Patio slope and drainage
A patio has two jobs and only one of them is visible. The surface has to be flat enough to set a chair on, and the water has to leave. Those pull in opposite directions, and the compromise is a slope of about 1½ to 2 percent — roughly 3/16 to 1/4 inch per foot.
How much fall
Slope is stated three ways and they are the same thing: a percentage, inches per foot, or a ratio. One percent is 1/8 inch per foot. Two percent is 1/4 inch per foot. The minimum that drains reliably on a hard surface is about 1 percent, and the practical target is 1½ to 2 percent.
| Slope | Per foot | Over 10 ft | Where it works |
|---|---|---|---|
| 0.5% | 1/16″ | 0.6″ | Too flat. Water sheets and ponds in low spots. |
| 1% | 1/8″ | 1.2″ | Minimum on pavers laid tight, if the subgrade is graded correctly. |
| 1.5% | 3/16″ | 1.8″ | The usual compromise. Reads as level underfoot. |
| 2% | 1/4″ | 2.4″ | Reliable drainage. Noticeable on a long axis. |
| 3%+ | 3/8″ | 3.6″ | Needed on clay or where the outlet is restricted. Starts to feel sloped. |
The residential code's surface drainage rule is written for the ground beside a foundation, not for the patio surface itself: the ground must fall away from the foundation at least 6 inches within the first 10 feet. That is 5 percent, and it is why the strip of ground along the house is steeper than the patio behind it. A patio that meets the house at 1 percent is relying on the wall and the flashing above it to shed what it sends sideways.
Where the water goes
Slope without an outlet is a design that moves water from one place to another and calls it drainage. Before setting the first edge restraint, answer three questions.
- What is the low point? Every patio has one. It should be at the open edge, not in the middle and not against the house.
- Where does it discharge to? A lawn, a swale, a dry well, a storm connection, or a daylighted pipe. If the answer is "the neighbour's yard", the answer is wrong.
- What happens when the ground is saturated? In clay, a patio at 2 percent still ponds at the edge if the soil beneath it will not accept the water. The slope only buys time.
On a patio that is enclosed on three sides, 2 percent toward the open edge is not enough on its own: water collects at the corners because the surface is a plane and the corners are the far points. A linear channel drain at the low edge, or a slight cross-fall to one corner plus a drain there, is what actually works.
A worked slope
A 16 × 20 ft patio sloping away from the house along the 16 ft dimension at 1.5 percent:
- Fall = 16 ft × 0.015 = 0.24 ft, or about 2.9 inches
- At 2 percent: 16 × 0.02 = 0.32 ft = 3.8 inches
- The difference between the two over the same width is about one inch — less than most people expect, and the reason 1.5 percent is usually enough.
Building the slope into the base
The slope belongs in the compacted subgrade and the gravel base, not in a thick sand bed. Sand is for bedding, an inch or so; a sand layer thick enough to create the fall will migrate and settle unevenly within a season.
- Set the fall on the subgrade by excavating deeper at the low edge. A laser level or a string line on stakes is the only reliable way to hold it.
- Carry the same fall through the base at 4 inches of compacted gravel, then keep the bedding sand at a constant 1 inch.
- Set edge restraint to the finished slope, not level. A level restraint on a sloped patio leaves a lip at the low edge that trips and holds water.
- Check with a 4 ft level and a tape as you go. 1.5 percent is 5/8 inch of fall across a 4 ft level — small enough that eyeballing it will not catch a reversal.
Where the slope goes wrong
| Symptom | Usual cause |
|---|---|
| Puddle in the middle of the field | Base not compacted in lifts, or a low spot from a stone that was never levelled. |
| Water against the house | Slope reversed at the wall, or a level first course used as a visual datum. |
| Ponding at the low edge | No outlet. The slope is doing its job and the water has nowhere to go. |
| Pavers heaving after one winter | Water trapped in the base. Usually a missing geotextile or a base that does not drain. |
| Efflorescence on the pavers | Water moving through the units from below, which is a drainage problem, not a cleaning problem. |
What slope alone will not fix
- It does not calculate a drainage system. Sizing a channel drain or a dry well depends on the tributary area, the rainfall intensity for your location, and the infiltration rate of your soil. Those are three inputs this page does not have.
- It does not cover a patio below a slope. Where the patio sits below the surrounding grade, you need a retaining edge and a waterproof detail, not just a fall.
- It does not replace a local drainage bylaw. Some jurisdictions restrict how much runoff may leave a lot, and a hard patio increases it.
Questions people actually ask
Questions people actually ask
What is the minimum slope for a patio?
One percent, or 1/8 inch per foot, on a hard surface with a well-graded base and a clear outlet. Below that, surface tension and small undulations hold water. Most builders use 1½ percent as the working figure.
Should a patio slope away from the house?
Yes, always, in both directions if it touches the house on two sides. The fall should run to an open edge or a drain, never toward the wall, regardless of what the flashing detail looks like.
How do I slope a patio that butts against the house?
Set the top of the patio at least one paver thickness below the damp course, then fall away from the wall at 1½ to 2 percent. The gap against the wall is filled with a flexible sealant, not mortar.
Does a paver patio need a channel drain?
Only where water cannot leave at an open edge — an enclosed courtyard, a patio against a wall on three sides, or a surface that meets a door threshold. On an open patio, slope to the edge is enough.