Minimum Slope for Drainage: The Numbers to Use

The One Number Most Drainage Failures Have in Common

You can buy the best pipe, the cleanest gravel, and the right filter fabric, and still end up with a drain that doesn't work — because water doesn't move uphill, and it barely moves on flat ground. Slope is the engine of every drainage system. Get it right and gravity does the work for free. Get it wrong and nothing else matters. A surprising share of failed Oregon drains come down to one cause: not enough fall.

This guide covers the slope numbers that actually matter and how to think about them. It's a companion to our Oregon drainage guide.

How Slope Is Measured

Slope, or grade, is just rise over run — how much the ground or pipe drops over a horizontal distance. It's expressed two common ways:

• As a percent — a 1 percent slope drops 1 foot over 100 feet (or about 1/8 inch per foot).
• As a ratio or inches per foot — handy for shorter runs.

The conversions worth memorizing: 1 percent ≈ 1/8 inch per foot, and 2 percent ≈ 1/4 inch per foot. With those two, you can check most drainage grades in your head.

Surface Grading: 1 Percent Minimum

For water to run across the ground — a lawn, a swale, a graded yard — aim for a minimum of 1 percent slope. Below that, water stalls, ponds, and sits. One percent is the floor; on lawns and broad areas, 2 percent (1/4 inch per foot) drains more reliably and is a common target.

The flatter you go, the less margin you have. A yard graded at exactly 1 percent leaves no room for the small settling and irregularities that happen over time, which can create low spots that pond. A bit more slope buys durability.

Drain Pipe: 0.5 to 1 Percent

Buried drain pipe needs a minimum fall of about 0.5 to 1 percent to carry water and stay self-cleaning. That self-cleaning part matters: at adequate slope, water moves fast enough to keep sediment suspended and flush it through. Too flat, and water creeps along, dropping sediment that slowly clogs the pipe.

There's an upper bound too. Very steep pipe can actually run too fast, letting water outrun the solids and leave them behind — though for most residential drainage, getting enough slope is the real challenge, not too much. Slope and pipe size work together; our drain pipe sizing guide covers how a flatter run may need a larger pipe to carry the same water.

Away From the Foundation: 2 Percent

This is the most important slope on most properties. The ground around a building should fall away at roughly 2 percent — a 6-inch drop within the first 10 feet from the foundation. That's enough to reliably shed rain away from the house instead of letting it pond against the wall and seep in.

When the grade is flat or, worse, tilts back toward the building, you have negative grade — one of the leading causes of foundation water in Oregon. Our negative grade foundation fix guide covers correcting it.

Quick Reference

Application	Target Slope	In Inches
Surface grading (minimum)	1%	~1/8 in per ft
Lawn / broad area (preferred)	2%	~1/4 in per ft
Drain pipe	0.5–1%	~1/16–1/8 in per ft
Away from foundation	2% (6 in / 10 ft)	~1/4 in per ft

These are general targets. Soil, outfall location, and the specific system can shift them, which is what a site assessment sorts out.

What Happens on a Flat Site

Oregon has plenty of flat valley-floor and built-out lots where getting slope is genuinely hard. When you can't establish enough fall:

• Excavate deeper at the outfall end to create artificial slope, where depth allows.
• Upsize the pipe so it carries the required water at a gentler grade.
• Use a pop-up emitter to discharge a barely-sloped line at the surface.
• Add a pump as a last resort, when terrain forces water uphill to any outfall.

The flatter the site, the more the design has to compensate for what gravity won't provide. This is exactly where professional grading earns its keep — laser levels and careful excavation can find or build slope that isn't obvious by eye. Our French drain cost in Oregon guide notes how difficult slope conditions affect cost.

Why "Close Enough" Isn't

Slope is unforgiving because it's cumulative and invisible once buried. A pipe that's a hair too flat doesn't fail dramatically — it just slowly silts up and quits, and by the time you notice, the fix means digging it back up. Measuring and setting grade correctly the first time, with proper tools rather than eyeballing, is the cheapest insurance in drainage.