Render Looking Tired? What Causes Cracks, Stains & Green Growth

If your property’s render is starting to crack, stain, or grow algae, you’re not alone. These are common problems, especially in older homes, and they’re not just cosmetic. Damaged or worn render can let water in, cause heat loss, and even lead to internal damp and mould. In this guide, we’ll explain what causes these issues, when to worry, and how professional repair or re-rendering can protect both your home’s appearance and structure for years to come.

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What Causes Render to Crack, Stain or Turn Green?

Render can crack, stain, or develop algae for several reasons. These are common issues that affect not only the appearance of your home but, if left unchecked, can also impact the protective function of your exterior walls. Below, we explain the most frequent causes, and what they might mean for your property. 

Cracks in Render

Render provides a decorative and protective layer to your home’s exterior. However, over time or due to specific conditions, cracks may appear. Here are the most common causes:

• Building movement: All buildings naturally shift and settle over time. This movement can place stress on rigid surfaces like render, leading to cracks. 
• Rapid drying: If render dries too quickly after application, often due to high temperatures or winds, it can shrink and crack.
• Moisture trapped behind render: When water becomes trapped between the render and the wall (usually due to poor sealing or water ingress), it can expand and cause the surface to crack or bulge.
• Brittle base layers: If the substance underneath the render isn’t stable or has existing cracks, it can weaken the surface and cause cracks. 
• Poor workmanship or incorrect materials: Cracks can result from skipped reinforcement mesh, wrong render mix, weak adhesion, or a lack of expansion joints. A professional renderer will always apply mesh around openings and stress points and ensure proper detailing around windows and sills. If your render has been failing prematurely, it’s worth reading more about common rendering mistakes and how a professional can help avoid them.
• Age and weathering: Traditional renders typically last 20-30 years before signs of wear appear. Older systems may lack breathable properties or reinforcement mesh, making them more vulnerable to frost, UV degradation, and movement. Over time, this can result in cracks, bulging, or render falling away.

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There Are Three Common Types of Render Cracks:

• Hairline Cracks: Very fine cracks, typically under 1mm wide. These are usually cosmetic and caused by shrinkage during curing, natural drying, or temperature changes.
  
• Shrinkage Cracks: Often appear as random or map-like patterns (known as crazing). They tend to be superficial, but if left untreated, they can widen over time and let in moisture.
  
• Structural Cracks: Wider and deeper cracks that may run through the render and into the underlying wall. These often indicate movement in the building and should be assessed by a professional.

Staining on Render 

Discolouration or patchy staining on rendered walls is a common complaint. It’s actually caused by water interacting with different materials. 

• Water run-off: Leaks or splashing from gutters, downpipes, and windowsills can cause dirty streaks or encourage algae growth. 
• Rust: Metal fixings or nearby iron components can cause rust if they come into contact with moisture. 
• Efflorescence: White salt stains (powdery deposits) often appear when water within the wall draws salts to the surface. 
• Hard water deposits: In areas with hard water, you may notice white mineral stains where water has repeatedly dripped or pooled. 

Algae and Moss Growth

Green or black staining on render is typically caused by algae, moss, or mould. This is not only unsightly, but can also indicate ongoing moisture problems. 

• Shaded or damp areas: Algae and moss thrive on walls that receive little sunlight, particularly north-facing elevations or where trees block airflow. 
• Trapped moisture: Damaged or poorly applied render can retain water, creating ideal conditions for organic growth. 
• Pollution: In urban environments, pollutants can settle on render and encourage staining or algae development. 

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Could it be Rising Damp?

If you’re noticing paint peeling, bubbling, or staining at the base of your walls, especially on both the inside and outside, it could be a sign of rising damp. This happens when posture from the ground rises through the mansory due to a failed or missing damp-proof course, more commonly in older properties. Damp and mould can pose serious health risks, especially to children, older adults, and people with underlying health conditions such as asthma or weakened immune systems.

Before planning any patch repairs or full re-rendering, it’s important to rule out rising damp. Applying new render over damp walls can trap moisture and worsen the problem over time. 

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When Should You Be Worried?

Most render issues are manageable, but certain signs suggest it’s time to seek professional advice: 

• Cracks wider than a hairline
• A hollow sound when you tap the render
• Render that’s flaking off or pulling away from the wall
• Damp patches or black mould inside your home
• Green or black staining that keeps coming back

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If you’ve noticed any of the above, it’s worth getting a qualified renderer to take a look. Acting early can help prevent further damage, protect your home’s structure, and avoid costly repairs down the line.

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Can You Repair External Render, or Is It Time to Re-Render?

Whether your render can be repaired or needs to be fully replaced depends on the type of render, the extent of the damage, and the conditions of the wall underneath.

Repair may be suitable if:

• The issue is localised – for example, small cracks, chips, or isolated stains
  
• The render base is still solid and well-bonded
  
• The existing render is relatively modern and repair materials can be matched

Re-rendering is often the best option if:

• The render is over 20 years old and showing signs of age
  
• Cracks are widespread or deep
  
• Moisture has penetrated behind the render
  
• You're experiencing damp problems inside the property

If in doubt, a professional assessment can help determine the most cost-effective and long-lasting solution.

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If your render is cracked, stained, or simply making your home look tired, don’t wait for it to get worse. We specialise in restoring and re-rendering homes across the Midlands. Whether you need a small repair or a complete re-render, our team will offer honest advice and professional results. Get in touch today for a free quote.