Control joints are shallow cuts in a concrete slab designed to create a weak point that absorbs stress and shrinkage. Instead of the slab cracking randomly as it dries, it cracks where you want it to — along a straight, planned joint line. But for control joints to actually work, they must be cut at the right time, to the right depth, and in the right layout. This guide breaks down exactly how to do it.
Before the first bit of concrete goes down, decide where every joint will be placed. Joints should form a grid pattern and divide the slab into equal, manageable squares or rectangles. A good rule of thumb is to place a joint every 24 to 30 times the slab thickness in millimetres. So for a 100mm slab, spacing should be no more than 2.4m to 3m in both directions. Avoid awkward shapes — long narrow strips are more likely to crack. Also mark joints around penetrations, columns, or changes in depth.
The two main types of cutting tools are early-entry saws and conventional wet-cut saws. Early-entry saws can be used within 1–4 hours after finishing, while the slab is still green. They have specialised blades and lighter weight. Wet-cut saws (with water suppression) are more common and typically used 6–24 hours after pour. For indoor or polished concrete, early-entry saws are ideal to reduce raveling. Outdoors, wet saws with diamond blades work well, but make sure water control is managed to avoid staining.
Timing is everything. If you cut too early, you risk tearing or chipping the surface (especially with wet saws). If you wait too long, the slab may already start cracking on its own. A good rule: as soon as you can walk on the slab without leaving an imprint — and it passes the thumbnail test (no paste on your nail) — you can cut. For hot weather pours, this can be as little as 4 hours. In cooler conditions, you might have a 10–12 hour window. Always check site-specific hardening before committing.
Control joints must be deep enough to actually work — that means cutting at least 25% of the total slab thickness. So for a 100mm thick slab, cut at least 25mm deep. Too shallow and the joint won’t relieve internal stress. Too deep and you risk weakening the slab unnecessarily. Use depth gauges on your saw, and double-check before starting the full cut. Depth is especially important for polished slabs, where improper joints can telegraph through to the surface finish.
Use a chalk line, laser line, or steel straightedge to mark your cut paths. Even a small wobble will show on exposed floors or polished finishes. Keep your saw steady, work at a controlled pace, and don’t stop mid-cut unless absolutely necessary. If you're using a walk-behind saw, ensure the blade is centred and sharp — a dull or bent blade will wander and drag.
Every control joint must start and end at a logical boundary — either an edge, a wall, or another joint. Don’t leave random floating cuts in the middle of the slab. Cracks follow the weakest line — so your cuts must give the stress somewhere to go. For slabs with re-entrant corners (L-shapes, columns), place joints coming off the corners at 45° or use diagonal cuts to reduce crack risk.
After cutting, remove any slurry, dust, or residue from the joint. For decorative or polished concrete, this is essential to avoid discolouration. If the slab is in a wet area or prone to freeze-thaw conditions, seal the joints with a flexible polyurethane or silicone filler. This prevents water ingress, dirt buildup, and joint deterioration. On indoor slabs, sealing helps reduce dust and improve hygiene.
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