How do I know if a render crack is structural?

Hairline cracks (< 0.5mm) and fine cracks (0.5-2mm) 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 structural movement. If you can see daylight through a crack, or the crack is wider at one end, consult a structural engineer before repair.

What is the best filler for render cracks?

For hairline and fine cracks, use a flexible exterior filler (such as Everbuild Flexible Filler or Sika MonoTop) 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 with fibreglass mesh reinforcement rather than filler alone.

Why do I need mesh over a render crack repair?

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.

How wide should I cut back render for a patch repair?

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.

How do I match the repair to the existing render finish?

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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Render Crack Repair Calculator — Assessment & Material Quantities

Assess crack width, select the correct repair method, and calculate material quantities by repair type.

Average Crack Length

Average length of each crack

Number of Cracks

Number of cracks to repair

Crack Width Category

Determines repair method and materials

Render Price Per 25kg Bag (optional)

For render patch repairs only

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
Structural (> 10mm): Hack off 300mm+ each side, full render system

Material calculation

Material = Total crack length × Rate per metre × Wastage factor

Each repair method has different material consumption rates per linear metre of crack. For example, a medium crack repair uses approximately 1.5 kg of render per metre (for a 300mm wide patch at 15mm depth), plus 0.3 m² of mesh and 0.05 L of bonding agent.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.