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

Cracking in the Render

Cracks are the #1 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.