Uses of Lime Based Products in Construction: Lime Render
Lime has been used in construction for millennia, with evidence of lime mortar being used as early as 6,500 BC. The word lime comes from Old English, where it referred to a sticky substance. Over time, the meaning shifted to include mortar, cement, and gluten. Lime is related to Latin words like limus (slime) and linere (to smear).
Lime was an important part of Roman construction, being used in everything from concrete to plaster. In the Middle Ages, lime was used to whitewash walls and ceilings, a practice that continues to this day. Lime mortar is especially well-suited for use in harsh weather conditions, like those found in the UK.
Today, lime is still an important part of construction, particularly in the conservation and restoration of heritage buildings. It is also used in agriculture, as a soil stabiliser, and in water treatment. With its many uses and centuries-long history, lime is truly a versatile and essential material.
Lime Render for Construction
What is Lime Render?
Lime render is a type of plaster made from lime and sand. It is commonly used on old buildings as it is breathable and can accommodate movement, unlike harder, more modern materials. External lime render also helps regulate humidity by managing moisture levels.
Materials that help mortars set more quickly include brick dust and ash. These are known as “pozzolans”, which means they produce strong bonds between lime plaster and aggregate particles in the mix.
Common Uses of Lime RendeR
- Aesthetically pleasing: Lime render creates a beautiful, whitewash aesthetic finish. Sharper corners, defined edges and a uniform appearance are just some of the key benefits for those looking to purchase a stylish home.
- Protection from the elements: Rain or snow, this external coating will thrive and survive. Lime render offers a waterproof layer and water-resistant properties. Whilst its breathability allows it to remain strong against potential weather damage.
- High vapour permeability or absorption of moisture: Hygroscopic allows lime render to regulate humidity, temperature and manage moisture levels. The higher the permeability of a material, the faster that water vapour will pass through the substance. If vapour can pass through easily, the structure will not be affected by trapped moisture.
- Gains strength over time through carbonating.
- Environmentally friendly: Lime render is manufactured at a lower carbon cost to the environment. The material reabsorbs carbon dioxide and creates less embodied carbon than the process used to manufacture cement.
- Lime render can accommodate subtle movements (without cracking).
Application, Materials and Tools
Lime Render Preparation
Preparation is very important whenever you are working with lime. Cleaning the surface for the render to be applied is essential, making sure it is free of all dust and debris.
The surface must be made slightly rough so that there is a good ‘key’ for the render first coat to adhere to.
Lime Render, Things you Should Note:
Lime plaster needs some level of moisture present, otherwise it can dry out very quickly making it susceptible to failing. During the curing process, using a garden sprayer to dampen the wall can do the trick. If the lime plaster dries almost instantly, it will likely need more moisture. Please be careful to not over-saturate the wall.
Application of Lime Render
The number of coats of lime render varies depending on the requirement. Generally, three coats is best practice, however, in vernacular or historic structures it is common to find two coats or even a single coat.
The scratch coat or render coat is the first coat applied to masonry or bricks in a three coat system. This has a thickness of about 10mm and is applied in a single application.
A steel trowel can be used to apply the render, or a harling trowel can be used to put it on the wall and then flatten it with the steel trowel. When the plaster has firmed but not hardened, it is keyed or scratched up to create a key for the subsequent coatings.
The keying is done using a lath scratcher or other toothed device. Please take care not to cut through the plaster coat all the way to the background. Keying up will often form a diamond shape with a spacing of about 30mm. Before applying additional layers of plaster, allow this coat to cure for at least 72 hours.
The first coat should be checked for shrinkage cracks before the second coat is applied. Before applying additional layers, these should be filled with plaster.
It’s also a good idea to brush down the initial layer to remove any dust that has flown onto the surface. Following this, damp down the first coat to ensure that the second coat is applied to a damp but not wet surface.
The second layer, referred to as the floating coat, is straightened to create a smooth and even surface. The surface of this coat is scrubbed up with a wood or polyurethane float after it has been levelled.
Scouring is used to consolidate the plaster and prevent it from shrinking. In most cases, this procedure must be repeated at least twice.
Before applying the final coat, any minor holes should be plugged during the scouring process. This coat should be allowed to dry and firm for 72 hours.
The last layer is applied after dampening down the floating coat once more. This coat is known as the setting coat and is carried out in two layers, coat on coat, with the trowel. This coating can be applied in a skimming-like manner.
This material should be sufficiently saturated to enable a lengthy and even distribution. After the setting has been placed, it is scoured in the same way that the floating was to prevent shrinkage. The surface is then left with a scratched up rendering finish or polished fine with a steel trowel.
About EcoRight
EcoRight supply mortars, plasters and renders throughout the UK construction industry. EcoRight products and services are equally suitable for large scale construction projects or local craftsmen working on individual or small projects. EcoRight’s expert team, who have over 100 years combined experience in lime based building products and conservation work, can also provide technical design support, prepare NBS specifications and provide CPD presentations.
Give us a call on 0845 873 3888 and work with one of the UKs leading suppliers.
