Lime plaster is one of the key materials that we manufacture here at Limebase. And, it forms a vital role in the restoration and renovation of historical buildings. In fact, the earliest known use of lime in buildings is as far back as 12,000 BC in Israel, meaning the benefits have long been recognised. Our guide will give you an insight into this impressive material while explaining best practices for its use too.
What Is Lime Plaster?
Made from a mix of water, quicklime, sands and soundusts, lime plaster is a building material that has been used to finish and strengthen buildings. It can be mixed with other materials, such as horsehair, to heighten its properties or alter the structure, as needed. This type of plaster can be used internally to improve strength and externally to help protect poorer materials. Some mixes include pozzolan additives which can help to speed up hardening and reduce the labour needed to apply it.
Pros & Cons
As one of the world’s oldest building materials, lime plaster offers a host of benefits. And, as technology continues to develop, it also falls short in certain areas.
In many historic buildings, lime plaster is used both internally and externally. It has an impressive versatility that allows it to be mixed with other materials to improve its performance. It can also be mixed with specific minerals to change the colour in accordance with the building that is being renovated or restored. This historic plaster can also move with the building without cracking as much as modern options.
One of the main reasons that modern-day building materials are unsuitable for historic buildings is due to their lack of permeability. In short, they don’t easily allow moisture to evaporate but instead trap it inside. Over time, this causes structural damage and corrosion which leads to significant repair works. Lime is vapour permeable and can be used with more delicate and traditional materials easily.
When it dries, lime plaster is impressively hardwearing. In order to achieve these benefits, it has to be left for an extended period of time to fully set. The exception here is when lime plaster has been mixed with specific additives that speed up this process or a hydraulic lime (which sets with water) is used.
- Less Brittle
Plaster made with lime is less brittle than modern cement plaster. This means it is less prone to cracking, reducing the access points for moisture and meaning there is less need for expansion joints when used.
- Good Adhesion
The particles found in lime plaster are considerably smaller than those found in modern plaster. This allows it to adhere to other materials in a quicker and deeper fashion than other materials.
Non-hydraulic lime plasters have a pH of 12, when wet. And, even once they dry, this drops to only 8.6. This high alkali rating means that mould growth is inhibited, meaning you’re less likely to struggle with contaminants within your walls. It essentially acts as a natural fungicide without needing additional chemicals.
While the high pH level offers fungicidal benefits to the building’s owners, it also makes lime plaster potentially dangerous during application. The caustic properties mean it can cause burns if it isn’t used with appropriate PPE.
- Long Drying Process
Modern cement takes between 1-2 days to dry fully. In contrast, lime plaster takes closer to 5 days to cure fully and get to its final hardness (depending on thickness and environmental factors such as weather). This means the entire process isn’t quick and requires a considerable time investment to get the best results.
- Specialist Application Process
Modern plastering can be learnt in an intensive day-long course. Applying traditional materials such as lime plaster requires very specific experience and, therefore, can be costly to have applied.
How Is It Applied?
The application process of lime plaster is specialised. It should be applied by someone with experience and training in the method to ensure you receive a good and matching finish on any historic building. Below details the rough method you can expect to be followed:
- Plaster Preparation
Before use, lime plaster should be mixed in large batches, using either a paddle or roller mixer. If you are using pre-mixed plaster don’t use any additional water until you have mixed it back up for some time. Lime that is older will sometimes begin to harden, even in airtight bags/containers, when it is re-mixed it will usually soften without the need for added water. If you have mixed your plaster and it is still far too stiff to use, then small amounts of water may be added.
- Wall Preparation
Lime plaster adheres best to a clean surface with a good mechanical key. Therefore, the background wall should be cleaned thoroughly, ensuring any moss or other vegetation has been removed. Any loose debris should also be brushed off.
- Control Suction
If the background wall has too much suction, it will impact the lime plaster’s ability to adhere and bond. This results in a sub-par finish and one that is likely to crack and fall off. This can be done by wetting down the surface with a mist spray or, if needed, by using a hose. You’re aiming to create an entirely damp surface but not one where water drips.
- No. of Coats
Generally, lime plaster is applied in 3 coats. The first, known as the scratch coat, is allowed to harden to a degree, but not entirely. It is then scratched into the shape of diamonds and allowed to harden for at least 72 hours. This should be checked for shrinkage cracks and dampened before the second coat is applied. This is known as the floating coat and needs to be finished to a flat, smooth surface. Again, a period of at least 72 hours, this should be dampened and then the final coat applied. The third setting coat is generally applied in 2 layers using a trowel and in a similar way to skimming.
Here at Limebase, we have been manufacturing lime plaster and other lime-based products for historic restorations for many years. During this time, we have worked on a great number of restoration projects using this specialised material, including Exeter and Canterbury Cathedral. If you have any questions about our range of products or would like to discuss the use of lime plaster on your existing project, feel free to get in contact today.