Control Joint Spacing for Concrete Slabs in Spokane: Driveways, Garage Floors and Shop Slabs (2026)
Short answer: in a 4-inch slab, control joints go about 8 to 12 feet apart; in a 5-inch slab, 10 to 15 feet; in a 6-inch shop floor, about 12 to 15 feet. That comes from the industry rule of spacing contraction joints 24 to 36 times the slab thickness, with a recommended cap of 15 feet. Cut them at least a quarter of the slab's depth, and cut them on time. Here's how that plays out for driveways, garage floors and metal shop slabs around Spokane.
Why concrete needs joints at all
Concrete shrinks as it sets and dries, and it is weak in tension. When that shrinkage is restrained by the ground, the forms, footings or columns, the slab cracks. The National Ready Mixed Concrete Association (NRMCA) puts it simply: cracks can't be prevented entirely, but they can be controlled, and joints are "simply pre-planned cracks."
There are three kinds you'll see on a slab drawing:
- Contraction (control) joints: saw cuts or tooled grooves that create a weak plane, so the slab cracks there instead of at random
- Isolation (expansion) joints: a full-depth break, usually with a compressible filler strip, that separates the slab from walls, footings, columns, steps and the garage slab where a driveway meets it
- Construction joints: where one day's pour meets the next
How far apart should control joints be?
NRMCA's guidance (CIP 6, which cites ACI 302.1R and ACI 224.3R) is:
- Maximum spacing: 24 to 36 times the slab thickness
- Cap: keep spacing to 15 feet or less
- Panel shape: square or nearly so. A panel's length shouldn't be more than 1.5 times its width, and avoid L-shaped panels
Converted to feet, that gives:
- 4-inch slab (walks, patios, many driveways): 8 to 12 feet, and NRMCA uses about 10 feet as its example
- 5-inch slab (heavier driveways, many garage floors): 10 to 15 feet
- 6-inch slab (shop floors, RV pads): 12 to 18 feet by the multiplier, but held to 15 feet by the cap
The American Concrete Institute's slab design guide, ACI 360, has a similar spacing chart for slabs that aren't designed with heavy structural reinforcement. As summarized by Vertex engineers, it puts a 4-inch slab between 8 and 13 feet and needs an 8-inch or thicker slab before joints can stretch to 20 feet.
What that looks like on a real shop floor
Two examples, worked from the rules above:
- 30x40 shop, 5-inch slab: two joints across the 30-foot width and three along the 40-foot length make twelve 10x10 panels, at the tight end of the 10 to 15 foot range
- 40x60 shop, 6-inch slab: two joints across the width (about 13 ft 4 in panels) and three along the length (15-foot panels) give panels that are close to square and inside the 15-foot cap
Joints also need to land where cracks want to start:
- Re-entrant corners: run a joint off any inside corner, like a notch for a stem wall or a step
- Columns: in a pre-engineered metal building, NRMCA recommends isolating columns with a boxed-out opening, turned 45 degrees if square, and running the contraction joints into the corners of that diamond
- Door openings and drains: line joints up with overhead door jambs and trench drains where the spacing allows
- Walls and footings: use isolation joint filler where the slab meets a stem wall or footing
If you haven't settled the slab itself yet, our shop slab thickness guide and PSI guide come first. For metal buildings, our PEMB foundation guide explains how the columns and anchor bolts tie in.
How deep should the cut be?
- Conventional (wet) saw cuts: at least one quarter of the slab depth, and never less than 1 inch. That's 1 inch in a 4-inch slab, 1-1/4 inches in a 5-inch slab and 1-1/2 inches in a 6-inch slab
- Early-entry (dry-cut) saws: at least 1 inch deep for slabs up to 9 inches, because they cut much earlier, while the concrete is still green. ACI 360 notes some designers have the joint deepened to a quarter of the depth the next day to make sure it activates
- Tooled joints: run early during finishing and re-run later so the groove doesn't bond back together
When do joints have to be cut?
Timing matters as much as depth. Cut too early and the edges ravel; too late and the slab cracks on its own first. Per NRMCA and ACI 360:
- Conventional saw cuts: within 4 to 12 hours after finishing, closer to 4 hours in hot weather and closer to 12 in cold weather
- Early-entry saws: about 1 to 4 hours after finishing, 1 hour in hot weather and 4 in cold weather
- Hard deadline: ACI 360 says sawing should be done before the slab starts cooling after its peak heat of hydration
- Raveling: if the edges ravel while sawing, wait, but not so long that random cracks beat the saw
In a Spokane fall or spring pour, the concrete sets slower in the cold, so the saw window shifts later. That's one more reason a cold-weather pour needs a plan. Our winter concrete guide covers the rest.
Do rebar and wire mesh replace control joints?
No. NRMCA says wire mesh "will not prevent cracking"; it mainly keeps cracks from opening up. It recommends cutting alternate wires, or better, stopping the mesh at contraction joints. ACI 360 adds that too much continuous rebar through a joint can keep it from opening and push the crack somewhere else. Slabs designed with enough steel to skip joints entirely exist, but Vertex notes that takes more than 0.5 percent of the slab's cross-section in steel and usually costs more than joints.
For heavy traffic, like a forklift or a loaded trailer crossing the same joint every day, dowels at the joints transfer load across them, and NRMCA recommends filling industrial floor joints with a material that supports the edges.
Joints and floor coatings
If you plan to epoxy or polyaspartic-coat a garage or shop floor later, the control joints should still be honored. Our epoxy vs. polyaspartic guide explains how coaters handle joints and cracks.
What we spell out before we pour
- Joint layout drawn on the plan, including columns, door openings and drains
- Slab thickness, PSI and reinforcement, and how the steel is handled at joints
- Saw type and timing, including the cold-weather plan
- Isolation joints at stem walls, footings, steps and where a driveway meets the garage
- Base and vapor retarder, covered in our vapor barrier guide
Cracks in an older driveway are a different question. Our driveway crack guide and repair vs. replacement guide cover those.
Get a slab quote
Planning a shop floor, garage slab, driveway or a full metal building with the slab included? Call 509-350-2016 or request a free quote, and we'll lay out the joints with you before anything is poured. See our concrete flatwork services.