygroscopic salts trapped within the masonry moving into the new plaster.

Accurate diagnosis is key, making it important for property owners to have a damp survey carried out which can provide answers backed by evidence, and recommendations for further work, where necessary.

Renovation plaster is a specialist product containing a salt retardant, designed for the replastering of properties that have salt contaminated walls, usually after rising damp has affected them, often where there was a missing or failed damp proof course. It is applied after a new damp proof course has been installed and prevents salt migration from the masonry into the newly applied plaster. The plaster will allow the walls to dry out, preventing moisture from being trapped within the structure.

It's an excellent choice for properties with salt contaminated solid walls and can be used on heritage buildings and restoration projects. 

The need for replastering depends on the type of damp affecting the walls, making it important to accurately diagnose the type of damp. Some damp walls may not even require replastering. A damp survey will provide you with answers and next steps to a damp problem.

Condensation can cause cold walls to become damp as warm humid air condenses on cold surfaces, which in time can lead to mould growth. This may not require damp proofing as the source of the moisture is from the internal environment.

Penetrating damp is another way walls can become damp, but fixing the external source of moisture may stop the problem, often without the need for replastering. If lateral penetrating damp from high ground levels is the source of damp inside the property, specialist replastering may be required.

Walls affected by rising damp will often need to be replastered due to ground salts present in the water that rises up through the masonry by capillary action. These hygroscopic salts migrate to the surface of plaster which causes it to deteriorate. This may cause damp patches (salt damp) and white powdery residue to form on the surface.

Because not all damp walls require replastering - it is important to accurately diagnose the type of damp.

The reason damp proof plaster is used when treating walls that are contaminated with hygroscopic salts is because they can prevent these salts from migrating to the surface.

If damp proofing is carried out, and a salt inhibitor is not used, over time the salts in the masonry can migrate to the surface, and cause damp patches to reappear as the salts absorb moisture from the air. This gives the illusion of a damp problem, despite successful damp proofing being carried out.

It is also possible the salts will form large crystals and pop the plaster off the wall. 

Lime plaster is commonly used for heritage, listed and historic buildings in keeping with the existing plaster.

It is permeable and allows walls to ‘breathe’ by allowing moisture to pass through and evaporate, helping walls to dry out. It cannot however prevent salt migration, so if walls are contaminated with hygroscopic salts, lime plaster may not be sufficient in preventing future deterioration of the plaster caused by salt damage.

Lime plaster is often cited as a fix for damp problems opposed to more modern methods of plastering, but this will depend on the type of property in question, and the root cause of the damp affecting the structure.

Older properties were designed with open fire places and single glazed windows that allowed draughts into the property, increasing ventilation which helps keep walls suffering from damp dry. As measures are taken by homeowners to draught proof properties to keep in heat, it can cause moisture to build up within the property, causing condensation on any cold surface, lime plaster or not.

The best course of action is to have a qualified surveyor investigate the source of damp causing the problem, and they can provide you with recommendations, including the application of lime plaster if required by the listed building officer.

Waterproofing slurries or tanking can be used to create a waterproof barrier between the high external ground level outside that allows moisture to penetrate the masonry and the drier internal space. Tanking consists of a cementitious product that is applied directly onto the walls to create a waterproof barrier.

Walls must be prepared correctly for tanking to work successfully, so existing plaster must be removed and any old mortar joints or deteriorated masonry must be repaired. Surfaces will then need to be dusted down to allow for the slurry to sufficiently bond. Salt and lime inhibitors may be required where hygroscopic salts are migrating through masonry into the internal space.

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  Does renovating plaster stop damp?

  No, you must carry out damp proofing to stop the source of damp causing the issues. A qualified professional should be used to correctly identify the type of damp. If the walls are contaminated by salts, an application of renovating plaster containing a salt inhibitor can be used to prevent salt migration through the newly applied plaster, preventing further damage.

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  Can mould grow behind plaster walls?

  If there is a severe condensation problem affecting walls, it is possible that mould can grow behind the plaster on walls. This is called interstitial condensation, as the dew point where condensation forms is within the structure rather than on a surface. Addressing the excess moisture in the property can help, while steps should be taken to remove the cold bridging on external walls by increasing the thermal efficiency.

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  Can you treat damp without removing plaster?

  Yes, plaster does not always need to be removed because of a damp problem. Often a damp problem is caused by leaking pipes or faulty appliances, and this will not require expensive repairs, the plaster will just need to dry out.
  
  If however the damp is caused by rising damp or high ground levels, there is the possibility that the walls are contaminated with hygroscopic salts which can cause damage to plasterwork. This is when walls will need to be replastered with specialist plaster with a salt retardant that creates a barrier that prevents salts from migrating to the surface.

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  What is the best renovation plaster?

  The best plaster to use will depend on the damp problem at hand and the source of moisture. It is important to have a qualified surveyor inspect the wall in question to fully understand if replastering is required, with their findings detailed in a survey report backed by evidence. Peter Cox use our own type of salt retardant plaster, and typically back our damp proofing treatments with a 20 year guarantee.