Slab Saw Cut and Joints
Sawn joint spacing at twice the slab thickness, the cut depth and window, the joint count and linear feet across your panel, dowel bars, and the sawing cost.
A 5 in slab gets joints at 10 ft, which is twice the thickness in inches read as feet. A 40 x 20 ft panel needs 3 saw cuts across the width, and the cut goes a quarter of the slab depth: 1.25 in on a 5 in slab.
How the math works
- Joint spacing (ACI 332.1R): sawn joint spacing is at most twice the slab thickness in inches, read as feet: 4 in slab is 8 ft, 5 in is 10 ft, 6 in is 12 ft, 8 in is 16 ft. Panels as close to square as possible, because joints at 2x thickness on a 20 ft width produce the 10 ft layout.
- Cut depth: one quarter of the slab thickness, with a 1 in minimum, so the sawn joint is a real hinge where the slab cracks. Under 1 in is a score line, not a joint, and the slab cracks at random behind it.
- Cut timing: the saw goes in once the mix can hold an edge and before shrinkage cracking starts. The window is 4 to 12 hr on a standard walk-behind saw and 1 to 4 hr with an early-entry saw, and the finish, the mix (a hot mix saws early), and the wind and sun set the actual hour.
- Dowels (ACI 332.1R and the joint-filler rule): smooth round dowels at 12 in on center, at mid depth, lubricated or staggered so the two panels slide against each other under shrinkage, and dowel bars are #4 on slabs to 6 in and #5 on 8 in slabs, cut 20 in long.
- Sealing: a sawn joint in a slab that carries traffic gets sealed with a silicone, polyurethane, or hot-pour sealant after the slab cures, and an unsealed joint in an exterior slab pumps water and fines and spalls the edge.
How far apart should saw cuts be in a concrete slab?
Twice the slab thickness in inches, read as feet: a 5 in slab gets 10 ft joints, a 6 in slab gets 12 ft, and an 8 in slab gets 16 ft, with panels as square as the layout allows. That is the ACI 332.1R rule and the reason a 4 in patio slab has joints at 8 ft.
How deep does a saw cut need to be?
A quarter of the slab thickness with a 1 in minimum: 1.25 in on a 5 in slab and 2 in on an 8 in slab. The cut is where the slab chooses to crack, so a shallow score leaves the crack random and the slab spalls at the surface.
When do you cut a slab after it is poured? Standard walk-behind sawing goes 4 to 12 hr after the finish, and 4 hr is only right on a cool, set-fast mix. An early-entry saw cuts at 1 to 4 hr, and past 24 hr you are sawing a slab that already cracked on its own.
Hot weather moves the window earlier and wind moves it earliest of all, which is why a fogging mist behind the saw keeps a late cut from becoming a cracked slab.