In the latest of its building defects series, Safeguard Europe looks at the problems that can arise when inappropriate replastering systems are used on damp walls.
One of the more overlooked aspects of rising damp treatment is the replastering. It is common knowledge that the best way to stop rising damp at its source is to create a new damp-proof course using high-strength damp-proofing rods.
However, even though the damp is no longer present, salts will have been left behind from the groundwater. These salts can contaminate plaster surfaces causing blistering paint, crumbling plaster and further damp-patches. They can also contaminate new plaster surfaces so standard gypsum plasters cannot be used.
The old ways are not always the best
The traditional answer to replastering salt-contaminated walls is to use a sand and cement render with a salt-resistant additive. This form of replastering is very good at resisting salts but it does have some significant drawbacks.
Sand and cement render needs to be very carefully gauged and mixed to make sure that it is effective. It also requires sharp washed sand, which can be difficult to source. This places a large emphasis on operator competency, which can be difficult due to skill shortages.
Sand and cement render is also a very cold and dense material. It is often stronger than the walls it is being applied to. This means that, if it is ever required down the line, removing the render can be very difficult and may cause damage to the masonry behind it.
Finally, sand and cement render is not breathable. It can drastically decrease the rate of evaporation on walls it has been applied to. This means that the wall will dry out more slowly and, in the event of a future damp-proof course failure, rising damp may return at a height above where it was before.
Not just a sticking plaster
There are now modern renovation plasters available that provide all the salt-resistance and damp-resistance of sand and cement render without any of the drawbacks. One such plaster is Dryzone Damp-Resistant Plaster, which is specifically blended and pre-bagged to ensure consistent performance on any damp or salt-contaminated wall. These modern renovation plasters are ideal for use on walls that have previously suffered from any form of dampness, not just rising damp.
Modern renovation plasters are designed to have a large pore structure and are applied at 20 mm thickness. This allows any residual salts in the wall to form behind the decorating surface, keeping it free from damage, whilst still allowing the wall to breathe and dry out naturally. It also means that the resulting surface is warmer and softer than sand and cement, which helps to preserve the original building materials underneath should the need for more renovations arise.
When dealing with renovations on heritage properties, traditional wisdom dictates that lime plaster should be used on internal surfaces because of its breathability. Whilst it is true that lime plaster is highly breathable, it does not deal well with salt-contamination and physically degrades over wetting and drying cycles.
Certain modern renovation plasters contain a blend of natural hydraulic lime and calcite that is ideal for use on heritage properties. For instance, laboratory testing shows that Dryzone Hi-Lime Renovation Plaster is actually more breathable than lime plaster whilst still being able to resist salt-contamination and damp.
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