Why Does Render Crack?

Render is a critical component in construction, as it serves to protect the underlying structure of buildings and enhances their aesthetic appearance. However, one of the most common issues that homeowners and contractors face is the cracking of render.

Understanding the factors that contribute to render cracking and how to avoid or mitigate this issue is key to ensuring the longevity and integrity of buildings.

Various reasons can lead to render cracking, including improper mix ratios, inadequate curing processes, and environmental factors. One of the primary causes is the incorrect balance of ingredients in the render mix, which can weaken the material and make it prone to cracking.

Another factor is the curing process; if the render dries too quickly, it may crack and lose its structural integrity.

repaired render

Understanding Render and Its Cracking

What is Render

Render is a term used to describe a mixture of cement, sand, and water that’s applied to the exterior of a building to provide a protective and decorative finish. It helps protect the underlying structure from weather, dampness, and other environmental factors. Rendering is the process of applying this material to the surface of a building.

Cracks in render can occur for various reasons, and it’s essential to understand these factors to maintain the integrity of the building and its appearance. In this section, we will discuss the common causes of render cracking and measures to prevent it.

One primary reason for render cracking is the natural aging process. Over time, the render on a building may deteriorate due to exposure to elements like sunlight, rain, and temperature fluctuations. This can lead to the formation of hairline cracks, which may grow and become more visible if left unaddressed. Regular maintenance can help mitigate these effects and extend the lifespan of your render.

Another cause of render cracking could be incorrect application or preparation of the render mix. The right proportions of materials must be used to create a durable, long-lasting finish. Too much water, for instance, can weaken the render and make it more susceptible to cracking. Additionally, poor workmanship, such as improper mixing or inadequate surface preparation, may also contribute to render failure.

Substrate movement can also cause render cracks. Buildings naturally experience slight movements due to factors like ground settlement, thermal expansion, and structural changes. When the substrate moves, the render may not have the flexibility to accommodate these changes, leading to cracks in the surface.

The use of appropriate render materials with good flexibility and proper installation techniques can help avoid cracking caused by substrate movement.

Lastly, issues with the underlying structure can contribute to render cracking. A weakened or damaged substrate can cause the render to separate from the surface, resulting in cracks. Surveying the structure for any deficiencies and addressing them before rendering can help prevent crack formation.

cement sand and water

Causes of Render Cracking

Water Ingress

One of the primary causes of render cracking is water ingress. When external render is exposed to water, it can lead to the formation of cracks. Moisture penetrates the render and affects the substrate beneath, weakening the overall structure. Render cracking caused by water ingress can often be seen after heavy rainfall or in areas with high levels of humidity.

Temperature and Shrinkage

Render cracking can occur due to temperature fluctuations and shrinkage. As the render dries and cures, the process can cause differential movement between the render and the substrate. This movement happens because the materials may have different contraction rates as they dry.

When exposed to extreme temperature changes, the render can expand and contract, leading to shrinkage and subsequent cracking.

Substrate Failure

Another cause of render cracking is substrate failure. The substrate is the foundation upon which the render is applied. If the substrate is inadequate, it will not provide enough support for the render, leading to cracks.

Substrate failure can be the result of insufficient preparation or an unsuitable substrate material. Moreover, if moisture or suction levels are not correctly managed, the render will not bond properly, causing it to pull away from the substrate and crack.

Incorrect Application

Lastly, incorrect application of the render can lead to cracking. If the render is applied too thickly or in layers with a high moisture content, it can lead to shrinkage and cracking. Additionally, inadequate curing times can result in weakness, ultimately causing the render to crack.

incorrect application

Types of Render Cracks

Hairline Cracks

Hairline cracks are very fine, narrow cracks that can appear on the surface of rendered walls. They usually develop due to various factors such as incorrect mix proportions, exposure to the elements, or drying too quickly.

It is important to address these cracks early on, as water can penetrate and cause larger, more severe issues over time. Although hairline cracks in render can be aesthetically displeasing, they typically don’t present a major structural threat. However, addressing the cause of these cracks can help prevent further damage and ensure the wall remains sound and weatherproof.

Shrinkage Cracks

Shrinkage cracks are a common issue in rendered walls, and they form due to the natural process of drying and curing of the render material. As the render dries, it can contract and lose volume, resulting in the formation of shrinkage cracks.

The severity and size of these cracks depend on several factors, including the render mix, drying conditions, and the quality of the substrate material.

Shrinkage cracks can be minimised by implementing good rendering practices such as using the correct render mix, ensuring adequate curing time, and applying appropriate control joints to allow for movement in the structure.

It is important to monitor these cracks for any signs of severity or further development, as untreated cracks can provide an entry point for moisture, leading to potential water damage and other issues in the long run.

hairline crack

Role of Materials in Render Cracking

Cement Render

Cement render is a widely used building material that consists primarily of cement, sand, and water. When applied to walls and surfaces, it provides an aesthetically appealing and durable finish. However, a significant issue with cement render is its tendency to crack.

This is mainly due to the high strength and rigidity of the material. Additionally, cement render has a high shrinkage rate upon drying, which makes it more prone to cracking. Moreover, inadequate or incorrect mixing ratios of cement and sand can lead to the creation of weak points in the render, making it more susceptible to cracks.

Sand and Cement Render

Sand and cement render is a mixture of Portland cement, sand, and water. This type of rendering is particularly popular because it offers a cost-effective solution for a variety of construction applications.

The sand helps to reduce the overall shrinkage of the render and increase its workability. However, the composition and quality of sand used can also contribute to render cracking. For instance, if the sand contains a high percentage of clay or silt, it might cause the render to become weak and crack upon drying.

Ensuring a well-graded, clean sand will help mitigate cracking issues in sand and cement renders.

Lime Mortar

Lime mortar is another type of render that consists of lime, sand, and water. Lime renders are known for their flexibility, breathability, and self-healing properties, which make them an ideal choice for preserving and restoring historic buildings.

One advantage of using lime mortar over cement-based renders is its lower propensity to crack. Its natural elasticity allows it to accommodate minor structural movements or changes in humidity without cracking.

However, one crucial factor in preventing lime mortar render cracking is the appropriate choice of sand. The sand must have a well-graded particle size distribution to ensure good workability and strength.

Additionally, adequate curing time is essential, as lime mortar takes longer to harden than cement renders. Proper curing will help the lime mortar reach its optimum strength and durability.

Additional Concerns and Tips for Renders

Importance of Breathability

Breathability is a crucial aspect to consider when applying render to a building. It allows moisture to escape from the structure, maintaining the integrity of the walls and preventing dampness. High-quality render is designed to ensure that water is not trapped within the material, reducing the risk of cracks and other damage occurring due to moisture build-up.

Properly mixing and applying render is essential in achieving this level of breathability, as well as selecting the correct type of render for the building’s structure and environmental conditions.

Render Aesthetics

Aesthetics play a significant role in the overall appeal of any rendered building. Choosing the right colour, texture, and finish for your render can greatly influence its appearance and impact on the surrounding environment.

It is important to keep in mind factors such as the architectural style of the building, the local landscape, and any planning restrictions that may influence your choice of aesthetics.

Additionally, when applying render – ensure that it is evenly distributed and properly adhered to the surface, as inconsistencies can affect the overall look and weaken the structure.

Insulation and Temperature Control

Render can significantly improve a building’s insulation and temperature control capabilities. By creating a barrier between the external environment and the internal walls of the building, render can help to maintain a comfortable and consistent indoor temperature.

This is particularly relevant in climates with extreme temperature fluctuations, as the insulation properties of the render can help to mitigate heat loss or gain. Further benefits include potential savings on energy bills due to more efficient temperature regulation of the building.