Drop is slope times run, and the run is longer than people think
Fall is quoted as a slope, usually in inches per foot, and the total drop is that slope multiplied by the length of the run. A quarter inch per foot over a sixteen foot patio is four inches of drop, which is a lot more than most people picture when they agree to a quarter inch per foot. Over a twenty-four foot run it is six inches, and now the far edge is half a foot below the near edge and something has to happen there: a step, a kerb, a regraded lawn, or a retaining edge.
This is why the drop should be worked out before the height at the building is fixed, rather than after. The height at the building, the length of the run and the level of whatever the far edge has to meet are three constraints and the slope is what connects them. Pick any two and the third is decided. Most people fix the building end first because it is the obvious starting point, then discover at the far end that the drop has nowhere to go.
Direction is not negotiable
A surface next to a building falls away from it. That is the whole of the rule and there is no version of a paved surface next to a house where falling toward it is acceptable. Water arriving at the wall goes to the foundation, into whatever detail sits at grade, and eventually under the siding or the threshold, because paving settles slightly and grade rises slightly over the years and the margin between them is what you built in on day one.
An existing patio that falls the wrong way is a repair. On pavers it is lift, regrade the base, relay. On concrete it is either replacement or cutting in a linear drain along the building edge and piping it away to somewhere it can be discharged, which costs more, needs an outlet, and still leaves you with a surface that is trying to send water at the house and a drain that has to catch all of it every time. If a contractor proposes to fix it by adding a strip of paving that slopes the other way, ask where the water in the middle goes.
How much fall, and the two ways to get it wrong
A quarter inch per foot, which is about two percent, is the figure most hardscape work is set to. The reason is not that water needs that much slope to move; it needs far less. The reason is tolerance. A screeded sand bed and a floated slab both come out of the work with variation in them, the paving settles a little in its first season, and if the design slope is smaller than that variation, the surface will have low spots that pond regardless of what the drawing said.
| Fall per foot | Percent | Drop over 16 ft | Character |
|---|---|---|---|
| 1/8" | 1.04% | 2" | Minimum many installers will accept, unforgiving of any settlement |
| 3/16" | 1.56% | 3" | Used where the total drop has to be limited |
| 1/4" | 2.08% | 4" | The common hardscape figure |
| 3/8" | 3.13% | 6" | Sheds hard, starts to be noticeable underfoot |
| 1/2" | 4.17% | 8" | Furniture will not sit level |
The other failure is too much. Above about four percent a patio stops feeling flat. Chairs rock, a table needs shimming, and a drink set down on it tells you exactly what the slope is. If a long run means the drop at a comfortable slope is more than the site can absorb, the answer is usually to break the surface into planes falling to a linear drain, or to introduce a step, rather than to flatten the whole thing until it ponds.
Cross fall, and the diagonal nobody checks
Adding a second fall across the width steers water toward a corner or a drain rather than spilling evenly along the whole low edge. It is useful, and it has a consequence people miss: with fall in two directions at once, the steepest line on the surface is the diagonal, and it is steeper than either component. Combining a quarter inch per foot away from the house with a quarter inch per foot across the width gives a diagonal of about a third of an inch per foot. That is what a level laid across the corner will read, and it is what furniture standing near the corner will feel.
The calculator reports that diagonal so it is not a surprise on site. If it comes out steeper than you want, reduce the cross fall rather than the main fall, because the main fall is the one keeping water off the building.
Getting the slope into the ground
Fall belongs in the base. The base is graded to the finished slope, the bedding layer goes on at a constant thickness over it, and the surface course follows. What does not work is building a flat base and trying to create the slope by varying the bedding depth, because a thick bedding layer consolidates unevenly under traffic and the surface goes wavy within a season. The bedding layer is a bedding layer, not a levelling compound.
On site the slope is usually held with string lines set to the drop at each station, or with screed rails pitched to the fall, and the station table above gives those figures directly. A screed board can also be run off a level by shimming one end, and the shim thickness for the board length you are using is in the results. Whichever method, check the finished surface with water rather than trusting the setout: a hose run over a finished patio finds the low spots that a level between two points does not.
Base depth and compaction, which decide whether the fall you set is still there in five years, depend on the soil, on how the ground drains and on what crosses the surface, and are not something a page can specify. Foot traffic and vehicle traffic are different specifications by a wide margin. Where the fall discharges also has to be answered, because moving water off a patio and into a spot that already floods has solved nothing. For the quantities under the surface, see the paver patio calculator; for what to do with the water once it arrives at the low edge, the french drain calculator and the permeable paving runoff calculator. If the drop at the far edge has become a retaining problem rather than a grading one, note that walls above a modest height are engineered structures, and this site gives no design guidance on them.
Questions people ask
What slope should a patio have?
A quarter inch per foot away from the building is the figure most hardscape is set to, which is about two percent. It is not a rule handed down from anywhere; it is the slope that reliably sheds water despite the tolerance of the work and the settlement that follows. Less than about one percent and normal variation in the finished surface produces ponding. More than about four percent and the surface stops feeling flat underfoot. Between those, the slope you choose is usually decided by how much total drop the site can absorb over the run.
My existing patio slopes toward the house. What are my options?
On a paver surface, lift the pavers, regrade and recompact the base to fall away, and relay. It is a weekend of work over a small area and it is the correct fix. On a concrete slab the choices are worse: replace it, or cut a linear drain along the building edge and pipe the water somewhere it can legally discharge. The drain is not a cheap fix and it depends on continuing to work, on being kept clear, and on having an outlet. Whichever route, do not let anyone talk you into a self-levelling topping to reverse a slope; the thickness needed at one end is not what those products are for.
Can I get the fall by making the bedding sand thicker at one end?
No. Bedding sand is laid at a constant thickness, typically about an inch, over a base that already carries the slope. A wedge of bedding sand consolidates by different amounts along its length under traffic, so the surface that started true develops a dip at the thick end within a season. The same applies to trying to fix a low spot in the base by adding bedding sand over it. If the base is not at the right grade, the base is what you fix.
How do I set the fall on site with a string line?
Set a level string line between two stakes at the high edge and the low edge, then measure down from the string by the drop figure for each station, which is the table in the results. Working from a level line and measuring down is more reliable than trying to set a sloped string, because a level line can be verified with a line level at any point along it. Check the finished surface with a hose before the joints are sanded. A hose finds low spots that a level laid between two high points will step straight over.
What about the height where the patio meets the house?
It has to be low enough that the paved surface stays clear of the sill plate, siding, weep courses and any threshold detail, and that clearance is set by how the wall is built and by the local building department rather than by a general figure. It is worth confirming before the base is graded, because a patio built too high against a wall cannot be lowered without lifting it. Remember to allow for the paving surface finishing above the base, and for the fact that the surrounding ground will not settle at the same rate the paving does.