Yard Drainage & Grading Planner
How much water your roof and yard actually shed, whether the grade survives settlement, and what a French drain can really hold.
What settlement does to each grade
Two inches of settlement reverses a 1% grade
Over the first ten feet a 1% grade is 1.2 inches of fall. Backfill around a foundation settles two to three inches over the first few years — so settlement doesn't reduce that slope, it reverses it, and the ground ends up draining toward the house. At 2% it survives two inches and fails at three. At the 5% code asks for (six inches in ten feet) there is still three to four inches of positive fall after the same settlement. The margin in the code number isn't for flow — it's for settlement. That's the answer to why the minimum constructible grade and the required grade are different numbers.
The roof is the problem, not the lawn. One inch of rain on 1,000 sq ft is 623 gallons — and on a 2,000 sq ft roof, 1,247 gallons, over five tons, delivered to four points within a few feet of the wall. In a 2 in/hr downpour that roof sheds 42 gallons a minute, two to four times the entire water supply of the house it sits on. Hold that number in mind when deciding whether a six-foot downspout extension is doing anything useful.
A French drain is a conveyance, not a tank. Clean angular stone is about 35% voids, so a 1×2 ft trench 40 ft long holds 209 gallons — under a third of one inch on a 1,000 sq ft roof. It works by carrying water somewhere; with nowhere lower to carry it to, it's a puddle with extra steps. A dry well sized to genuinely hold a 1″ storm off a 2,000 ft roof needs 476 cu ft of gravel — a cube nearly eight feet on a side.
- Gutters overflowing at the corners means you're short of downspout, not gutter. Adding one is usually the cheapest fix on this page.
- The area a downspout serves depends on the storm. A "serves 600 sq ft" rating is tied to one assumed rainfall rate — halve the intensity and it serves twice that; double it and half.
- Get water past the drip line. Extensions want to discharge four to six feet out, further on clay. A splash block moves water about eighteen inches and hands it straight back.
- Perforated pipe goes holes down, so it picks water up off the trench bottom. Holes up means the trench must fill before the pipe does anything. Wrap the stone in filter fabric — a silted drain isn't repairable, it's a dig-it-up-again job.
- Solid pipe near the house, perforated further out. Perforated pipe is designed to leak, and beside a foundation is the last place you want that.
This sizes the obvious things; it does not diagnose a wet basement. If the grade is right, the gutters are clear and water is still arriving, the cause is below ground — a failed footing drain, a high water table, a spring, or water tracking along a service trench — and that wants someone who can survey the site rather than more arithmetic.
How to use
- Measure the fall over the first ten feet from the wall with a level and a string.
- Enter the roof footprint, which is usually the larger source of water.
- Check the grade against settlement, not just against the code number.
- Size a trench for what it must carry, not for what it can hold.
Frequently asked questions
How much should the ground slope away from a house?
Six inches over the first ten feet, which is five per cent. That is the published code minimum and it is a great deal steeper than the one to two per cent quoted for general lawn drainage — for a good reason that has nothing to do with how fast water flows.
Why does code require 5 per cent when 1 per cent drains fine?
Because of settlement rather than flow. Over ten feet a one per cent grade is 1.2 inches of fall, and backfill around a foundation settles two to three inches in the first few years — so settlement does not reduce that slope, it reverses it, and the ground ends up draining towards the house. At five per cent there is still three to four inches of positive fall after the same settlement. The margin is for settlement.
How much water comes off my roof in a storm?
One inch of rain on a thousand square feet is 623 gallons, so a 2,000 square foot roof sheds about 1,247 gallons — over five tons — in a one-inch storm. All of it arrives at four or five downspouts within a few feet of the wall, which is what makes a roof a drainage problem in a way that an equal area of lawn is not.
What flow rate does a roof produce in heavy rain?
A 2,000 square foot roof in a two-inch-an-hour downpour sheds about 42 gallons a minute. For comparison, the entire water supply of a typical house is ten to twenty gallons a minute — so during a real storm the roof is moving several times more water than the plumbing inside could. It is a useful number to hold in mind when judging whether a short downspout extension is achieving anything.
How many downspouts do I need?
Enough to carry the peak flow: a 2 by 3 inch downspout handles about 12 gallons a minute and a 3 by 4 about 25. Gutters that overflow at the corners rather than along their length are short of downspout rather than of gutter, and adding one is usually the cheapest fix available. The often-quoted coverage figures are tied to one assumed rainfall rate.
How much roof does one downspout serve?
It depends on the storm, which the rule-of-thumb figures quietly hide. A 2 by 3 inch downspout serves about 600 square feet at two inches an hour — but about 1,200 at one inch an hour and only 300 at four. Any published area rating is really a statement about a particular design rainfall, and if your storms are heavier than the one it assumed, so is your shortfall.
Does a French drain store water?
Barely. Clean angular stone is about 35 per cent voids, so a one-by-two-foot trench forty feet long holds around 209 gallons — under a third of what a single inch of rain puts on a thousand square feet of roof. A French drain is a conveyance: it works by carrying water somewhere lower, and if there is nowhere lower it is a puddle with extra steps.
How big does a dry well need to be?
Larger than almost anyone expects. Holding the runoff from a one-inch storm on a 2,000 square foot roof means about 476 cubic feet of gravel — a pit nearly eight feet on a side — because you need roughly three cubic feet of stone for every cubic foot of water. That arithmetic is why most domestic dry wells overflow, and why a route to daylight beats storage wherever the ground allows it.
Which way up do the holes go on perforated drain pipe?
Down. The pipe is there to pick water up out of the bottom of the trench, and holes at the bottom let it start doing that immediately. With the holes up, water has to fill the trench before any of it enters the pipe, which defeats the point. Wrap the stone in filter fabric as well — a silted drain is not repairable, it is a dig-it-up-again job.
How far from the house should a downspout discharge?
Four to six feet at minimum, and further on clay, onto ground that slopes away. A splash block moves water about eighteen inches and then hands it back to the same soil beside the foundation. Where the pipe runs near the house it should be solid rather than perforated, because perforated pipe is designed to leak and a foundation is the last place you want that.
Does my lawn or my roof cause the drainage problem?
Usually the roof, and not because it is larger. Runoff coefficients tell the story: a roof sheds about 95 per cent of what lands on it, flat clay about 20 per cent and sandy soil about 10. And the roof delivers its share to a handful of points rather than spread across an area, so the same volume arrives concentrated. Paving counts as roof for this purpose, which is why an added patio changes things.
My basement is still wet after fixing the grade. What now?
That is the point at which arithmetic stops helping. If the grade is right, the gutters are clear and water is still arriving, the cause is below ground — a failed footing drain, a high water table, a spring, or water tracking along a service trench — and it wants somebody who can survey the site. A persistently wet basement is a structural and health problem rather than a landscaping one.
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