Drainage Slopes That Actually Move Water

Two contractors stood on the same back patio and disagreed about whether it drained. One had a laser. The other carried a 4 foot level with a scrap of shim taped to one end, set it down four times, and settled the argument in ninety seconds. The shim was 1 inch thick, which on a 4 foot level means the bubble centers at exactly 2 percent. Three of the four readings came up short.

The numbers that actually appear on drawings

Slope gets written three different ways and the confusion is avoidable. One percent equals 1/8 inch of fall per foot. Two percent equals 1/4 inch per foot. A 4 foot level with a 1 inch shim is a 2 percent gauge; a 1/2 inch shim is a 1 percent gauge. That is the whole conversion.

1 percent is the usual stated minimum for a broom-finished concrete surface. It is a floor, not a target.
2 percent is what most designers specify for driveways, patios and lot surfaces, and what experienced crews build even when the plan says less.
5 percent over the first 10 feet away from a foundation is the residential grading rule most jurisdictions take from the model codes published by the International Code Council. That governs the soil around the house, not the slab.
2 percent maximum cross slope applies where a surface is part of an accessible route, which makes drainage a ceiling as well as a floor.

Why one percent is a paper number

Placed concrete is not a plane. Flatness tolerances allow real deviation across a slab, and a finisher chasing a bull float on a warm afternoon can leave a quarter inch of relief in a 10 foot stretch without doing anything anyone would call wrong. Build to 1 percent and a single flat spot eats the whole slope.

Then the ground moves. Settlement at the low end of a driveway steals slope from the entire run. A finisher I interviewed said he treats the drawing number as the number he is not allowed to fall below, and pours at 2 percent unless something physically prevents it. Over 20 feet, 1 percent is 2.4 inches of total fall and 2 percent is 4.8 inches.

Where the water is supposed to end up

This is the part bids skip. Slope is half a drainage design; the other half is a discharge point, and I have read plenty of quotes that specify the first and never mention the second. Water pushed off a driveway still has to go somewhere that is not a neighbor’s yard or the base course under your own slab.

Daylight to a graded swale, cheapest by far when the lot allows it. A swale wants 1 to 2 percent along its length and a rounded bottom, not a V.
Trench drain across the low edge, $60 to $150 per linear foot installed, with an outlet that actually goes somewhere.
Area drain and buried pipe, $25 to $60 per linear foot, which works until the pipe silts up and nobody has cleaned it in eight years.

Where the drive meets the street the discharge question stops being yours alone, which is the subject of aprons, gutters and the public right of way. Half the water arriving at that low edge came off the roof first, so a Lake Oswego roofing contractor will move a downspout before anyone pays to cut a trench drain to catch it.

Checking it before anyone bids

A 4 foot level with a taped shim costs about $40 and tells you slope anywhere you can set it down. A garden hose running for two minutes tells you where water actually goes, which is often a surprise. Above roughly 3,000 square feet, a topographic survey at $500 to $1,500 pays for itself, because the gap between a workable design and an expensive redo is usually 3 inches of elevation nobody measured.

Frequently asked questions

How much fall does a driveway need per foot? Plan on 1/4 inch per foot, which is 2 percent. The stated minimum for concrete is usually 1/8 inch per foot, but tolerance and later settlement both eat into it, so building at the minimum means finishing below it.

Can a trench drain fix a driveway that slopes toward the garage? Only if there is an outlet with enough fall to carry water away by gravity. A trench drain that dead-ends into a flat pipe is a holding tank. Where the garage floor sits below the street, expect regrading or a pump, at $3,000 to $9,000.

What if there is nowhere downhill to send the water? That is when dry wells, infiltration trenches or a permeable surface enter the picture, sized against how fast the soil absorbs. On tight clay the absorption rate can be low enough that infiltration is not real, and the answer becomes storage with a slow release.

Code reference: American Concrete Institute.