Cracks in Rendered Walls: How to Spot the Difference Between Cosmetic and Structural Damage

Goal - Learn how to identify, repair, and repaint cracks in exterior render the right way. Discover when it’s just surface wear and when it could signal something deeper. If you’ve spotted cracks appearing on your exterior walls, this post is for you. Rendered surfaces naturally move and shift over time, especially in the UK with our constant cycle of British rain, sun, and frost. The key is knowing which cracks are harmless and which ones need proper attention before you grab the filler and paintbrush or panic and mix up a barrow full of render that you might not need.

Let’s break down how to tell the difference, how to repair each type, and how to get your walls looking flawless again: 1. Cosmetic vs Structural Cracks — What’s the Difference?

Surface-level (cosmetic) cracks: These are usually hairline or shallow cracks that appear in the outer layer of render. They’re often caused by minor movement in the building materials, paint, temperature changes, or simple age. Cosmetic cracks tend to:

• Be less than 2 mm wide

• Follow straight or random thin lines

• Not continue through the wall underneath

While they don’t affect the structure, they do let in moisture if ignored — which can lead to further damage down the line.

Example of a cosmetic crack

Structural cracks:

Structural cracking, on the other hand, points to movement in the actual wall or foundation behind the render. These cracks are:

• Wider than 2–3 mm

• Diagonal or stair-stepped in pattern

• Often appear around windows, doors, or corners

• Sometimes reappear even after filling

Example of a structural crack

If you can fit a coin in the gap, or if the crack keeps returning after repair, it’s worth getting it checked by a builder or structural specialist before decorating over it. 2. Dealing with Cosmetic Cracks: Fill and Prepare Properly

Once you’ve confirmed the damage is only surface-deep, you can move ahead with repairs:

• Clean and open up the crack — Use a scraper, angle grinder (Remember your PPE) or bolster chisel to widen it slightly so the filler can properly bond.

• Brush away dust and debris — A clean surface is essential for good adhesion.

• (OPTIONAL EXTRA STEP) - Use a sealer/adhesive or bonding agent like PVA or FEB - we find PVA works well and is the most cost efficient product to use. Simply mix 50/50 PVA - water in a bucket and then apply liberally with a brush to the scored cracks/damaged areas. Not strictly necessary but can help with adhesion and may even make stubborn reappearing cracks less likely.

• Apply exterior filler — Choose a flexible, weather-resistant product made for render or masonry. Apply in layers if it’s deep. If doing this in the heart of winter, ensure you're applying the filler between 10am-4pm - you do not want frost getting to it before it's had a chance to cure.

• Sand flush — Once fully dry, sand the repair smooth to blend it with the surrounding surface.

• Prime before painting — Always apply a stabilising primer or sealer over repaired areas. This prevents the paint from flashing (showing uneven sheen or colour where filler was used).

• 

  Filling in cracks - you can use a damp sponge to simulate the render finish of your property.

  Finish with quality exterior paint — We recommend using a breathable masonry or exterior wall paint, not strictly necessary unless your properties render includes lime but can help to prevent future reappearance of stubborn cracks because it will protect against weather but also allow trapped moisture to escape. 3. Addressing Structural Cracks: DON'T JUST FILL AND FORGET
  

  If cracks are wide or keep reappearing, filling won’t fix the cause. In these cases:

  
  

• Call in a professional (Like us) to assess movement or subsidence.

• Allow any structural work to settle before re-rendering.

• Once stable, have the affected areas raked out, repaired with mesh reinforcement, and re-rendered before painting.

  
  

  -Sometimes, whole areas of render will need to be removed and replaced in order to fix stubborn cracking - like this bungalow in Tiverton that we recently repaired & decorated.

Tiverton bungalow - structural crack identified, damaged render removed, dust cleared and sealed, ready for re-rendering.

Tiverton Bungalow - after repair

BOTTOM LINE - Trying to decorate over unstable surfaces = throwing your money and time away, the cracks will return every time. 4. Finishing Touches Matter

After repairs, your wall should be:

• Smooth or relevant to the render finish of your property (You can use a damp sponge on wet filler to get some surprisingly visually convincing results)

• Properly sealed and primed

• Painted with at least two full coats for lasting protection

  A properly repaired and repainted render not only looks new again, it also helps shield your home from damp and frost damage for years to come.

  
  

  Sound like a bit of a headache?

  Let us do it for you! If you’re in Devon, Somerset, or the surrounding areas, we offer full exterior surface repairs, rendering touch-ups, and painting services at very reasonable prices. Whether it’s house decorating, small hairline cracks or even full re-renders, every project gets the same level of attention and care.

  Head to our contact page and get in touch for a free quote!