How to Repair Cracks in Render – Step-by-Step UK Guide

Noticing cracks in your external render? You're not alone. Cracks in render are one of the most common problems faced by UK homeowners – especially in older properties or homes exposed to harsh weather. The good news? Most cracks can be repaired easily if caught early.

In this guide, our expert George Edward explains why render cracks, the types of cracks, how to repair them properly, and when it’s best to call in a professional.

Why Does Render Crack?

Cracks in render usually occur due to:

• Movement in the wall (settling, expansion, vibrations)

• Poor initial application (wrong mix, no mesh, weather problems)

• Water ingress causing freeze-thaw cycles

• Old render systems that are brittle or outdated

• Structural issues (less common, but serious)

George explains: “Not all cracks mean serious damage – but ignore them, and they’ll let in moisture, which leads to bigger problems.”

Crack Decoder: George’s Identification Guide

🔍 George’s Trick: "Press Blu-Tack into cracks. If it comes out clean, it’s active movement. If dirty, it’s stable."

Types of Render Cracks

Understanding the crack helps determine the right repair:

How to Repair Render Cracks – Step-by-Step

Tools & Materials Needed:

• Stanley knife or crack chaser tool

• Wire brush

• Exterior filler or render repair mortar

• Render mesh (for wider cracks)

• Trowel, float, or putty knife

• Sandpaper

• Primer/sealer

• Paint/render finish (to match)

Method 1: Repair Hairline or Fine Cracks

Ideal for: Hairline cracks < 1mm

1. Clean the Area
   Use a brush to remove dirt, loose render or algae.

2. Widen Slightly (Optional)
   Use a knife to gently open the crack for better filler grip.

3. Apply Flexible Filler
   Use exterior-grade filler (masonry filler or silicone-based).

4. Smooth & Leave to Dry
   Follow product instructions. Usually dries in 2–4 hours.

5. Sand & Paint
   Use fine sandpaper and match your paint/render finish.

George’s Tip: “Don’t skip the prep. Even a small crack can return if dust or algae is left behind.”

Method 2: Repair Larger Cracks (2mm – 5mm)

Ideal for: Bigger or deeper cracks that may reopen if not reinforced.

1. Chase the Crack
   Use a crack chaser or angle grinder to widen it slightly (5–10mm deep).

2. Brush Out Dust
   Use a wire brush and rinse with clean water.

3. Apply Bonding Primer
   Helps new render adhere to the surface.

4. Use Render Repair Mortar
   Apply with a trowel and press in firmly. Blend into surrounding area.

5. Embed Mesh (Optional)
   On long cracks, press mesh into the wet mortar to prevent movement.

6. Finish Smoothly
   Use a sponge float or trowel to blend. Let it cure fully (24–48 hours).

7. Paint or Re-render Area
   Use matching exterior paint or render finish.

George’s Horror Story: The £42,000 Repair

"A London client ignored diagonal cracks above windows. When the lintel failed, it took out two floors of render. Structural repairs cost £37k before we even touched the façade. Moral: Small cracks scream big warnings."

Method 3: Structural or Recurring Cracks

If you notice:

• Wide cracks (>5mm)

• Cracks returning after repair

• Cracks near windows or corners

• Stepped or diagonal cracks

You should call a qualified builder or structural engineer.

“We’ve had clients paint over deep cracks only to find internal damp patches six months later. Always check the source if you’re unsure.” – George

When to Re-render Instead of Repair?

If your wall has:

• Dozens of cracks

• Aged or blown render

• Mismatched patch jobs

• Damp, moss, or bulging sections

…it’s probably time to strip and re-render the whole wall.

George says:

“Sometimes it's better to bite the bullet and re-render properly. It adds curb appeal and protects the home for 25+ years.”

Render Crack Repair Products (UK)

Some trusted products George recommends:

• Toupret Masonry Repair Filler (for small cracks)

• EWI Pro Render Repair Mortar

• Weber Rend Aid (as a bonding primer)

• K Rend Scratch Render Repair Kit

• Silicone-based sealants (for flexible joints)

Final Thoughts

Cracks in render are common – but they don’t have to mean disaster. With the right tools, technique, and materials, you can repair them quickly and affordably.

But remember: good prep + quality filler = long-lasting repair. If in doubt, contact a rendering expert like George at HouseRenderPro.

"Render cracks are like toothaches – ignoring them gets exponentially more painful. My toolkit has £3,000 of diagnostic gear, but your best tool is vigilance. Spot cracks early, fix them properly, and sleep soundly. At HouseRenderPro, we warranty repairs for 10 years because we hunt the root cause, not just the symptom."