Why External Rendering Fails (And What You Can Do About It)

Cracking in the Render

Cracks are the number one visible sign that something’s wrong with your rendering system.
Not all cracks mean failure — but most cracks, left untreated, will grow and let in water.

Causes of Render Cracks:

• Building movement or settlement
• Thermal expansion and contraction
• Inadequate mesh/reinforcement during application
• Drying too fast due to hot weather

Types of Cracks:

• Hairline cracks: superficial but allow water in
• Spider cracks: often from fast drying or weak mix
• Large structural cracks: may indicate deeper substrate issues

Fix or Replace?

• Hairline: flexible filler + sealant (temporary)
• Spreading or deep cracks: professional render repair or full re-render

Water Ingress and Damp Behind Render

Render is meant to repel water. But once cracked or poorly sealed, it can trap moisture inside your walls, leading to:

• Mould growth inside the home
• Damp patches on internal plaster
• Rotting timber or insulation underneath

Common Causes:

• Blocked or missing weep vents
• Gaps around windows or wall joints
• Render applied in wet conditions

What You Can Do:

• Identify and fix any sources of water ingress
• Replace damaged render sections
• Re-render with breathable systems (e.g., silicone-based)

The Energy Saving Trust explains how breathable external render systems can improve insulation while preventing trapped moisture — ideal for solid wall properties. 

Algae, Staining and Discolouration

Green or black staining is very common on north-facing walls.  Algae loves damp, shaded surfaces — and poorly maintained render is the perfect host.

Causes:

• Poor drainage or splashback from the ground
• Lack of overhang on the roofline
• Blocked gutters

Solutions:

• Softwash or pressure wash render (if safe for material)
• Treat with biocide cleaner
• Apply self-cleaning topcoat or re-render with algae-resistant finish

Peeling, Flaking or Powdery Surfaces

Render that’s coming off in flakes or chalking when touched has likely degraded due to:

• UV exposure breaking down the top layer
• Moisture trapped behind the surface
• Incompatible paint or coatings applied

Signs of serious failure:

• Topcoat detaching from the base layer
• Soft, crumbly texture when rubbed
• Pieces falling away after heavy rain

Fix or Re-render?

• Small sections may be patched
• Full wall failure = time to re-render using a modern system (e.g., monocouche or silicone)

Poor Application by Previous Contractors

Not all renderers are equal — and sadly, poor workmanship is a leading cause of early render failure.

Common Mistakes:

• Thin, uneven application
• No mesh used to reinforce edges or stress points
• Rendering done in freezing, damp, or hot conditions
• Wrong render type for the substrate

 Red Flags to Look For:

• Visible trowel marks or texture inconsistency
• Uneven colour fading or patches
• No expansion joints on long walls

What to Do:

• Get a proper inspection (we do free site visits)
• Strip back poorly bonded render
• Re-render using correct method + high-quality materials

Movement in the Wall or Substrate

Sometimes it’s not the render’s fault — the wall underneath may be moving or deteriorating.

This is common in:

• Old stone properties
• Homes built on clay or shifting soil
• Houses with historic damp damage

What to Look For:

• Long diagonal cracks from corners of windows or doors
• Bulging or concave render sections
• Separation between render and skirting

If the substrate moves, even the best render won’t hold. Fixing the wall structure comes first then re-rendering can be done properly.