Slope & Grade
How to Slope a Patio for Drainage
Slope a patio 1/4 inch per foot (2%) away from the house: how to set form heights, screed to the fall, handle two-direction slopes, and fix an existing patio that drains toward the wall.
Patio drainage is decided on form-setting day. Concrete and pavers both follow whatever plane you establish, and re-establishing a plane later is somewhere between annoying and demolition. The good news: building the fall into the forms is elementary once you see the numbers.
The numbers
| Patio width | Fall at 2% | Outer form sits lower by |
|---|---|---|
| 8 ft | 2 in | 2 in |
| 10 ft | 2.5 in | 2.5 in |
| 12 ft | 3 in | 3 in |
| 16 ft | 4 in | 4 in |
| 20 ft | 5 in | 5 in |
Convert any other slope or check an odd dimension with the slope calculator; the field measurement technique with a string line is covered in the percent grade guide.
Setting the slope, step by step
- Fix the high edge. At the house, the patio surface should sit 1-2 inches below the door threshold or siding line (4+ inches below siding in wet climates). Mark it on the wall.
- Compute the low edge. Width × 1/4 inch below the high mark. Transfer it with a string line and line level or a laser.
- Set forms to the marks. Stake every 3-4 feet; check each form top against the string as you go. The subgrade and gravel base below should parallel the finished plane so slab thickness stays constant (base rules in the base layers guide).
- Screed straight between forms. A straight screed board riding both forms produces the slope automatically. Do not "help" the middle: flat spots pond.
- Check before it sets. Lay a 4-foot level with a 1-inch block under one end anywhere on the surface; the bubble should center. For pavers, screed the bedding sand off sloped rails the same way.
Where the water goes matters too
A 2% fall delivers every gallon to the patio's low edge, so give that edge a plan: lawn or bed that can absorb it, a gravel strip, or on paved-in yards, a channel drain feeding solid pipe away. Water shed toward a low fence corner just relocates a puddle; the choice between a basin and other fixes is in catch basin vs. French drain.
Common mistakes
Sloping the forms, screeding flat. If the screed spans a sloped form and a flat ledger strip at the wall, the surface twists. Both screed rails must follow the plan.
Chasing perfect level for furniture. 2% is imperceptible for seating and tables; wobbly furniture comes from surface texture, not designed slope.
Trapping water at a step. A step-down landing at the door needs its own tiny fall, or it becomes a bird bath against the sill.
Forgetting expansion and edge restraint. Not a drainage item, but the two other decisions locked in on form day; get all three right in one setup.
Sources & standards: quantities and factors in this guide follow the references documented in our methodology, including ACI 302/318 (concrete practice), OSHA 29 CFR 1926 Subpart P (excavation safety), ASTM aggregate designations, and the International Residential Code. Local codes, climate, and site conditions govern.
Frequently Asked Questions
How much slope does a patio need?
1/4 inch per foot, a 2% grade, falling away from the house. A 12-foot patio sits 3 inches lower at its outer edge. Less than 1/8 inch per foot risks ponding; much more than 3/8 becomes noticeable underfoot and under furniture.
Which direction should a patio slope?
Away from the house and toward somewhere that can take the water: lawn, a planting bed, a channel drain, or a catch basin. Never toward the foundation, and not toward a neighboring structure or property line.
Can a patio slope in two directions?
Yes. Large patios often crown or tilt diagonally so water leaves on two sides. Keep every path a consistent 2% and make sure the receiving edges can absorb or carry the flow.
How do I check the slope of an existing patio?
Set a 4-foot level on the surface, lift the low end until the bubble centers, and measure the gap: 1 inch under a 4-foot level is 2%. Or use the slope calculator with any rise and run you can measure.
How do I fix a patio that slopes toward the house?
Options in rising cost: cut a channel drain at the wall and pipe it away, overlay the surface to re-establish fall (pavers make this easy, concrete needs a bonded topping), or demolish and re-pour. A French drain beside a slab-on-grade patio treats the symptom, not the surface.