Repairing cracks in concrete foundations
2 April 2026
When you spot cracks in a concrete foundation during a renovation, it’s essential to know when it’s a minor cosmetic job or a major structural issue. Small surface cracks can be harmless, but anything that points to poor drainage, significant soil movement or structural failure isn’t something you can just carpet over.
Assessing the damage
So, how do you tell what sort of repair is needed? Start by working out what’s causing the cracking. Hairline cracking in concrete is common and often not a major issue. But wider cracks, movement, uneven floors, doors or windows sticking, or signs the slab has dropped should ring alarm bells. If water, poor soil compaction, tree roots or subsidence are behind it, simply filling the crack won’t solve the problem. If movement has caused the cracking, has that movement stabilised or is it likely to continue?
BRANZ has some useful techniques to check whether a concrete slab is moving. Also ask whether water can get into the crack, or are there rust stains that indicate the reinforcing steel is corroding? Are floor tiles cracking, signalling a lot of movement? If there is any uncertainty about the cause or the seriousness of cracking, seek advice from a structural engineer.
According to MBIE, you should measure the slope of the floor between two points more than 2 m apart. If it’s more than 0.5%, or the variation in level over the whole floor is more than 50 mm, the foundation probably needs to be relevelled. If a concrete slab foundation is more than 150 mm out of level, a rebuild is suggested.
Is an expert needed?
As a simple rule of thumb, minor non-structural cracks that don’t exceed 0.3 mm in an exterior environment, or 0.4 mm for a ground floor slab protected by a damp-proof course, can be repaired by builders. Otherwise, repairs generally need to be carried out or supervised by a Licensed Building Practitioner (LBP). Cracks in heavy-use industrial floors will also have different requirements, so always consult an expert in these cases.
Making minor repairs
Cleaning the area around the crack is the first step. Use a water blaster or compressed air to get rid of any loose debris and failed concrete, and remove oil staining or algae with solvents, detergent or herbicide. Wait until it’s dry before filling.
Portland cement is a popular and cost-effective choice. Patch the crack with a slurry mix of one part cement to two parts of filler for gaps from 10 to 50 mm wide. Brush with a dry mix that’s one part cement to two parts sand, and keep damp for seven days. If you want to match the colour to the existing floor, mix grey cement with white cement or finely ground pumice.
If using epoxy resins, make sure to choose the right product for the job and read the manufacturer’s instructions. Temperature variations can have a major effect on curing time. You can also purchase polymer modified cement products such as the Cemix range. Simply apply until the crack has been filled.
The general guidance when repairing concrete is, if there’s anything you’re not sure about, consult a structural engineer or LBP, or visit the BRANZ website.
