Concrete shrinks as it cures, and without room to move, it will crack — guaranteed. Movement joints are a planned way to control that cracking by giving the concrete space to shrink, expand, or shift. On large slabs, driveways, polished floors or exposed surfaces, they’re essential. This guide explains how to plan, space, and install movement joints in a concrete slab so it looks good and lasts for decades.
As a rule of thumb, control joints should be placed every 24–30 times the slab thickness. For example, on a 100mm slab, joints should be no more than 2.4–3.0m apart. For slabs over 6m long or wide, use a grid to divide them into smaller panels. Large uninterrupted pours are prone to random cracking — spacing joints correctly keeps cracks predictable and out of sight.
Mark out where joints will go before you pour the slab. Use chalk lines, tape, or pegs. Joints should be straight, continuous, and run through re-entrant corners or stress points like doorways, pillars, or penetrations. Poor joint layout — like sudden offsets or tight L-shapes — causes stress and cracks. If you’re planning to saw cut, the markings will guide you post-pour.
Expansion joints allow thermal movement and should be placed at slab perimeters or where the slab meets other structures (like walls, columns, drains). Use compressible material like foam, cork, or rubber filler strips and install them vertically before the pour. These joints are full-depth and usually sealed after curing. Without them, slabs can lift, heave, or push against buildings and fail over time.
Once the concrete is poured and set firm (but before it fully hardens), control joints are cut using a floor saw or joint cutter. This is usually done 6–24 hours after pouring, depending on slab thickness and weather. Cuts should be at least 1/4 the depth of the slab and match your pre-marked layout. These joints control cracking and should be straight and uniform across the surface.
If the slab is poured in phases or sections, each edge between pours becomes a construction joint. These must be planned and reinforced correctly — typically with dowel bars or keyways to allow movement while maintaining load transfer. Always align construction joints with control joints where possible. Otherwise, the slab will crack unpredictably across the mismatch.
After curing, joints can be sealed with flexible filler or left open depending on slab use. For internal floors, a PU sealant or silicone joint filler is ideal to stop debris and moisture ingress. For polished or decorative slabs, joints can be hidden with coloured sealant or incorporated into the final finish. Periodically inspect joints — especially external ones — to ensure they haven’t failed or let in water.
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