Where to Put Downspout Water: The Cheapest Fix in Home Ownership

If we could persuade every homeowner in Union County to do exactly one thing, it would be this one, and it is not a job we would be paid for.

Start with the volume, because nobody believes it

One inch of rain on a square foot of roof produces about 0.623 gallons. That is the whole calculation; everything else is multiplication.

• 1,200 sq ft roof, one inch of rain: about 748 gallons

• 2,000 sq ft roof, one inch of rain: about 1,247 gallons

• 2,400 sq ft roof, one inch of rain: about 1,496 gallons

• 2,000 sq ft roof, three inches of rain: about 3,740 gallons

Split a 2,000 square foot roof between four downspouts and each one delivers roughly 312 gallons in an ordinary one-inch rain. That is about five full bathtubs, arriving in an hour or two, landing wherever that downspout lets go.

People underestimate this by roughly a factor of ten, which is why a downspout emptying at the corner of the house looks harmless and is not.

Where it should go

Far enough that the water is leaving rather than soaking into the backfill against your foundation, and to a place where it can keep moving.

The relevant reference point is the building code requirement for the ground itself: grade must fall a minimum of six inches within the first ten feet away from the foundation, and paving within ten feet must fall at least two percent away. That ten-foot zone is the ground the code cares about — so getting concentrated roof water past it, onto ground that is still falling away, is the target.

In practice that means: past the backfill, past any bed you have built up, and onto turf or a route that continues downhill. On many lots that is six to ten feet. On a tight side yard with a fence and a neighbour it can be considerably further, and that is where the job stops being simple.

The options, cheapest first

Flexible extension on the ground

Ugly, effective, costs almost nothing, and you can move it. If you do nothing else, do this. The failure mode is that people disconnect them to mow and forget to put them back, so check them in the autumn.

Rigid extension, buried shallow, to daylight

Solid pipe — not perforated — in a shallow trench with continuous fall, opening onto ground that is visibly lower. This is the standard fix and it works for decades if the outlet stays clear. Two rules: the pipe must be solid so it does not leak the water back out along the way, and you must be able to see the outlet.

Pop-up emitter

A spring-loaded cap that opens under flow and sits flush when dry. Tidy, mowable, and it works when the line has enough fall and the emitter is above the surrounding grade. It fails when it is set in a low spot, because then it becomes an inlet in reverse and silt gets in.

Dry well

Last resort on Piedmont clay, and often the wrong tool. A dry well relies on water soaking away. If your ground drains poorly — and much of Union County's does — you are building a buried container that fills and stays full. Before anybody sells you one, dig a hole, fill it with water, and see whether it drains. If it is still holding the next morning, a dry well is not your answer.

The four mistakes we see most

• Perforated pipe used for the carry. It lets the water out along the route, which is precisely what you did not want.

• No fall. A buried line that runs level silts up and then backs up to the house. One percent minimum, two percent target — over fifty feet that is twelve inches of drop.

• An outlet nobody can find. An outlet buried in grass is a drain that no longer works. Mark it. Walk to it once a year.

• Discharging onto the neighbour. Concentrating your roof water and putting it across a boundary is how disputes start.

Do this in the rain

Go outside during an actual storm and watch each downspout. You will learn in five minutes what no dry-day inspection can tell you: whether the gutter is overflowing before the water even reaches the downspout, whether the extension is doing anything, and where the water goes once it lands.

Photograph it. If you later need a contractor, those photographs are the most useful thing you can hand them.

The Outside-the-House Water Calendar builds this into a twelve-month routine, and the Dry Creek and Swale Build Plans covers the buildable version if a surface route is what your lot needs.

If this is your yard

If the downspouts are sorted and the water is still there, that is the point where the problem is genuinely worth looking at on the ground. We work in Waxhaw, Weddington, Marvin, Wesley Chapel, Indian Trail, Matthews, Monroe, Mint Hill, Stallings and south Charlotte. Call 980-280-7638 or use the drainage page to ask for a look.

Every guide we have written is sorted by symptom on the Start Here page.

Who wrote this. Alex Purdy owns Carolina Terrain LLC in Waxhaw and holds North Carolina Landscape Contractor Licence CL.1872, the NCSU Stormwater Control Measure Inspection and Maintenance Certification (#5970), and NDS Certified Drainage Contractor status. He is not a Professional Engineer and not a Registered Landscape Architect. Nothing here is an engineered design, a legal opinion, or a substitute for either.

Sources

• Roof runoff arithmetic — 0.623 gallons per square foot per inch of rainfall; every figure above is that number multiplied out

• IRC R401.3, Drainage — grade falls a minimum of 6 inches within the first 10 feet from the foundation; impervious surfaces within 10 feet fall at least 2 percent

• Trade practice on fall — 1 percent practical minimum, 2 percent target for solid drain lines

Rules and published figures change. Everything here was checked when this was written. Before you rely on any of it for a permit, a purchase or a build, confirm it with the agency that issues it.