Control joint spacing

The three rules

Spacing. No more than 2.5 times the slab thickness in inches, read as feet. A four-inch slab gets a joint every ten feet at most; a six-inch slab every fifteen.

Depth. A quarter of the slab thickness. The cut is what creates the weak plane the crack will follow, and a cut that is too shallow simply gets ignored — the slab cracks where it wants to instead, usually a foot away and visibly.

Timing. Between six and eighteen hours after the pour, sooner in hot weather. Cut too early and the edges ravel; cut too late and the slab has already chosen its own crack. This window is the reason jointing gets skipped by people who poured on a Saturday afternoon.

Panels want to be square

Keep the length-to-width ratio of each panel near 1 and never past 1.5. A long thin panel cracks across its middle no matter how carefully the joints were cut, because concrete shrinks along its longest dimension and a narrow panel has nowhere to relieve that.

This is why the spacing figure is a maximum rather than a spacing to lay out from one edge. Divide the slab into equal panels that come in under the limit. The concrete calculator does that division for you and reports both the maximum and the spacing it actually landed on.

Concrete is going to crack

That is not pessimism, it is the material. Concrete shrinks as it cures, shrinkage creates tension, and concrete is weak in tension. Jointing does not prevent cracking — it decides where the crack goes, and puts it at the bottom of a straight saw cut where nobody sees it.

Reinforcement does something different: it holds the two sides of that crack together so the slab keeps acting as one piece and the edges do not lift. You want both. A patio can use welded wire mesh; anything a vehicle sits on wants bar in a grid, held at mid-depth on chairs rather than laid on the sub-base and hoped for.