What Slope Do You Need for Patio Drainage?
The short answer
A patio needs a minimum slope of ¼ inch per foot - about a 2% grade - falling away from the house. On a 12-foot patio that’s 3 inches of total drop (about 7.6 cm over 3.7 m). What the bands mean in practice:
- 2% and steeper (¼ in per foot and up) - good drainage. This is the standard rule for hard surfaces: patios, walkways, driveways.
- 1% to 2% - marginal. Water moves, but slowly, and any low spot or rough texture will hold it.
- Under 1% - too flat. Water ponds instead of shedding.
So ¼ inch per foot isn’t a nice-to-have - it’s the floor. Anything flatter and you’re planning for puddles, algae, and (in cold climates) ice sheets right where you walk.
How much fall your patio needs
The arithmetic is one line: fall (inches) = patio run (feet) × ¼. Measured along the direction the water should travel:
| Patio run (from house outward) | Fall needed at ¼ in/ft |
|---|---|
| 8 ft (2.4 m) | 2 in (5.1 cm) |
| 10 ft (3.0 m) | 2.5 in (6.4 cm) |
| 12 ft (3.7 m) | 3 in (7.6 cm) |
| 14 ft (4.3 m) | 3.5 in (8.9 cm) |
| 16 ft (4.9 m) | 4 in (10.2 cm) |
| 20 ft (6.1 m) | 5 in (12.7 cm) |
For metric planning, 2% is simply 2 cm of fall per meter of run.
Worked example (12-foot patio)
Say the patio runs 12 feet out from the back door. Here’s the full slope math:
- Fall needed: 12 ft × ¼ in = 3 in of drop from the house edge to the outer edge.
- As a percent grade: the run is 12 ft × 12 = 144 in, so 3 ÷ 144 × 100 = 2.08% - just over the 2% line, which is exactly where you want to land.
- As a ratio: 144 ÷ 3 = 48, so the slope is 1:48.
- As an angle: arctan(3 ÷ 144) ≈ 1.2° - far too small to see by eye, which is why you measure it (see Watch-outs).
👉 The Drainage Slope Calculator does this instantly - enter the run in feet and the fall in inches and it returns the percent grade, fall per foot, ratio and angle, and flags whether your slope clears the ¼ inch per foot rule or falls into the marginal or ponding band.
Slope conversion cheat sheet
The same slope shows up on plans as inches per foot, a percent, a ratio, or an angle. Here are the common drainage grades in all four forms (computed over a 10-foot run):
| Fall per foot | Percent grade | Ratio | Angle | What it’s for |
|---|---|---|---|---|
| ⅛ in/ft | 1.04% | 1:96 | 0.6° | Ponding threshold - the bare minimum for water to move |
| ¼ in/ft | 2.08% | 1:48 | 1.2° | Standard minimum for patios, walks, driveways |
| ½ in/ft | 4.17% | 1:24 | 2.4° | Fast-shedding surfaces, splash zones |
| 6 in over first 10 ft | 5.00% | 1:20 | 2.9° | Ground against a foundation (US IRC R401.3) |
Handy rules of thumb from that table: 1% is about ⅛ inch per foot, and 2% is about ¼ inch per foot.
Which way should the patio slope?
Always away from the house - toward the lawn, a swale, or a drain, never toward the foundation or a neighbor’s lot. Two related targets:
- The patio surface: ¼ in per foot (about 2%) minimum, sloping outward in one consistent plane.
- The soil past the patio and around the foundation: US IRC R401.3 calls for the ground to fall 6 inches within the first 10 feet from the foundation - about a 5% grade (15 cm over 3 m). If your patio butts against the house, its 2% slope handles the surface water, but the ground where that water lands still needs somewhere to go.
Local grading rules can be stricter than either number, so check before you dig.
Watch-outs
- You cannot eyeball 2%. A 1.2° tilt is invisible. Stretch a string between two stakes, level it with a line level (a small bubble level that hangs on the string), then measure down from the string to the ground at each end - the difference is your fall.
- Measure along the flow direction. The run that matters is the horizontal distance water travels, not the patio’s longest side.
- Steeper isn’t automatically better. ½ in per foot sheds fast but the tilt starts to be noticeable, and tables and chairs may need shimming. For a sitting area, ¼ in per foot does the job without feeling sloped.
- Watch for low spots, not just the average. A patio can average 2% and still pond in a dip. Check the grade in a few lanes across the surface, especially on paver patios where settling is common.
- A safety note on regrading: moving soil near the foundation is fine DIY work, but never bury siding, weep screed, or a basement window below grade - keep the required clearance and route the water to daylight.
Related
- Drainage Slope Calculator - percent grade, fall per foot, ratio and angle from one run and fall, checked against the ¼ in/ft rule.
- Landscaping tools - the full landscaping and garden toolset.
- How far can a 2x8 deck joist span - the next number to pin down if the patio plan grows a deck.
- How to calculate shadow length - figure out how much sun that patio spot actually gets.
- Minimum roof pitch for shingles - the same slope math, applied to the roof shedding onto your yard.