Lime Rendering

Breathable external render for solid walls, specified to the exposure rather than to habit.

External render is the most demanding lime work there is. It takes the full weather, it has to shed water while remaining vapour open, and it has a narrow season in which it can be applied safely. Getting the binder wrong here does more damage than anywhere else on a building, because the material it damages is the brick or stone underneath.

Three photographs from external render work, before and after.

Overview

A render coat on a solid wall has two jobs that pull against each other. It has to shed the majority of driving rain, and it has to let whatever moisture does get into the wall evaporate back out. Modern impermeable renders do the first job well and the second not at all, which on a cavity wall is acceptable and on a solid wall is not.

Lime render does both. It is not waterproof, and it is not meant to be. It slows water entry, holds some of it in its pore structure, and releases it back to the air when conditions allow. The wall stays within a moisture range it can tolerate rather than being sealed at whatever moisture content it happened to hold on the day the render went on.

It is also sacrificial by design. Lime render is weaker than the masonry it protects, so where something has to weather, decay or crack, it is the render rather than the wall. Render can be repaired. Spalled brick cannot.

Why Cement Render Fails on Old Walls

Cement render on soft Victorian stock brick or on rubble stone is one of the most damaging things done to British buildings in the twentieth century, and it is still being applied.

The tell-tale is usually sound rather than sight: tapping a cement-rendered elevation and hearing hollowness across large areas means the bond has already gone and water is moving freely behind it.

Removing failed cement render is a delicate job in itself. It is bonded harder than the brick, and careless removal takes brick faces with it. It is worth budgeting for properly rather than treating it as a stripping exercise.

Specifying the Render

Binder Choice and Exposure

External render is the case where a hydraulic set genuinely earns its place. A non-hydraulic lime putty render can be used externally in sheltered situations by an experienced hand, but it carbonates slowly and is vulnerable during the first winter. Most external work in London uses a natural hydraulic lime, and the question becomes which grade.

The temptation is always to go harder. A higher NHL grade sets faster, is less anxious in poor weather and feels more robust on the trowel. It is also less permeable, stronger and more brittle, and specifying it on a soft substrate reproduces the cement failure mode in slower motion.

The grade should be matched to exposure and to the strength of the background, not chosen for convenience. A sheltered London rear elevation in soft stock brick and an exposed gable taking driving rain are not the same specification. This is precisely the decision our binder paper was written to inform:

The Strength Gradient

The governing principle in all lime work, and the one most often broken, is that each layer should be no stronger than the one beneath it.

The masonry is the strongest element. The base coat is weaker than the masonry. The next coat is weaker again. The finish is weakest of all. Get that order right and stresses release harmlessly outward through the sacrificial layers. Get it inverted, with a hard finish over a soft base, and the assembly delaminates.

Coats and Build-Up

1. Preparation. Old render removed, joints raked out to provide a key, defective bricks cut out and replaced, the background dampened down to control suction. Suction control matters more than any other single step: a dry, thirsty background pulls water out of the render before it can cure and produces a weak, dusty coat.
2. Base coat. Pushed hard into the joints to key mechanically. Scratched to receive the next coat.
3. Straightening coat. Brings the elevation to plane. Thickness kept sensible, since thick coats shrink more.
4. Finish coat. Thin, weaker than what it sits on, worked to the chosen texture.

Each coat needs time and protection before the next. That is the fundamental difference from a cement programme, and it is why lime rendering cannot be compressed into a fortnight because a scaffold hire is running.

The Mixes and Systems We Use

“Lime render” covers a wide range of specifications that behave differently on the wall, so here is what we actually reach for and what governs the choice.

Why the Insulating Route Is Worth Its Published Numbers

An insulating render is easy to oversell, so judge it on figures. Diathonite Evolution, the system we use most often in this role, is documented at a thermal conductivity of 0.045 W/m·K, giving roughly 0.222 m²K/W per 10mm of thickness. Its compressive strength is around 2.95 N/mm², which puts it in class CS II, and its adhesion to brick is specified at 0.10 N/mm² or better with cohesive failure in the mortar rather than at the interface. It is class A1 for reaction to fire.

The number that matters most on an old wall, though, is the vapour resistance factor of µ 4, combined with a porosity of about 71 per cent. That is a very open material. It is the reason a 40mm insulating coat can go onto solid masonry without setting up the trapped-moisture problem that a sealed system would, and it is the number to ask any supplier for when a render is described only as breathable.

Practical Limits That Shape the Programme

• Thickness per pass. 40mm maximum on walls, 20mm on ceilings and soffits, with a 40mm total on ceilings and mesh mandatory there.
• Mesh. Required from 60mm total thickness, and always over panels, timber, plasterboard and any substrate expected to move. At structural beams and columns it runs at least 150mm onto both sides.
• Recoat window. The previous coat needs to be firm to the touch and visibly lighter, typically 12 to 24 hours, and dampened again before the next goes on.
• Application conditions. Between 5°C and 30°C, and not above 70 per cent relative humidity. Corner beads bedded in the render itself rather than in cement, so the bead does not become a thermal bridge.
• Do not over-compress. Ruling off is to bring the coat to plane, not to polish it. Closing the pores defeats the material.

Full layer-by-layer assemblies, including where these renders sit in a complete build-up, are on the build-ups page.

The Gates We Will Not Pass

On a south London commercial elevation we wrote down four conditions and would not start the render until all four were true: lintels installed and inspected, masonry stable and not shedding, all beads and profiles on site, and an acceptable weather window.

A render applied over unstable masonry looks correct on handover and fails within a season or two, and by then nobody can tell whether the render or the wall was at fault. Holding the start date is cheaper than that argument.

Read the full case study.

Finishes and Decoration

Texture is a specification decision as much as an aesthetic one. A floated finish reads flat and even and suits most London terraces. A wood-floated or sponged finish gives an open texture that dries fast. A thrown finish, harling, is exceptionally durable in exposed positions because it sheds water without relying on a smooth plane.

Decoration must be at least as permeable as the render. Limewash is the traditional and best-matched option: it bonds chemically with the lime beneath, it is repairable, and it weathers rather than peels. Mineral and silicate paints are a reasonable alternative where a specific colour or a longer maintenance interval is wanted.

Masonry paint is not an alternative. A plastic film over a lime render defeats the entire specification, and once it is on, removing it without damaging the render is difficult.

We carry out limewash and distemper decorating as well, so render and decoration can be handled as one programme rather than handed over mid-cure to a decorator who may not know the constraints.

Season, Weather and Curing

Lime render has a working season, and pretending otherwise is how failures are built in.

Manufactured systems put numbers on this. An insulating lime render is quoted at 10 to 15 days to dry at 23°C and 50 per cent relative humidity, which is a mild indoor condition rather than a London elevation in October, and thickness, cold or damp all extend it. In hot or windy weather the opposite problem applies and the surface has to be misted, two or three times a day for the first two or three days, and roughly every two hours once the temperature is above 28°C, or it shrinks and cracks.

That aftercare is a real line in the programme, not a courtesy. On our own jobs it means someone returning to the scaffold specifically to wet a wall that looks finished, and it means air movers and ducting left running on internal work when we leave site.

We will decline to start an external render in the wrong conditions. It is a short conversation that avoids a long problem.

Cracking and Maintenance

Fine shrinkage cracking in the first months is normal, and much of it closes as carbonation proceeds. Lime's limited self-healing capacity is real and is one of the practical arguments for the material.

What matters is distinguishing that from cracking that indicates a problem: cracks tracking a structural line, cracks that open and close seasonally at a widening rate, cracks concentrated at a junction where water is getting in. The diagnostic framework is set out in full in our crack paper.

Lime render is a maintained surface rather than a fit-and-forget one. An annual look at the elevation, particularly around openings, at the base and where rainwater goods discharge, catches almost everything early. Limewash is refreshed periodically rather than stripped, which is a considerably cheaper maintenance cycle than repainting a failing masonry paint.

Cost and Scope

External render is priced on survey. The variables are elevation area, access and scaffold, whether failed cement render has to come off, the condition of the masonry underneath, the number of coats and the finish specified. Removal of existing cement render is frequently a significant share of the total and is worth establishing early.

Removing cement render does not always mean re-rendering. On a building whose brickwork was never intended to be covered, cleaning and repointing the exposed face is often the better outcome, and usually the cheaper one. We will say so if that is what the elevation wants.

Common Questions

Why does cement render fail on an old solid wall?

The giveaway is often sound rather than sight: tapping a cement-rendered wall and hearing hollowness across a large area means the bond has already failed and water is moving freely behind it, often before any cracking is visible. The mechanism, in short: cement is far less permeable than the masonry it is protecting, so moisture concentrates at the interface and eventually spalls the brick face, which cannot be undone.

Which grade of natural hydraulic lime should be specified?

There is no single right grade. It is set by exposure and the strength of the substrate together, never picked for convenience. A sheltered rear elevation in soft stock brick and an exposed gable taking driving rain are different specifications on the same street. The chemistry behind why a harder grade can damage a soft wall is set out in our binder paper.

Can lime render be applied at any time of year?

No, and the risk runs both ways. Frost on green render has no remedial fix, it simply has to come off; at the other extreme, direct sun and wind dry it too fast and cause shrinkage cracking. In London that leaves roughly spring to early autumn as the working season, with scaffold sheeting up through the whole cure rather than just the application.

Further Reading

• Build-ups, the specified assemblies and the materials named in full
• Hydraulic vs Non-Hydraulic Lime in Retrofit Projects
• Why Breathable Walls Crack
• Brickwork restoration, the alternative to re-rendering
• Wall remediation, where the render is the source of a damp problem
• Lime plastering, for internal work
• All technical white papers

Guidance on lime render and the repair of traditional buildings is published by Historic England, the Society for the Protection of Ancient Buildings and the Building Limes Forum. Our white papers carry full references.