Drainage Requirements for Concrete Work in New Orleans

Why Does Drainage Control Whether a Slab Lasts in Greater New Orleans?

Water trapped beneath or against a slab edge is the single largest cause of concrete failure across the metro. South shore and west bank soils are alluvial clay and organic silt deposited by centuries of river flooding. These soils hold water rather than draining it. When a slab is poured over clay that stays saturated, two things happen. First, the organic material in the soil compresses under the weight of the concrete, and compression is not uniform. One section of slab settles 1 inch while the adjacent section settles half an inch, and the result is a crack along the differential line. Second, saturated clay expands and contracts with seasonal water-table fluctuations. A water table that sits at 2 feet in April and drops to 5 feet by October creates a cycle of heave and settlement that no unreinforced slab survives.

On the north shore, St. Tammany's sandy soil drains quickly, which sounds like an advantage until surface water washes sand out from under a slab edge. The failure mode inverts: instead of settlement from below, a Slidell or Mandeville slab loses support at the perimeter as runoff erodes the subgrade. The slab cantilevers over a void, and a 4-inch residential pour has no tensile capacity to span the gap. The corner cracks and drops.

What Does Concrete Failure from Poor Drainage Look Like?

Drainage-related failure shows up as uneven settlement, edge cracking, or standing water that will not drain off the surface. On south shore residential driveways, the most common sign is a 1 to 3 inch depression at the garage apron where the original fill beneath the transition has consolidated. The slab separates from the garage floor, water ponds in the gap, and the settlement accelerates because the ponded water further saturates the subgrade.

On patios and pool decks, poor drainage shows as a slab that has tilted toward the structure instead of away from the structure. A patio poured flat rather than sloped at a minimum 1% grade sends storm runoff against the foundation wall. In Orleans Parish and Jefferson Parish, where annual rainfall averages 64 inches, that volume of water against a foundation causes interior moisture problems within 2 to 3 years.

Commercial slabs in the river parishes along St. Charles show a different pattern. Parking lots poured over river-deposited silt near the levee develop long longitudinal cracks along the path of subsurface water flow. The silt migrates with the water, creating voids 6 to 12 inches below the slab that are invisible at the surface until a loaded truck punches through.

What Drainage Specifications Does a Concrete Pour Require on the South Shore?

A south shore slab requires both surface slope and subsurface base drainage designed for soil that does not drain on its own. The standard residential specification in Jefferson Parish and Orleans Parish starts with removing organic topsoil down to stable clay, typically 8 to 14 inches of excavation. The exposed subgrade gets proof-rolled with a loaded wheelbarrow or plate compactor to identify soft spots. Any area that deflects more than 1 inch under proof-rolling gets over-excavated and replaced with compacted fill.

The drainage base is 6 inches of compacted crushed limestone, size #610 or equivalent, placed in 3-inch lifts and compacted to 95% Standard Proctor density. Limestone serves two purposes: the angular fragments interlock under compaction to create a stable bearing layer rated at 3,000 PSF or higher, and the void space between fragments allows trapped water to migrate laterally rather than sitting against the bottom of the slab. The base extends 6 inches beyond the slab edge on all sides so water exits the base freely rather than pooling at the form line.

Surface slope runs at a minimum of 1%, which is 1 inch of fall per 8 linear feet, directed away from any structure. Driveways slope toward the street. Patios slope toward the yard. Where a property line or fence prevents drainage away from the slab, a 4-inch PVC channel drain set flush with the slab surface at the low edge collects runoff and directs the water to an approved discharge point.

How Do Drainage Requirements Differ on the North Shore?

North shore drainage design prioritizes edge protection against erosion rather than subsurface water management. St. Tammany Parish soil is sandy and porous, so the water table sits deeper, often 8 to 15 feet below grade in Mandeville and Covington. Subsurface saturation is rarely the problem. Surface runoff is.

A residential slab in Slidell gets a 4-inch compacted sand base rather than the 6-inch limestone base used on the south shore, because the native soil already drains. The critical detail is what happens at the slab edge. A concrete apron or turned-down edge beam extending 8 to 12 inches below grade at the perimeter prevents runoff from undercutting the slab. Without that edge detail, rain washes sandy soil away from the slab perimeter at a rate of 1 to 2 inches per heavy storm season, and 3 to 4 years of erosion creates a void large enough to cause a corner failure.

Surface grading around a north shore slab should maintain a 2% minimum slope for the first 4 feet away from the slab edge, steeper than the 1% minimum on the south shore, because sandy soil absorbs water close to the surface and softens if the drainage gradient is too shallow. Splash blocks, downspout extensions, or French drains set 3 to 4 feet from the slab edge intercept roof runoff before the runoff reaches the concrete.

What Subsurface Drainage Systems Protect a Slab Long Term?

A French drain installed at the time of the pour costs $12 to $22 per linear foot and prevents damage that costs $4,000 to $12,000 to repair after the slab has settled. The standard subsurface drain for residential concrete in Greater New Orleans is a 4-inch perforated PVC pipe bedded in 8 to 12 inches of washed #57 stone, wrapped in filter fabric to prevent silt migration into the stone. The trench runs 12 to 18 inches deep along the low side of the slab or along any edge where water ponds.

The pipe runs at a minimum 0.5% slope to a discharge point. On the south shore, discharge options are limited because the surrounding grade is often flat. A pop-up emitter at the property line or a tie-in to the municipal storm drain, where permitted, handles the volume. In Jefferson Parish and Orleans Parish, tying into the public storm system requires coordination with the Sewerage and Water Board or the parish drainage department. Property owners should confirm discharge requirements with the parish office before installation.

For commercial pours, area drains set into the slab surface at 20 to 30 foot intervals connect to a below-grade storm pipe network. Commercial drainage plans in St. Charles Parish near the levee can require review by both the parish permitting office and the levee district. These reviews add 2 to 4 weeks to the project timeline, and concrete contractors in the river parishes account for that lead time in scheduling.

How Can a Property Owner Tell Whether Existing Drainage Is Failing?

Three visible signs indicate that drainage around a concrete slab is inadequate or has failed. The first sign is standing water against the slab edge for more than 30 minutes after rain stops. Properly graded and drained concrete sheds water within minutes. Persistent ponding means the surface slope has reversed through settlement, or the subsurface base has clogged with silt and no longer drains laterally.

The second sign is a white mineral deposit along the lower 2 to 3 inches of a slab edge, called efflorescence. Efflorescence forms when water migrates upward through the concrete by capillary action, carrying dissolved calcium carbonate to the surface. The deposit itself is cosmetic, but the water movement that causes the deposit means the slab is sitting in saturated soil. On south shore properties in Metairie, Kenner, and Chalmette, efflorescence on a slab less than 5 years old almost always indicates missing or failed base drainage.

The third sign is differential settlement visible as a gap between the slab and an adjacent structure. A driveway that has dropped 1 inch at the garage apron, a patio that has separated from the house slab by half an inch, or a sidewalk panel that has tilted 1 to 2 inches relative to the neighboring panel all indicate that water has caused uneven soil compression beneath the concrete. Once settlement reaches 1.5 to 2 inches, mud-jacking or polyurethane foam injection can sometimes lift the slab, but the cost of injection, $5 to $15 per square foot, approaches the cost of replacement, and injection does not solve the underlying drainage problem.

What Does Correct Drainage Add to a Concrete Project's Cost?

Proper drainage adds 15% to 25% to the cost of a residential concrete pour and eliminates the most common cause of early failure. On a typical 600 square foot driveway in Jefferson Parish, the concrete work alone runs $7,200 to $10,800 in 2026 pricing. Adding correct base preparation, surface grading, and perimeter French drains adds $1,200 to $2,700 to that total. The combined project comes in at $8,400 to $13,500.

Skipping drainage saves money on the initial pour. Repairing the result does not. A settled driveway that needs demolition, subgrade correction, new base, new drainage, and a new pour costs $12,000 to $18,000 for the same 600 square feet, because demolition and haul-off alone add $2,400 to $3,600. Every concrete contractor in this market has torn out slabs that were less than 8 years old because the original pour had no base drainage and no surface slope. The concrete itself was fine. The water underneath was the problem.

Commercial projects carry higher drainage costs per square foot because parish permitting, engineered drainage plans, and storm-system tie-ins add design and review fees. A 5,000 square foot commercial parking slab in St. Charles Parish with a full storm drainage system runs $18 to $28 per square foot installed, with drainage accounting for $4 to $8 of that per-square-foot cost.

A site visit before any concrete work begins identifies the soil type, the drainage pattern, and the parish requirements that apply to the specific property. New Orleans Concrete Contractors evaluates subgrade conditions, existing drainage, and grading on every project before proposing a scope of work. Contact the office to schedule a site assessment for any planned concrete pour or to evaluate drainage around an existing slab that shows signs of settlement.