Cracks often crop up seemingly overnight — from small, hairline fractures that are simple to solve to large holes that require a bit more effort. That said, you can sometimes repair them yourself, but only if you have the tools and follow the right processes.
In this blog, we’re going to provide you with a bit of background on building crack repair, including the types of cracks, what causes them, how they are assessed, what you can do and when you should ask for help.
Non-structural vs structural cracks
A non-structural crack doesn’t threaten the structural integrity of a building. These cracks are mainly caused by concrete shrinking during the curing process. These cracks are normally harmless; but some, such as non-structural vertical cracks, can cause leaks in basements.
Structural cracks, on the other hand, threaten a building’s structural integrity. These are cracks in concrete and foundation block walls that — if left unchecked — could be dangerous. For example, large diagonal cracks, cracks that run across a ceiling and down a wall, horizontal foundation cracks in walls, and stair-step cracks in blocks or brick walls, are structural cracks. To avoid these happening in your building, it is important to keep up with a building maintenance program.
What causes structural building cracks?
Building cracks are caused by several factors, but the most common include:
- Unsuitable design
Inadequate thickness, insufficient reinforcement, incorrect geometry, improper materials — all of these can affect the structural integrity of a building. At the design stage, it’s important to provide a blueprint that takes into account all the variables and doesn’t focus on just the aesthetic of the build. Failure to do so can result in “latent defects” — issues that only become apparent over time.
- Poor construction practices
Carelessness, ignorance, greed and/or negligence can lead to workers selecting the wrong materials for the job and not following construction best practice. The result is that buildings may not be designed correctly or resilient enough to withstand certain scenarios.
- Subsidence
The sudden sinking or gradual downward settling of the surface beneath a building can cause cracks as it leads to the walls and floors shifting.
- Thermal changes
Thermal movement is one of the most potent causes of cracking in buildings. All materials — more or less — expand when heated and contract when cooled. As a result, the rapid heating and cooling of concrete, for instance, can cause cracks to form. Of course, this depends on several variables: temperature, insulation, the physical properties of the material. In buildings, mainly thin walls and the roof are subject to thermal changes that could cause cracks.
- Vegetation
Fast-growing trees in the vicinity can cause cracks in walls as their roots grow out under the building’s foundation.
- Chemical reactions
Depending on the materials used, there may be chemical reactions that cause cracks in the walls or foundation of a building. For example, expansive reactions between the aggregate (coarse to medium-grained particulate material) and alkalis in the cement paste can cause cracks.
- Creep movement
Under sustained loads or pressure, concrete structures will deteriorate and cracks will form. This is known as creep movement. As creep increases, structures will distort and bend under the weight. To counteract this, walls are often fitted with multiple movement-joints.
How are structural cracks assessed?
Any cracks will be assessed as part of a building inspection. However, if a category three crack (or greater) is identified, you should consult a structural engineer. Building cracks fall into the following five categories:
- Hairline cracking: under 0.1 mm
- Fine cracks that don’t need repair: less than 1.0 mm
- Noticeable cracks that are easily filled: 1.0 mm to 5.0 mm
- Cracks can be repaired, but some of the wall sections will need to be replaced: 5.0 mm to 15.0 mm
- Extensive repair work required (involves breaking out and replacing sections): 15.0 mm to 25.0 mm
This should give you a good idea of whether or not you can repair any building cracks yourself or if you need to call in an expert.
How to repair building cracks (or stabilise walls)
For serious cracks, we would recommend you speak to a construction expert — someone who can assess the structural integrity of your building. However, for minor building crack repair, i.e. hairline cracks, fine cracks or noticeable cracks that just need filling, there are plenty of things you can do (still, it’s worthwhile talking to an expert to make sure they’re not structural cracks in disguise).
- Crack stitching
Crack stitching is a permanent and inexpensive way to repair building cracks. It involves embedding stainless steel helical bars into slots that are cut into the wall (usually in the bed joints) to restore structural integrity. These bars are typically high tensile rods with enough flexibility to accommodate the movements of walls while ensuring continued reinforcement.
- Filling small and minor cracks in interior walls and plasterwork
Start by clearing the crack and its surrounding area of dust or debris using a small brush, and then vacuum clean.
Next, use Polyfilla or a similar building crack repair filler and spread it over the crack using a flat tool. Apply a little bit of pressure to work it into the crack. Smooth it out and get it as flat as possible. Leave it for a few days and repeat if any pits/dips or holes are still visible.
- Cracks in sills and copings
The amount of building crack repair you can do here will depend on the size of the crack and whether the coping is load-bearing (i.e. an active structural element of a building and holds the weight of the elements above it).
In most cases, you will need to widen the crack slightly to be able to insert some mortar compound. This will help “sure up” the crack. Push it in as far as you can. Once it’s filled up, make it as flush (flat) as you can to ensure a natural finish.
- Caulking
For cracks larger than a hairline’s width, you can fill them with polyurethane, silicone or latex concrete caulk. Use a caulking gun to force the caulk into the crack along its entire length. This type of caulk is effective as it fills the space and allows for the expansion and contraction of the foundation during extreme changes in the weather.
Larger cracks may need foam-backer rods in addition to caulking. These rods are used to fill cracks and gaps between building materials and are excellent at controlling the amount of sealant used or the depth of the sealant in the crack or gap.
- Hydraulic cement
A special type of cement that sets quickly and hardens with the addition of water to form a water-resistant product. It’s generally used to stop water (which can cause cracks in structures over time due to expansion) and widely used in the construction industry. Push this cement into the crack using a putty knife and let it dry for an hour or two before adding another layer. Use a trowel to smooth out the patch so it’s level.
- Carbon fibre straps
To fix walls that are bowing or bulging and causing cracks, carbon-fibre straps can be bonded to the walls to increase their strength. This won’t fix the cracks but will stabilise the walls.
When should you call an expert?
- If the crack accompanies a bulge in the foundation
This indicates that the walls are being pushed inward by external pressure and eventually that pressure will cause those walls to crack and snap. It’s best that you immediately contact a structural engineer to assess the situation.
- If the crack is wider than a ½ inch
Larger cracks are difficult to fill with conventional, run-of-the-mill solutions. Instead, a construction expert will be able to provide you with cost-effective, long-term alternatives.
- If large horizontal cracks are present
These cracks are usually caused by horizontal pressures pushing in on the outsides of walls, causing them to bend and separate.
- If large vertical cracks are present
Major vertical cracks on the exterior walls of your property can indicate settlement or foundation problems. This could be due to a drop in the foundation due to subsidence, causing the walls to pull away from the rest of the building.
At First Response, we specialise in building maintenance. This includes painting, landscaping and grounds maintenance, refurbishment, carpentry, electrical systems installation and design, roofing repairs, flooring, plastering and more. This means that no matter what kind of building crack you need to repair, we have the experience and expertise to fix it and anything else in the process.
If you want to find out more about our services and get a tailored quote, please click the button below. We’d be glad to help.
