Lime render is an external wall coating made from lime (natural hydraulic lime or lime putty) mixed with sharp sand and water, applied in two or three coats over masonry or timber lath to provide weatherproofing and a finished surface. Unlike cement render, lime render is breathable - it allows moisture vapour to pass through the wall and evaporate - and flexible, so it accommodates the natural movement of old buildings without cracking and falling off. It has been used on buildings for centuries and remains the correct material for historic and traditional masonry.

Why should you use lime render instead of cement render on an old building?

Pre-1920 buildings were designed to manage moisture by allowing it to move through the fabric and evaporate - a breathable system. Cement render creates a hard, impermeable skin that traps moisture in the wall behind it. As moisture builds up, it causes frost damage, salt crystallisation, and spalling of the masonry behind the render, and the rigid cement render eventually cracks, debonds in large sheets, and falls off. Lime render, being softer and breathable, allows the wall to breathe, accommodates movement without cracking, and maintains a stable moisture equilibrium in the wall.

How many coats of lime render are applied?

Lime render is applied in two or three coats. A three-coat system is traditional: a coarse scratch coat (10-15mm) applied to the primed substrate and keyed with a comb; a second float coat (8-10mm) to level and smooth; and a thin finishing coat (3-5mm) in a finer mix. Each coat must be allowed to partially set and carbonate before the next is applied. The scratch coat must be kept damp and cured slowly - rapid drying causes shrinkage cracking. Rushing between coats is the most common cause of lime render failure.

Heritage & Conservation noun

Lime Render

/ laɪm ˈrɛndə /

Also known as: traditional render, lime roughcast, lime plaster (external)

Definition

Lime render is an external wall coating made from lime (natural hydraulic lime or lime putty) mixed with sharp sand and water, applied in two or three coats to provide weatherproofing and a finished surface. It is the traditional external finish for old stone, cob, and softer brick buildings. Unlike cement render, lime render is breathable - allowing moisture to migrate through the wall and evaporate - and flexible, accommodating the natural movement of traditional buildings without cracking.

In practice

Lime render is typically applied in three coats. The scratch coat (10-15mm of coarse lime and sharp sand) is applied directly to the primed or dampened substrate and keyed before setting. The float coat (8-10mm) brings the surface to a true, level plane. The finish coat (3-5mm of finer mix) provides the final surface texture - smooth, sand-faced, roughcast, or pebbledash depending on the building's tradition.

Each coat must be kept damp and cured slowly for several days - rapid drying from sun, wind, or frost causes shrinkage cracking that is impossible to repair invisibly. The mix for each coat is typically 1 part NHL3.5 to 2.5 parts sharp sand. The aggregate source and colour significantly affect the appearance of the finished render and should match the original where the work is on a listed building or in a conservation area.

Building Regulations

Relevant PartPart C - Resistance to weather and ground moisture

Lime standardBS EN 459 - Building lime (NHL classes)

Conservation areasRender colour and finish may be controlled by LPA

Approved Document C requires external walls to resist weather. Lime render provides this protection while remaining compatible with traditional breathable construction. Applying cement render over lime render, or to a traditionally constructed wall, is a common and damaging error. On listed buildings, the render specification typically requires agreement with the local planning authority's conservation officer. Lime render is covered in detail in Historic England's technical guidance on external wall coatings.

Full Building Regulations guidance

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