Lime render is mainly made up of different kinds of calcium, typically calcium oxide and/or calcium hydroxide, which is an element. If you’ve ever been by the coast and noticed the cliffs are a pale, lemon-yellow tone, there is probably lime in the rock formations. You might also be familiar with limestone, which can be seen in certain UK cities, notably in Bath and York.
Lime is a mineral with the chemical formula CaO, according to the International Mineralogical Association. The meaning of the word “lime” derives from its early use as mortar in construction, where the Old English word “lim” meant ‘sticky’.
Limestone and lime render are similar in their make-up, though there might be different amounts of chalk and other substances in order to make the lime render viscous and sticky.
Did you know: Though it does have limestone cliffs, the name Lime Regis actually comes from the Welsh word for river (llif).
Lime used in construction can be either natural or synthetic, and it can be generally categorised as “pure,” “hydraulic,” or “poor” lime. Lime can also be further classed based on its magnesium concentration, such as dolomitic or magnesium lime. Lime is actually used for a variety of purposes, including lime mortar, lime plaster, lime render, lime-ash flooring, tabby concrete, whitewash, silicate mineral paint, and several sorts of limestone blocks. How different forms of processed lime are used depends on their individual characteristics. Roman concrete was created using two different types of lime mortar, allowing the Romans to revolutionise buildings, often known as the Concrete Revolution.
Lime and volcanic ash were combined to produce concrete by the Romans using a pozzolanic process. If this were combined with volcanic tuff and submerged in seawater, the exothermic reaction of the seawater hydrating the lime cemented the mixture. Those Romans, pretty clever!
For starters, using lime instead of cement is often a requirement of listed/period properties. However, lime also offers some benefits over cement. This has meant that its use isn’t just limited to listed buildings but is now a preferred option in non-listed buildings, especially given its breathable properties.
Lime however does have some disadvantages over cement. It has a longer drying time, and as such has a lengthier process in rendering (more time is needed in-between coats). It’s also softer and so can’t be used “structurally” in the same way cement can. It also doesn’t do well in any standing water as the water can soak through the render.
Lime render is often applied in to the exterior and interior of houses, particularly to older listed buildings, as a waterproof barrier protecting the structure of the building. The breathable properties of lime render stop moisture build-up within the house, preventing damp issues.
Sand, water and lime form the main ingredients of lime renders, with local variants, on the proportion that these take, cement is often used in the mixture to increase drying time (to prevent cracking) and make the render harder. Though lime is breathable one of the disadvantages of lime is its “softer” than cement. Adding cement into the mix makes the render harder but sacrifices the breathability of lime to a degree. If cement is used, we recommend using “snowcrete” or a white-dyed cement as it avoids giving the cement a greyish/cement look.
Though horse hair was traditionally mixed in with lime mixes, these days polyprolene/flock fibres are used as a slightly cheaper option (but equally sufficient as horse hair!). This helps the lime render bind and set as a single unit, preventing cracks in the render. Animal hair is still sometimes used if a homeowner wants to stay true to the tradition of the building
After rendering- particularly on external walls- hessian is often dropped over the lime to prevent the lime render from drying too quickly and causing cracks.
Sand choice is often important when mixing lime render, with brown coarse sand the preferred choice for the undercoat/foundation layer and finer brown sand used on the top layer.
To make sure that everything is blended well, the render mix’s components should be mixed well in a cement mixer or via a paddle mixer. Use a trowel to apply the mixture in thin, even layers. To achieve a nice finish, a finishing wash or topcoat can be softly applied with a sponge, trowel, or brush.
We never make the rendering mix stronger than the material to be rendered is a straightforward rule to follow. The render will fracture as a result of the wall’s inevitable contraction and expansion, which will cause the wall to slide. The use of too much cement also contributes to render damage by causing it to shrink back and break.
Lime render is often used on listed buildings, as it is a traditional building material, used for hundreds of years on British and Cornish homes. If your listed building is in need of a new coat of lime render, get in contact with us to sort that out today!