Lath and Lime Plasterwork

Unless you have a listed building or a period property, chances are that you’ve never heard of lime plaster. This is because other materials and ways of working came to the fore over time and saw the use of lime almost completely disappear in the world of plastering.

Young apprentices are often no longer taught these kinds of traditional skills, but given that we still have plenty of listed buildings in this country, and our period properties are only going to get older, it’s important that there are those in the trade who do know how to work with lime plaster.

Walls in some buildings need to be able to breathe, which is why lime may be a better option than modern gypsum plasters, which can result in condensation and mould growth if not used in the right places. Lime plaster differs from modern materials in that it sets quite slowly by absorbing CO2 from the air, but also in that it shrinks as it dries.

Traditionally, lime plaster would have been applied in three coats in order to ensure a flat finish. Before any work can be carried out on a building where the use of lime plaster is appropriate, it would be worth working out how many coats had originally been applied.

When applying three coats of lime plaster, the first layer is known as the scratch coat, because the surface is etched with lines to give the second coat more purchase. Interestingly, sand or hair is often added to the mix to provide tensile strength, helping to bridge over possible gaps if the plaster moves away from the substrate or if the building shifts.

It’s not always essential to use hair when plastering onto stone or brick but if you’re plastering on timber, you will need to use it as plaster doesn’t stick well to wood once it dries.

Another key point to bear in mind when working with lime plaster is that you need to keep the substrate wet before the plaster is applied. If you know you’re going to be working with a very dry or porous wall, you may need to make sure it’s sprayed down a few times before work can begin to ensure there is enough moisture in the wall for the plaster to adhere.

There are many benefits to working with lime, since it allows the building to breath and because it’s flexible and porous. That means water can escape from the walls of the building itself and if any fine cracks do appear in the walls, they’ll self-heal - unlike cement, which will crack and let water in.

If you’d like to find out more about lime plaster and renovating period properties in London, get in touch with us here at Ovolo today.

For more information on the history of lime plaster, check out the Building Conservation website now.