Why slope matters
Drainage performance comes from the entire system. Gravity moves water off the surface, the turf backing lets water through, and the base aggregates carry it away. A consistent 1 to 2 percent fall prevents puddles, odors, seam stress, freeze-thaw damage, and premature wear.
Recommended slopes by use case
- Residential and commercial lawns: 1 to 2 percent away from buildings and hardscape.
- Dog runs and high-rinse areas: 1.5 to 2 percent to speed up washdowns.
- Putting greens: 1 to 2 percent overall fall for drainage, with micro-contours as desired for play. Avoid dead-flat zones.
- On concrete or roof decks: 1 to 2 percent toward drains or scuppers. If the slab is flat, use drain panels and create directional fall to the nearest drain.
- Engineered sports fields with underdrains: often 0.5 to 1 percent cross slope per project specs. Follow stamped plans.
How to calculate slope
Quick conversions
- 1 percent = 0.12 inches per foot = 1.2 inches per 10 feet.
- 2 percent = 0.24 inches per foot = 2.4 inches per 10 feet.
- Drop = Run (feet) x 0.12 inches for 1 percent. Double it for 2 percent.
Example
For a 25 foot run at 2 percent, target 6 inches of total fall because 25 x 0.24 inches = 6 inches. For a 30 foot run at 1.5 percent, target 4.32 inches because 30 x 0.18 inches = 5.4 inches; verify the math to match your site plan.
Build the base to drain
- Subgrade: Shape the native soil to the target fall away from structures. Remove organics, proof-roll, and compact to a firm, stable plane.
- Separation fabric: Install a non-woven geotextile over soils that migrate or pump to keep fines out of the base.
- Drainage aggregate: Place 3 to 4 inches of free-draining angular stone, commonly 3/8 to 3/4 inch. For drive-on or heavy-use areas, use 4 to 6 inches. Compact in lifts to 90 to 95 percent and screed to final grade.
- Screed layer: If needed, top with 1/4 minus to fine-tune the plane without sealing off drainage. Keep the designed fall.
- Edges and restraints: Set perimeters so the low side does not trap water. Leave weep paths where curbs meet the low edge.
- Optional drains: Add French drains, perforated pipe, or slot drains at the low side or swales to handle large catchment areas or tight soils.
Working on flat or tight sites
- Create a shallow swale at the low edge to a safe outfall.
- Install drain panels or board over flat concrete and tie to existing drains.
- Use a narrow French drain where grade cannot be changed near foundations.
- Never pitch turf toward a structure. If necessary, split the area and slope each half away from the building.
Verification and tolerances
- Tools: laser or rotary level, digital level, string line, and a 10 foot straightedge.
- Water test: Hose down the base before turf and again after infill. Water should move to the low edge with no standing puddles.
- Tolerances: Keep a consistent fall within the 1 to 2 percent target and eliminate birdbaths. Correct dips before turf goes down.
Common mistakes to avoid
- Relying on turf perforations alone without a graded, free-draining base.
- Letting the low perimeter curb block discharge.
- Creating bellies at seams, around sprinklers, or under stepping pads.
- Over-compacting fines on top so the surface seals and sheds water randomly.
- Ignoring clay soils or high water tables that need underdrains.
When to involve a pro
Bring in an experienced installer or engineer when you face long runs over 50 feet, clay or expansive soils, high water tables, ADA transitions, complex hardscape interfaces, or sports field specifications. Getting the slope and base right the first time saves rework and protects your investment.
