Key takeaways
- Concrete shrinks as it hardens and dries, and the ground restrains the slab, so tension builds up and the slab cracks, and a control joint gives that crack a planned place to form.
- Joint layout should keep panels close to square and should pick up re-entrant corners, openings and changes in slab thickness, because cracks start at those points when no joint has been placed there.
- Joint depth is set as a proportion of the slab thickness by the engineer or the design, and a joint cut too shallow will not do the job it was placed to do.
- The saw cut has to be made after the concrete has hardened enough to hold a clean edge and before shrinkage cracks start, so the timing depends on the mix, the weather and the curing.
- Isolation joints separate the slab from columns, walls, pits and footings, while construction joints mark where a pour stops, and each has to be detailed as a separate decision on the plan.
A slab will crack. Almost every residential slab that is poured and cured properly will show at least one crack at some point in its life, and most of them are harmless. The question that matters to the owner, and to the builder in the defect period, is whether the crack ran along a line that was planned, in a neat joint that can be filled and forgotten, or whether it ran at random across the middle of a panel where it spoils the look of the floor and invites questions. The difference is settled before the concrete goes in and in the hours after it is finished.
Concrete shrinks as it hardens and dries. The water that is not needed for the chemical reaction leaves the paste, and the material gets smaller. The slab is not free to shrink, because it is held on the ground by friction and by whatever sits against its edges. That restraint produces tension inside the slab, and when the tension exceeds what the concrete can carry, a crack forms. The crack is not a sign of bad concrete. It is the normal result of a material that shrinks on a surface that will not let it move.
The organising idea of this article is that a control joint does not stop a slab cracking. It decides where the crack goes. The sections below cover where the joints belong, how deep they should be, how the cutting window works, the other joints a slab needs and the safety side of the saw. The article is about the layout and timing of joints. Slab edge levels and curing practice are separate subjects, and they are covered in their own articles.
Why a slab cracks and why the crack should be planned
A crack forms in the weakest line across a panel. Without a joint, the weakest line is wherever the concrete happens to be thinnest, most restrained or most disturbed, and that location is not predictable. A control joint is a deliberate weakening of the section. It reduces the cross-section of the slab along a chosen line, so the stress concentrates there and the crack forms along the cut. The joint does not reduce the shrinkage. It makes the crack occur where the builder wants it, and in a line that is simple to seal and easy to explain.
Weather changes the picture. Hot, dry and windy conditions speed up the loss of water from the surface, and the slab shrinks faster and more unevenly than it would on a mild day. That is why a slab poured in summer often cracks sooner and more widely than one poured in cooler weather, and why the timing of the joint matters more in heat. A builder who treats the joint as a fixed task in the programme will miss the window on a hot afternoon.
Choosing where the joints go
The joint layout starts with the panel shape. A panel that is close to square behaves better than one that is long and narrow, because the shrinkage has a shorter distance to act over and the stress is more evenly spread. A long thin strip of slab between two joints is a crack waiting to happen. The layout should break the slab into panels whose proportions suit the design, with the spacing set by the engineer or the design rather than by a rule of thumb picked up on site.
Re-entrant corners need a joint. A re-entrant corner is the inside corner where the slab steps in, such as the corner between a rear addition and the existing house or the corner where a garage meets a house slab. The stress concentrates at that corner, and a crack almost always starts there, usually running diagonally across the panel. A joint placed from the re-entrant corner into the slab gives the crack a line to follow. The same logic applies to large openings such as a garage door or a wide doorway, where a joint should line up with the opening rather than cut across it at a random point.
Changes in slab thickness need care too. Where the slab steps from a thicker section to a thinner one, the shrinkage and the stiffness change, and the step is another place where a crack will start. The joint should be placed at the step where the design shows it, and the marked plan should show it clearly before the pour, so the saw crew is not making the decision on the day.
The plan should be marked on the slab before the concrete is placed, using chalk lines or survey pegs that show each joint. The people who will cut the joint should have the plan in hand and should be able to see the lines on the ground. A joint that has been agreed on paper and never marked on the slab will end up wherever the saw happens to be pointing when the window closes.
Joint depth and the types of joint
The depth of a control joint is set by the design, as a proportion of the slab thickness. A joint cut too shallow does not create the weakened section it was placed to create, and the crack runs on past it. A joint cut too deep is wasteful and can damage the reinforcement where it is placed. The engineer or the design sets the depth, and the saw crew should be given the depth in writing and should check it with a depth gauge on the first cut.
Not every joint is a saw cut. The table below sets out the main types a residential slab uses, and what each one does. The control joint is the one most builders think of first, but the isolation and construction joints do different jobs, and a slab that relies on a control joint to do all of them will fail in at least one place.
The main types of joint in a residential slab
| Joint type | What it does and where it goes |
|---|---|
| Saw-cut control joint | Cut into the hardened surface to a set depth, so shrinkage cracks form along the line of the cut. |
| Formed or tooled groove | Pressed into the fresh surface as the slab is finished, used where a saw cut is impractical or as a decorative control. |
| Isolation joint | Separates the slab from columns, walls, pits and footings so each can move without transferring stress into the other. |
| Construction joint | Marks where a pour stops, and is detailed so the two pours bond or transfer load as the design requires. |
Isolation joints deserve particular attention, because they are the joints most often omitted or detailed wrongly. The slab must be able to move separately from the columns, walls, pits and footings it meets. A slab tied rigidly to a footing or a wall has nowhere to take up its movement, and it cracks where it is tied. The isolation joint is filled with a compressible material and sealed, so the slab can move without dragging against the element beside it. Check the detail on the plan rather than assuming the trade will know it.
Timing the cut so the crack follows the line
The cut has to be made in a narrow window. Saw too early and the concrete is too soft to hold a clean edge, so the cut ravels and the joint is ragged and weak. Saw too late and shrinkage has already started a crack somewhere else, possibly in the wrong place, and the joint is doing nothing useful. The window sits between the two, and the timing depends on the mix, the temperature, the wind and the curing conditions.
The stages below set out how the window runs from finishing to curing. The point of the sequence is that the saw crew should be ready before the slab is ready, because in hot or windy weather the window can close faster than a crew that is only called after the finishing is complete will expect.
The cutting window, from finishing to curing
- FinishingThe surface is finished to the level it will be kept at, so the saw does not tear into soft concrete.
- HardeningThe concrete sets enough to hold a clean edge, and the crew checks it by pressing the surface rather than by guessing.
- The cutThe saw cut is made to the depth the design sets, before shrinkage cracks start and along the marked line.
- CheckEach joint is checked for depth, alignment and any early cracking, and anything out of line is recut while the window is open.
- CuringThe joint area is cured with the rest of the slab, so the surface does not dry out at the cut.
The crew should test the concrete rather than guessing. Pressing a finished surface, or running the saw blade over a test strip, will show whether the concrete is ready to hold an edge. A crew that waits for the surface to look ready will often be late, because the concrete can be set enough to cut long before it looks it. Keep a spare blade and a fuel supply on site so a saw problem does not close the window.
Construction joints and the places a pour stops
A construction joint marks where a pour stops and the next one begins. It is a plan element, not an accident, and it should sit where the design shows it. A pour that stops at a point not on the plan, because the concrete ran out or the pump moved, leaves a joint that nobody designed, with no key or dowel and a rough edge that will not transfer load. The position of the pump, the hose path and the timing of the truck deliveries all affect where a pour can stop, which is why the construction joints should be agreed with the concrete supplier and the pump operator before the day.
Safe Work Australia’s material on concrete pumping covers the hazards of the pump and hoses on a pour, and those hazards decide where people can stand and work during the placement. The saw crew and the pump crew should agree the working zones before the pour starts, so the cutting does not begin while the hose is being moved across the slab.
Saw cutting and the dust it makes
Cutting hardened concrete produces respirable crystalline silica, which is a serious hazard to the lungs when it is breathed in. The controls are well established: wet cutting to keep the dust down, dust extraction where the saw allows it, and suitable respiratory protection for anyone working near the cut. Keep bystanders clear of the cutting line, and check that the saw’s guard is in place and working. The model code of practice for construction work sets out the general duties for managing these risks, and the crew should know them before the first cut.
Wet cutting creates its own problem, because the slurry runs off and has to be managed. Plan where the water will go before the saw starts. A slurry that runs into a drain, onto a neighbour’s land or into a stormwater system is a problem the builder will be asked about, and it is much easier to plan for than to explain afterwards.
Repairing a crack that has already formed
When a crack appears in a slab that was not jointed where it ran, the first question is whether the crack is still moving. A crack that has stopped moving can be filled with a material that suits its width and the conditions on the slab. A crack that is still moving needs a material that will flex with it, or the repair will crack again beside the line of the filler. The assessment belongs to a person who understands the movement, not to whoever has the tube of filler to hand.
The Building Defects Library from Building Commission NSW is a useful reference when a crack needs describing to an owner. Read the entries on concrete slabs before deciding what to tell the owner about a crack, and keep the description factual. A crack is not automatically a structural defect, and it is not automatically a normal shrinkage crack either. The description should say what was seen, where, when and what has been done about it.
A worked example from a rear addition slab
A builder in the Sutherland Shire pours the slab for a rear addition that steps out from the existing house. The re-entrant corner where the addition meets the old slab is marked on the plan, and a joint is set to run from that corner into the new panel. The pour goes well, but the afternoon is hot and the finishing runs long, and the saw crew arrives after the window has started to close.
The first cut is made late, and the joint at the corner is shallower than the design requires. Within a few weeks a diagonal crack appears from the re-entrant corner, running across the panel beside the joint line rather than along it. The owner calls the builder, and the builder’s first reaction is that the slab is cracking normally. On inspection, the crack runs where the joint should have controlled it, and the joint did not do its job because it was cut late and shallow.
The builder repairs the crack with a filler suited to a slab that is still settling, records the joint depth and timing for the next pour, and changes the crew’s roster so the saw is on site before the finishing starts. The owner accepts the repair, and the next addition in the same street is jointed to the plan without a second conversation. The mistake was small on the day and expensive in the defect period.
Lead in. The mistake that costs the most is cutting joints too late, too shallow or in the wrong place, because the saw crew was not ready when the window opened. A late joint leaves a random crack, the owner sees it first and the builder is left arguing that it was normal.
Checking the joint layout before the pour
The pre-pour inspection is the last chance to catch a layout problem before the concrete is placed. The check should confirm that the joints are marked on the slab, that the isolation joints are detailed around every column, wall, pit and footing, that the re-entrant corners have joints running from them and that the construction joints match the plan. The free pre-pour inspection checklist covers the wider slab checks, and it is a sensible starting point for building a joint check into the routine.
Joint layout is one part of a slab that performs well. The curing of the concrete in hot weather is covered in the article on concrete curing in hot weather pours, which matters because a slab that dries too fast will crack regardless of its joints. For a broader view of how builders keep their knowledge of slab work current under the NSW CPD scheme, the PointCPD guide to how NSW builder CPD works sets out the topics and the records that go with them.
Questions builders ask
Why does my concrete slab crack even when it was poured properly?
Concrete shrinks as it hardens and dries, and the ground holds the slab back, so tension builds up and a crack forms once that tension is too high. This is normal behaviour, not necessarily a sign of poor concrete. Control joints give the crack a planned place to form.
Where should control joints go in a residential slab?
Joints should break the slab into panels close to square, and should run from re-entrant corners, line up with large openings and sit at changes in thickness where the design shows them. The spacing is set by the engineer or the design. Mark the joints on the slab before the pour so the saw crew can see them.
How deep should a control joint be cut?
The depth is set by the design as a proportion of the slab thickness, and the engineer or the design should state it. A joint cut too shallow will not control the crack. Check the depth on the first cut with a depth gauge and keep the record.
When is the right time to saw a control joint?
The joint has to be cut after the concrete has hardened enough to hold a clean edge and before shrinkage cracks start. The window depends on the mix, the temperature, the wind and the curing. Hot and windy conditions shorten the window, so the saw crew should be ready before the finishing is complete.
What is the difference between a control joint and an isolation joint?
A control joint is a weakened line that makes the slab crack in a planned place. An isolation joint separates the slab from columns, walls, pits and footings, so each element can move without transferring stress into the other. The two do different jobs, and a slab usually needs both.
Is a crack in a slab a structural defect?
Not automatically. Many cracks are shrinkage cracks that have stopped moving and do not affect the strength of the slab. The answer depends on the width, the movement, the location and the cause. Describe what was seen, where and when, and assess the crack before deciding how to repair it or report it.
Sources
- Building Defects Library · Building Commission NSW · 2025-06-25
- Concrete pumping · Safe Work Australia
- Construction work: code of practice · Safe Work Australia
Published 5 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
