External render protects the substrate beneath from wind-driven rain, but once it starts failing the damage can accelerate quickly. Water gets behind cracked or blown render, accelerates freeze-thaw spalling, and can drive damp into the wall before any visible signs appear inside. Catching problems early — and choosing the right repair approach — is considerably cheaper than waiting until large sections need hacking off.
The correct repair method depends entirely on the type of render already on the wall. Applying a hard cement patch to a lime render building, or a lime patch to a polymer system, will create differential movement and fail within a few years.
Types of External Render
Understanding what is already on the wall is the essential first step before any repair.
Sand and cement render (typically a 3:1 or 4:1 sand:cement mix) has been the UK standard from the mid-twentieth century onwards. It is rigid and relatively impermeable, which suits modern cavity-wall construction but can be problematic on older solid-wall properties where it traps moisture.
Lime render uses natural hydraulic lime (NHL) as the binder. NHL 2 is the softest and most breathable, suited to very old or soft brick and stone; NHL 3.5 is a general-purpose choice for most pre-1919 solid-wall properties; NHL 5 approaches the hardness of weak cement and is used in exposed coastal or upland locations. Lime render allows walls to breathe, accommodates minor movement, and is the appropriate choice for any building constructed before cavity walls became standard (broadly pre-1920).
Monocouche (through-colour) render is a factory-blended single-coat product applied at 12–20mm thickness. It is pigmented throughout, so the colour is integral rather than painted on. Lifespan is typically 20–30 years. K-Rend, Parex, and Weber are common UK brand families, though generic polymer monocouche products are also widely available.
Silicone and acrylic renders are thin-coat systems (1.5–3mm) applied over a reinforcing mesh and basecoat. Silicone renders are hydrophobic, flexible, and highly vapour-permeable. They are now the dominant choice for new external wall insulation (EWI) systems and for refurbishing existing rendered facades. Acrylic renders are similar but slightly less breathable.
Common Render Failures
Crazing — a fine network of surface cracks, often forming a map or spider-web pattern — is caused by the render skin drying and shrinking too fast during application (low suction substrate, hot dry weather, or a mix that was too wet). Crazing cracks are typically less than 0.3mm wide. They rarely cause structural problems but can allow water ingress if deep.
Structural cracking — cracks wider than 1mm, diagonal cracks following mortar joints, or cracks that widen and narrow with the seasons — indicate differential movement between the render and the substrate, or in the substrate itself. These require investigation before repair; simply filling them will not hold.
Hollow or drummy render describes areas where the render has debonded from the wall behind. Tapping with a wooden mallet or the knuckle produces a hollow drumming sound rather than a solid thud. Drummy render may be visually intact but has lost adhesion and will eventually fall off in sheets.
Blown render is render that has fully delaminated, often in a visible bulge or with sections already fallen away. Frost is a common final cause, but the underlying failure is usually debonding triggered by moisture behind the render.
Staining — rust-coloured streaks, green algae growth, or tide-mark water staining — indicates either inadequate weathering details (missing or damaged bell beads, window reveals, abutments) or a render that has become porous over time.
How to Diagnose Render Problems
A systematic tap test covers the whole facade in sections, marking hollow areas with chalk. Hollow areas larger than roughly 0.5m² are generally not worth patching; the surrounding adhesion is usually marginal and the patch edges will crack as the remaining loose render continues to move.
Crack width is assessed with a crack gauge (a card template with graduated slots). The threshold between hairline and repairable is 0.3mm; cracks wider than 1mm should prompt investigation of the substrate and structure before any render repair begins.
Check window and door reveals, bellcast beads, and render-to-soffit junctions — these are where water most commonly gets behind the render. Missing or corroded movement beads, or render taken solid across movement joints, are common causes of cracking.
Causes of Render Failure
| Cause | Likely symptom | Typical location |
|---|---|---|
| Thermal movement (no movement joints) | Vertical or stepped cracks at 6–8m intervals | Large uninterrupted elevations |
| Sulphate attack (from brick) | Render pushed off, horizontal cracking | Near DPC level, under sills |
| Poor suction/key | Drummy render, wide areas of debonding | Whole elevations |
| Hard cement on soft brick (pre-1919 buildings) | Brick face spalling behind render | Victorian/Edwardian solid walls |
| Moisture ingress via failed details | Blown render, staining | Around windows, near ground level |
| Substrate movement (lintel failure, settlement) | Stepped diagonal cracks | Above openings |
Movement joints should be present at maximum 6-metre intervals on sand and cement renders on flat elevations, and at all re-entrant corners. Their absence is one of the most common causes of cracking on post-war rendered houses.
Repair Approaches
Patch repair is viable where hollow areas are small, isolated, and surrounded by well-bonded render. Hack out the defective area to a clean edge (undercut the perimeter slightly so the patch keys in), brush out dust, dampen the substrate, apply bonding agent or slurry if needed, and build up the patch in the same render type and mix as the original. Feathered edges fail; patch edges must be hard, clean, and well-bonded. Colour match on sand and cement is difficult once the original has weathered; monocouche patches are nearly invisible when matched correctly.
Full hack-off and rerender becomes economical when drummy areas cover more than 30–40% of the elevation, when the substrate needs repair or tanking, or when a change of render type is warranted. On pre-1919 properties where hard cement render is causing brick spalling, a full hack-off followed by lime render is the correct conservation approach. The substrate should be allowed to dry before applying lime render; NHL renders should not be applied in temperatures below 5°C or in direct strong sunlight.
Tanking and damp treatment may be needed before rerenderig if the substrate is contaminated with hygroscopic salts (white crystalline deposits on the masonry face indicate this). Render applied over salt-contaminated masonry will blow again within a few years.
Costs in 2026
| Render type | Typical coat thickness | Lifespan | Cost per m² (patch repair) | Cost per m² (full rerender, installed) | Suitability |
|---|---|---|---|---|---|
| Sand and cement (3:1) | 15–20mm (2 coats) | 15–25 years | £35–65 | £40–90 | Post-1920 cavity walls |
| Natural hydraulic lime (NHL 3.5) | 20–30mm (3 coats) | 25–50+ years | £55–80 | £60–120 | Pre-1919 solid walls, listed buildings |
| Monocouche / through-colour | 12–20mm (1 coat) | 20–30 years | £50–80 | £60–120 | Modern cavity walls, large elevations |
| Silicone thin-coat (on EWI basecoat) | 1.5–3mm | 20–25 years | £45–70 | £70–150 | EWI retrofit, flexible bases |
Scaffold allowance for a typical three-bedroom semi-detached (front elevation only): £600–1,400. Full perimeter scaffold: £1,800–2,500. Scaffold is usually the biggest single cost variable on a render project and can dwarf the material and labour costs on a small patch job — combining render work with other external works (fascia, gutters, windows) makes practical and financial sense.
Planning and Building Regulations
Render repairs to a matching specification and colour are generally permitted development and do not require planning permission. However, changing render colour on a property in a conservation area can require planning approval, as can removing render on a building where it forms part of the recognised character. Always check with your local planning authority before changing colour in a conservation area or when the property is curtilage-listed.
Render materials for external use should conform to BS EN 998-1 (specification for masonry mortars for rendering and plastering). For historic buildings, the Society for the Protection of Ancient Buildings (SPAB) and Historic England publish guidance on appropriate lime mortars that should be followed before specifying any repairs.
A structural engineer’s opinion is advisable when cracks are wider than 1mm or are associated with other signs of movement such as sticking doors, sloping floors, or cracking to internal plasterwork.
