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How to Repair Cracks in Walls for a Smooth Finish

Repairing cracks in walls is a vital maintenance task that improves the appearance of your home and protects it from potential damage. The size and scope of cracks can vary, from hairline fractures that are mostly cosmetic concerns to larger cracks that may indicate structural issues. Addressing these problems promptly can prevent further deterioration.

Diagnosing if a crack is structural or not

When confronted with cracks in walls, it’s essential to accurately diagnose their cause and severity. This diagnosis is crucial not only for the correct repair strategy but also for the structural integrity of the building. The Building Research Establishment (BRE) provides a categorisation system ranging from 0 to 5, which helps professionals and homeowners alike understand the severity of wall cracks and whether they are structural.

Category 0: Hairline Cracks

These are hardly noticeable and very fine cracks, measuring less than 0.1 mm in width. They are considered superficial, often caused by the initial shrinkage of materials and do not indicate a structural concern. Simple cosmetic repairs are usually sufficient for these types of cracks.

Category 1: Fine Cracks

Cracks that fall into this category are up to 1 mm wide. They can appear in plaster and wall finishes, often due to slight settlement or seasonal changes in temperature and humidity. Like Category 0, these are not typically structural and can be easily filled and painted over.

Category 2: Visible Cracks

Visible cracks measure between 1 mm and 5 mm in width. They might be a sign of more significant settlement or thermal movement. While these cracks may not immediately signal structural damage, they warrant a closer inspection to determine the underlying cause. Repairs might involve filling and addressing potential moisture, thermal movement, or minor settlement issues.

Category 3: Service Affecting Cracks

These cracks are more serious, ranging from 5 mm to 15 mm in width. They can indicate a structural issue that may affect the building’s usability, such as doors and windows not functioning correctly. At this stage, professional assessment is crucial to determine the extent of structural damage and the necessary repairs, which might involve structural reinforcement.

Category 4: Extensive Damage

Cracks wider than 15 mm fall into this category and often denote significant structural damage, requiring immediate professional intervention. These cracks may be accompanied by other signs of structural distress, such as sagging floors or roofs. Repairs can be extensive, possibly involving underpinning or major structural alterations.

Category 5: Structural Failure

This category is reserved for severe structural damage with a risk of collapse. It’s characterised by wide cracks where you can see through to the other side or where parts of the building have moved or fallen. Immediate action is required to secure the building and prevent further damage or collapse.

Identifying the category of a crack is the first step in diagnosing the problem and planning the appropriate repair or intervention. While Categories 0 to 2 might only require minor repairs, Categories 3 to 5 signify more serious issues that demand professional assessment and possibly significant structural work. Gyproc offers an effective solution for small cracks and holes. It’s easy to apply and sands down to a smooth finish, blending seamlessly with the surrounding wall. Larger cracks and damage might necessitate a stronger solution, like EWI Pro Levelling Mortar. This mortar is designed for more substantial repairs, offering durability and a smooth surface once cured.

The steps to repair all forms of cracks will follow the same pattern. If you damage a rendered surface and the insulation below, you must use the same basecoat, insulation, and render that is already on the wall. The process is detailed below, courtesy of our partners at EWI Pro, who now run a render repair training session.

Repairing cracks in walls – rendered walls

Step 1: Assess the Damage

Start by assessing the crack to determine the repair’s scope. Understand whether you’re dealing with surface damage or if the issue is deeper and requires more extensive intervention. This step helps you choose the appropriate materials and tools for the job.

Step 2: Mark the Area

Using a carpenter’s pencil and a long spirit level, mark out an area around the damage that’s twice the size of the damage itself. This ensures you have enough room to work without extending beyond the necessary area.

Step 3: Remove Damaged Render

Carefully remove the damaged render using a chisel, multi-tool, or angle grinder. Work from the centre of the damage outwards, ensuring not damage to the underlying basecoat and mesh.

Step 4: Re-score and Remove the Mesh

After removing the render, re-score the basecoat and mesh layer, then peel it off. This step is crucial for ensuring a seamless repair.

Step 5: Cut and Remove Insulation

Match the insulation removal to the mesh removal area. Use a saw for precision, maintaining the integrity of the clip-on profile.

Step 6: Apply New Insulation

Measure the area for the new insulation carefully, then cut the insulation board to fit. Apply adhesive using the dot and dab technique, ensuring at least 40% coverage for a strong bond.

Step 7: Fix the Insulation in Place

Depending on the insulation board’s size, you may need multiple fixings. Make sure these are evenly spaced and flush with the insulation surface.

Step 8: Apply Basecoat and Mesh

Install a new mesh layer, ensuring it overlaps by 100mm on all sides for continuous reinforcement, then cover the area with a basecoat.

Step 9: Prime and Render

Once the basecoat dries, prime the surface and apply render matching the existing wall’s colour and texture. Use rendering tape to protect the surrounding area and ensure a neat finish.

Step 10: Blend the Repair

Remove the rendering tape and blend the new render into the existing wall using a damp brush for a seamless finish when the render is still wet.

Considering “self-healing” materials

Small cracks can sometimes self-heal over time for walls covered with lime render. Lime render has the unique ability to reabsorb carbon dioxide and, in the presence of water, can fill in minor cracks as part of its natural carbonation process. This feature makes lime render an excellent option for historic buildings or structures looking for aesthetic appeal and durability.


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