How to Form and Pour a Concrete Curb

What you're building, and how big

A curb is a narrow, upright ribbon of concrete. The common dimensions for a landscape or driveway-edge curb are 6 inches wide at the top and 6 inches of exposed height, though a lighter garden curb or mow strip can run 4 to 5 inches wide. Contractors forming curb and gutter with wood typically set the back board from a 2x12 and the front from a 2x6, spacing them about 6 inches apart, and excavate roughly a foot deep to fit the gravel and concrete (Hunker).

Depth is where people cut corners and regret it. In a mild climate, a garden curb only needs to sit on 4 to 5 inches of concrete over compacted gravel. If you get real winters, the base should reach below your local frost line or frost heave will crack it and push it out of line. Frost depth swings from about 5 inches in California to 42 to 60 inches in Minnesota, and the rule for anything structural is to dig 4 to 6 inches deeper than the frost line (Family Handyman). A decorative curb is not a house footing, but a deeper, wider footing at the base is cheap insurance in cold ground.

Tools and materials

• Marking paint and mason's string line
• Shovel, mattock, and a hand tamper or plate compactor
• Gravel (3/4-inch crushed) for the base
• Concrete curb forms: bender board, 1/4-inch plywood ripped into strips, or flexible plastic form material for curves; 2x lumber for straight runs
• 1x2 or 2x wood stakes, deck screws, and a drill
• Rebar (#3 or #4) or wire for reinforcement on longer or load-bearing curbs
• Bagged high-strength concrete or ready-mix
• Wheelbarrow or mixing tub, a margin trowel, an edger, a float, and a broom
• Curing plastic or a spray bottle

Step by step: how to form and pour the curb

Follow these in order. The forming steps matter as much as the pour.

1. Lay out the line. Mark the curb path with spray paint, then set a mason's string line for straight runs and to guide smooth curves. Contractors treat layout and ground prep as most of the job, so take your time here (Curb Depot). Mark both edges so you know your trench width.
2. Dig the trench. Excavate along your line to the depth your climate needs, 4 to 5 inches for a mild-climate garden curb, deeper and below frost for a driveway or cold-climate curb. Keep the trench a couple inches wider than the finished curb so the forms have room.
3. Build and set the forms. For straight sections, screw 2x lumber to stakes driven every 2 to 3 feet. For curves, bender board or 1/4-inch plywood strips flex to the shape; back them with stakes and a bit of packed dirt. Set both faces to your finished width (about 6 inches) and to the height you want the curb to stand, then coat the inside faces with form oil or cooking oil so the concrete releases cleanly. Check the tops with a level, since the form tops are your screed guide.
4. Add and compact the base. Lay 4 to 6 inches of 3/4-inch crushed gravel in the trench and compact it hard with a hand tamper or plate compactor. A solid, level base is what keeps the curb from settling and cracking later (Hunker).
5. Set reinforcement (optional but smart). For a short garden curb, fiber-reinforced mix is usually enough. For a driveway curb, a long run, or anywhere it takes wheel loads, run one or two lengths of #3 or #4 rebar along the curb, held up on small chairs so the steel sits in the middle of the concrete, not on the gravel. Wire is a lighter alternative for crack control.
6. Mix the concrete. Use a curbing concrete mix in the 3,500 to 4,000 PSI range and keep it thick but workable, not soupy (Curb Depot). A low slump holds its shape on the narrow curb top. If you are hand-mixing bagged concrete, do small batches to about the consistency of stiff cake batter, so you can pour before it stiffens (Woodshop Diaries). A 200-foot curb runs roughly $340 to $500 in bagged material.
7. Place and consolidate. Shovel the mix into the forms and work it with the shovel or a piece of rebar to knock out air pockets, especially against the form faces. Fill to slightly above the form tops.
8. Screed the top. Drag a straight board across the form tops to strike off the excess and bring the top flush. Fill any low spots and screed again.
9. Float and edge. Once the surface water sheen is gone and the concrete has firmed up, float the top smooth, then run an edger along both top corners to round them. Rounded edges shed water and resist chipping.
10. Tool in control joints. Cut or tool a control joint every 8 to 10 feet so the curb cracks where you want it to, not randomly (Curb Depot). On decorative curbs, a shallow trowel cut about 1/4 inch deep does the same job (Woodshop Diaries).
11. Finish the face and top. For grip, drag a broom lightly across the top. For a cleaner look, tool it smooth with the float. Touch up the face after you strip the forms.
12. Cure it. Keep the concrete damp for the first several days. Cover it with plastic or mist it, since curbing benefits from a moist cure of up to 7 days for full strength (Curb Depot).

When to strip the forms and use the curb

Do not rush the forms off. Most DIY curbs are left to cure about 3 to 4 days before the forms come off, so the green concrete does not slump or spall at the edges (Woodshop Diaries). After stripping, patch any small voids on the face with a slurry and keep curing.

Concrete reaches roughly 70 percent of its strength in the first week and keeps gaining for about 28 days. A garden curb can take foot traffic and light landscaping within a few days. A curb that takes vehicle tires, like a driveway edge, should wait the full month before you drive on or against it.

Formula for how much concrete you need

Curb volume is cross-section times length. Multiply width by height for the square-foot cross-section, multiply by the length in feet, then divide by 27 to convert cubic feet to cubic yards. For a 6-inch wide by 8-inch tall curb (0.5 ft by 0.67 ft) running 40 feet, that is 0.5 x 0.67 x 40, or about 13.4 cubic feet, roughly 0.5 cubic yards. Add about 10 percent for the base footing and spillage.

Rather than run the math by hand, drop your dimensions into the concrete curb calculator and it will give you the exact yardage and bag count. If you are mixing this with other pours, the how much concrete do I need guide covers the full method.

Sources

• Hunker: How To Form A Concrete Curb and Gutter Using Wood
• Woodshop Diaries: How to Make a Concrete Landscape Curb
• Curb Depot: How to Pour Concrete Curbing DIY Guide
• Family Handyman: How to Pour Frost-Proof Concrete Footings