How Much Should a Patio Slope for Drainage?

At least 1.5 percent, and often 2 percent on the clay and shale ground in this region.

ICPI Tech Spec 2, the Interlocking Concrete Pavement Institute's construction standard, gives 1.5 percent as the minimum slope for paved surfaces, and 2 percent cross slope for roads. A patio is not a road, but the logic is the same: water has to leave the surface quickly enough that it does not sit in the joints, soak the bedding sand and find its way into the base.

Less than that and water lingers in any slight low spot. Much more than that and a table starts to feel as if it is on a hill. The useful range for a patio is narrow, which is why levels are set with an instrument, not by eye.

Percent slope is inches of fall per hundred inches of run, so it converts easily into what you would measure on the ground.

On a patio eighteen feet deep, 2 percent means the far edge sits about four and a third inches below the edge at the house. That is invisible when you are standing on it and decisive when it rains.

Away from the house, toward somewhere the water can legitimately go.

Fayetteville's residential grading guidance is a good reference even outside the city. It asks for final grade next to a structure to fall away at five percent for at least ten feet where possible, which is six inches of fall. Where that is not possible, it asks for five percent to a swale that then carries the water away at a minimum of two percent. It also says grading should not redirect runoff onto a neighbouring property unless there is an easement or an agreement in place.

A patio against the back of the house is part of that grade. It should carry water away from the foundation, and the lawn or bed beyond it should keep carrying it, rather than the patio delivering water to a flat spot at its edge where it sits and soaks back toward the house.

Where the natural ground falls toward the house, the patio cannot fix that on its own. That is when a swale, a channel drain or a small wall comes into the design.

Because it rains a lot here and the ground does not absorb it quickly.

The National Weather Service's 1991 to 2020 normals for Drake Field in Fayetteville put average yearly precipitation at 46.96 inches. Every square foot of patio turns that rain into runoff that used to soak into the lawn, and it arrives in heavy spring and autumn storms rather than as a steady drizzle.

The ground underneath slows everything down. On the Springfield Plateau, from Springdale north, the soil is often red clay full of chert fragments left behind where the Boone Formation limestone dissolved. From Fayetteville south, the Fayetteville shale swells when it gets wet and shrinks as it dries. Neither drains freely, so water that is not carried off the surface ends up sitting in the base.

Winter adds the last reason. The same normals show the average overnight low at or below freezing on essentially every day from December through February, with the average high above freezing. Water left standing on or under a patio freezes and thaws over and over, and that is what lifts pavers and opens joints.

Into the subgrade and the base. The surface only follows what is underneath it.

ICPI's standard calls for bedding sand screeded to a uniform nominal inch, and for the finished base to be flat within three-eighths of an inch under a ten-foot straightedge. That means the fall has to exist in the compacted base before the sand goes down. Making it up in the sand layer, thick at one end and thin at the other, produces uneven settlement.

The subgrade matters for the same reason. Water that gets through the joints reaches the bottom of the base and moves along the subgrade. If the subgrade is shaped with the same fall, that water drains out. If it has a low spot, water collects there and softens it, and the patio sinks in exactly that place.

A good crew sets string lines or a laser at the planned fall, checks the subgrade against it, then checks every lift of base against it again.

Then the water has to be collected and carried away rather than simply shed, and there are several honest ways to do that.

• Channel drain. A slot drain along the house edge or across the low end, piped to daylight or to a planned discharge point.
• Swale. A shallow grassed channel beyond the patio that picks up the water and moves it along the lot.
• Area drain. A catch basin at a designed low point, piped away.
• Permeable pavers. Units with open joints over an open-graded base that stores water. Fayetteville's grading guidance lists permeable pavement or pavers among the low impact features it encourages.

Each has a cost and a maintenance habit attached, and each only works if the water it collects ends up somewhere it is allowed to go. A drain that empties onto the neighbour's lawn has moved the problem, not solved it.

Somewhere spread out and deliberate, because on the plateau water tends to go down into the rock rather than across the surface.

The Arkansas Geological Survey describes the Boone Formation as the most widespread and karst-susceptible formation in the Ozark Plateaus, with sinkholes, springs and caves, and a weathered, fissured zone near the top of the rock. Concentrating all the runoff from a large patio into one spot on that ground is not the same as spreading it across a lawn.

In practice that means discharging drains to a stable, vegetated area, spreading outlets rather than stacking them, and keeping discharge away from foundations and walls. If a lot has an existing depression or a known soft spot, it is not a drain outlet.

With a long level or a straight board and a tape, and then by watching it in the rain.

Lay a straight eight or ten-foot board across the patio, running away from the house, with a level on top. Lift the low end until the bubble is centred and measure the gap under it. Divide that gap in inches by the board length in feet and multiply by about 8.3 to get the percent. A gap of an inch and a half under a ten-foot board is about 1.25 percent, which is on the flat side.

Repeat it in a few places. A patio that is correct at one edge and flat in the middle has settled, and the settlement is usually over a soft spot in the subgrade.

The rain test is the honest one. After a proper storm, go out and look. Water should be gone from the surface within a short time. Puddles that stay, stains that mark where they sat, or joint sand washed into a heap at one corner all tell you where the water actually goes, whatever the drawings said.

The minimum stays about the same, but smoother, tighter surfaces shed water faster and rougher ones hold it longer.

Concrete pavers with sand joints shed most water off the top and let a little through the joints, which is why the base under them still needs to drain. Natural flagstone is rarely flat, so it needs the fall set generously or water sits in the dips of each stone. A poured or stamped slab sheds water well, but any low spot in a slab is permanent, because it cannot be lifted and relaid.

Pavers with turf or gravel joints behave more like a permeable surface. Water goes into the joints readily, so the base and subgrade beneath have to be built to take it and move it on.

Whatever the surface, the slope is set before the surface is chosen, because it belongs to the base.

Around a pool, the choice also affects comfort and grip, so the surface texture matters as much as how fast it sheds water. A smooth, sealed finish that drains beautifully can still be slippery underfoot when wet.

The minimum is the same, but the direction changes with the use.

A pool deck falls from the coping outward, so rain and splash carry debris away from the water, with deck drains or a channel at the outer edge on larger decks. A walkway usually falls across its width, to one side, so water leaves it along its whole length rather than running down it and collecting at the bottom step.

Driveways follow the same idea with more at stake. A driveway that carries street water toward the garage is a common problem here, and Fayetteville's guidance specifically asks for the drive to fall away from the garage where the lot sits below the road.