Lime renders, plasters, washes and tadelakt

Lime is a historic building material that was used in many parts of the world as a traditional mortar, render, plaster and paint. It is formed by burning limestone or shells in a kiln, to create highly reactive burnt lime, or quicklime. The quicklime is then slaked with water to create lime putty, hydrated lime powder, or hydraulic lime powder.

Pure natural air limes set or cure slowly with exposure to the atmosphere, whereas hydraulic limes can set quickly with water. Pozzolans can be added to natural air limes to create hydraulic lime. This is what the Romans did, creating “Roman cement”. Portland cement is another product that does this. Portland cement is a very specific type of hydraulic lime, that has some advantages but also many disadvantages to traditional lime.

External lime renders can provide water resistant finishes when directly applied to protected natural wall systems, such as LEM, straw bale, hempcrete, cob and adobe. According to the NZ Earthbuilding standard NZS 4299-2020, and best practice, this protection can take the form of big eaves or full verandahs, depending on the level of exposure to wind driven rain for the site. A sacrificial, multi coat lime wash finish can be applied over the top of the render, pigmented to taste and reapplied as required. Traditionally in the UK this lime wash was reapplied yearly in Spring.

Lime plasters can be used internally, but in this situation clay plasters are more suited. Although lime will help regulate humidity, clay is much better at this. Clay also has lower embodied energy and is nicer to the touch.

Tadelakt is a shinny water repellant finish to lime plaster that was traditionally used in Moroccan bathing room interiors. The lime plaster is compressed and polished with a hard stone and olive oil soap, in a very labour intensive process.

Lime use waned in the early 20th century, and by mid century it had all but been replaced with Portland cement. Although these two materials are similar in some regards, traditional lime has many benefits.

Benefits include

● Lower firing temperature so less embodied energy than Portland cement
● Vapour permeable or breathable
● Softer and more flexible, so you don’t need control joints unless a control joint in the substrate is being bridged
● Self healing i.e. small cracks will be filled when it rains

Lime renders and plasters are covered in the Earth Building Standards.

Points to consider

● Safety precautions must be taken as lime is highly alkaline, and can cause serious injury. Personal protective equipment (PPE) must be worn by all individuals who come in contact with lime
● Bagged lime should be obtained fresh from the plant as they are bagging it, to avoid the curing process beginning in the bag
● All fixings and metal exposed to lime to be stainless steel
● Eaves/ verandahs are required as the primary weather protection for durability
● Lime render can be more expensive than some types of cladding
● Only apply render, plaster or wash to dry walls, although the substrate does need to be misted down first
● Must cure slowly in a moist, protected environment, avoid direct sun, freezing temperatures and wind as it cures
● Needs a good key. This varies by substrate
● May need plastic mesh in the render
● Plaster needs to mature for at least 3 months before use
● Lime is anti-fungal and anti-microbial
● Pigment can be added for aesthetic effect
● If applying a mineral silicate paint, the lime render must be fully carbonated first
● Use well graded sand, and animal or plant fibre in renders and plasters
● Lime render needs to be 25-30mm thick, and applied in 3 coats according to the NZ Earthbuilding standard NZS 4299-2020

Lime renders, plasters, washes and tadelakt

Benefits include

Points to consider

Further reading