Slabs and cracking
Control joints vs welded wire mesh: how they work together
Control joints tell concrete where to crack; welded wire mesh holds those cracks tight once they open. Slabs need both. For contraction joints to activate properly, saw cuts must penetrate at least 1/4 of the slab depth within 4 to 12 hours of pouring. Under WRI Detail WWR.7, wire mesh should either stop 50 mm (2 in) back from the joint or have alternate wires cut across the joint line.
Two distinct systems working in partnership
A common misconception among builders is that placing welded wire mesh allows you to skip cutting control joints, or that cutting control joints makes wire mesh unnecessary.
In reality, they perform complementary engineering functions:
- Control joints (contraction joints): Intentionally weaken the slab along straight, predetermined lines so that drying shrinkage cracks occur neatly at the bottom of the cut rather than meandering across the surface.
- Welded wire mesh (WWR): Bridges the crack below the saw cut, holding the two fractured concrete faces together under tension to maintain aggregate interlock and prevent vertical slab faulting under wheel traffic.
Without control joints, concrete will crack randomly in wide, unsightly spiderweb patterns. Without wire mesh, the controlled joints and any intermediate cracks can open wide, fill with dirt, lose aggregate interlock, and allow one slab panel to drop below the other.
Joint spacing and saw-cut depth guidelines
According to ACI 360R and CSA A23.1 Section 7.4, contraction joints must follow strict geometric limits:
| Slab Thickness | Maximum Contraction Joint Spacing | Minimum Saw-Cut Depth (1/4 Slab Depth) | Early-Entry Saw Cut Timing | Standard Diamond Blade Timing |
|---|---|---|---|---|
| 100 mm (4 in) | 2.4 to 3.0 m (8 to 10 ft) | 25 mm (1.0 in) | 1 to 4 hours after finishing | 4 to 12 hours (before final set) |
| 125 mm (5 in) | 3.0 to 3.6 m (10 to 12 ft) | 32 mm (1.25 in) | 1 to 4 hours after finishing | 4 to 12 hours |
| 150 mm (6 in) | 3.6 to 4.5 m (12 to 15 ft) | 38 mm (1.5 in) | 2 to 6 hours after finishing | 6 to 14 hours |
| 200 mm (8 in) | 4.5 to 5.5 m (15 to 18 ft) | 50 mm (2.0 in) | 2 to 6 hours after finishing | 6 to 18 hours |
The rule of thumb for joint spacing
Multiply slab thickness in inches by 2 to 2.5 to determine maximum joint spacing in feet. For example, a 4 in (100 mm) slab should have joints spaced no further than 8 to 10 ft (2.4 to 3.0 m) apart in both directions, forming roughly square panels. Avoid aspect ratios greater than 1.5 to 1 (such as 10 × 20 ft panels), which frequently crack across their midpoint.
How to detail welded wire mesh at control joints
When heavy, continuous welded wire mesh passes uninterrupted through a saw-cut joint, high tensile steel resistance can restrain the joint from opening freely. This restraint can force the slab to crack elsewhere in the panel field.
The Wire Reinforcement Institute Detail WWR.7 provides two approved detailing methods:
Option 1: Stop mesh 50 mm (2 in) clear of the joint line (Preferred for light slabs)
In residential driveways, garage pads, and unbonded toppings with joint spacings under 3.6 m (12 ft), stop the welded wire mesh sheets 50 mm (2 in) back on either side of the joint line. This leaves unreinforced concrete directly beneath the saw cut, ensuring the shrinkage crack opens reliably within the joint. Aggregate interlock beneath the 25 mm cut transfers shear loads between panels.
Option 2: Cut every second wire across the joint line (50% cut detail)
For commercial slabs and thicker floors (125 to 150 mm · 5 to 6 in) subject to forklift or heavy truck traffic, continuous shear transfer is necessary. Under WRI Detail WWR.7, run the wire mesh continuously across the joint line, but use bolt cutters to clip every alternate longitudinal wire crossing the joint line before the pour. This reduces steel restraint by 50%, allowing the joint to crack cleanly while maintaining 50% steel continuity to keep the joint tight.
Timing is critical
Cutting too early causes aggregate raveling and ragged joint edges. Cutting too late allows internal tensile stresses to accumulate, causing uncontrolled wild cracks to form before the saw blade touches the surface. In warm, dry weather, early-entry dry-cut saws allow cutting within 1 to 4 hours of finishing, significantly reducing wild cracking risks.
- Wire Reinforcement Institute, Detail WWR.7: Saw-Cut Control Joint Details. Link
- Wire Reinforcement Institute, Tech Fact TF 705-R-21: Welded Wire Reinforcement for Slabs-on-Ground. Link
- ACI 360R, Guide to Design of Slabs-on-Ground. American Concrete Institute.
- ACI 302.1R, Guide to Concrete Floor and Slab Construction. American Concrete Institute.
- CSA A23.1, Concrete materials and methods of concrete construction. CSA Group.
Questions
Can I skip control joints if I use wire mesh?
No. Welded wire mesh does not stop concrete from shrinking or cracking; it only holds cracks tight once they form. Without control joints, the slab will crack randomly across the surface.
Should wire mesh be cut at control joints?
In standard residential flatwork with close joint spacing, yes: stop the mesh 50 mm (2 in) back or clip every second wire per WRI Detail WWR.7 so the joint opens cleanly.
How deep should a saw cut be in a concrete slab?
Exactly 1/4 the depth of the slab: 25 mm (1.0 in) for a 100 mm (4 in) slab; 38 mm (1.5 in) for a 150 mm (6 in) slab.
When should you saw cut concrete joints?
As soon as the surface is firm enough to walk on without leaving footprints and without the blade raveling aggregate, typically 4 to 12 hours after finishing (or 1 to 4 hours with an early-entry saw).