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Render Crack Repair Calculator — Assessment & Material Quantities
Assess crack width, select the correct repair method, and calculate material quantities by repair type.
Average length of each crack
Number of cracks to repair
Costs render bags only — filler, mesh and bonding agent are not priced.
15% standard for repair work
Depth of the render patch — mainly relevant to medium/wide/structural patches. Fine cracks take a thin reinforcing skim, so depth has little effect there.
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How We Calculate This
This calculator helps you assess render cracks by width, recommends the appropriate repair method, and calculates material quantities. The repair method escalates with crack severity — from simple flexible filler for hairline cracks to full patch replacement for structural cracks.
Repair method selection
- Hairline (< 0.5mm): Flexible exterior filler only
- Fine (0.5-2mm): Filler + 150mm mesh tape patch
- Medium (2-5mm): Raked out, bonding agent, 300mm mesh, render patch
- Wide (5-10mm): Cut back 150mm each side, full mesh, 3-coat render
- Wide / possible structural (> 10mm): Hack off 300mm+ each side, check substrate, full render system
How wide is ‘structural’?
Crack-width severity here follows BRE Digest 251(‘Assessment of damage in low-rise buildings’): cracks up to about 5mm are generally aesthetic (Categories 0-2); 5-15mm is a serviceability concern that needs opening up and patching (Category 3); 15-25mm (Category 4) may need sections of wall replaced; and cracks over about 25mm (Category 5) point to structural movement and possible rebuilding. A crack over 10mm is not automatically structural — but it is worth having the substrate and any movement assessed by a structural engineer before re-rendering.
Material calculation
Material = Total crack length × Rate per metre × Wastage factor
Each repair method uses a different material consumption rate per linear metre of crack. These per-metre rates are indicative trade estimates for planning — there is no UK standard that fixes kg of render per metre of crack — set to reconcile with manufacturer consumption data (repair mortar ≈ 1.6 kg/m² per mm of thickness, e.g. Isomat Durocret-Plus ≈ 16 kg/m² per 10mm; SBR bonding agent ≈ 3-4 m²/L). For example, a medium crack repair works out at roughly 1.5 kg of render per metre over a 300mm-wide patch, plus about 0.3 m² of mesh and 0.05 L of bonding agent. Always confirm against the Technical Data Sheet for the products you buy.
Filler is converted from mass to 300ml cartridges using a filler density of about 1.5 kg/L (typical for filled flexible exterior fillers), so each tube is treated as holding roughly 0.45 kg rather than 0.3 kg.
Frequently Asked Questions
BRE Digest 251 grades cracking by width: under about 5mm is generally aesthetic, 5-15mm is a serviceability issue (needs opening up and patching), 15-25mm may need sections of wall rebuilt, and over about 25mm points to structural movement. So hairline (< 0.5mm) and fine (0.5-2mm) cracks are usually cosmetic — caused by shrinkage, thermal movement, or minor settlement. Cracks wider than 5mm, cracks that follow a stepped pattern along mortar joints, or cracks that continue through the substrate may indicate movement. If you can see daylight through a crack, the crack is wider at one end, or it is wider than about 10mm, consult a structural engineer before repair.
For hairline and fine cracks, use a flexible exterior filler (such as Everbuild Decorators Flexible Filler or Toupret Fill-Flex) that can accommodate slight movement. Do NOT use rigid fillers (like Polyfilla) on external render — they will crack again. For wider cracks (2mm+), use a polymer-modified render repair mortar (for example a Sika MonoTop cementitious repair mortar) with fibreglass mesh reinforcement rather than filler alone.
Fibreglass mesh bridges the crack and distributes stress across a wider area, preventing the crack from reappearing through the repair. Without mesh, even a well-filled crack will typically re-crack along the same line as the building moves with temperature changes. Use minimum 150mm wide mesh for fine cracks, 300mm for medium, and full-area mesh for wide or structural repairs.
For wide cracks (5-10mm), cut back the render at least 150mm each side of the crack to sound, well-bonded material. For structural repairs (10mm+), cut back a minimum of 300mm each side. Always cut clean edges (use an angle grinder with a diamond blade) rather than feathering — a clean edge gives a better bond for the repair. Sound the surrounding render with a hammer to check for hollow areas. Once the patch area is cut back, use our rendering calculator to get the render mix ratio and material quantities for the replacement coats.
Matching texture is the biggest challenge in render repair. For smooth renders, use the same product type and trowel technique. For textured renders (roughcast, pebbledash), try to source the same aggregate. New render will always look lighter initially — it takes 3-12 months to weather to match. For a closer colour match, consider applying a render paint to the entire elevation after repair.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.