Drainage
Downspout Drainage: How to Move Roof Water Away
A 1,500 sq ft roof sheds about 935 gallons per inch of rain. Move it away from the foundation with extensions, splash blocks, buried solid pipe, and pop-up emitters: sizing, slope, and winter rules.
Most wet basements and soggy foundation beds are roof water problems wearing a disguise. A modest roof concentrates hundreds of gallons per storm at a few discharge points, and if those points sit beside the foundation, the soil there never dries. Fixing downspouts is the cheapest drainage work you will ever do, and it comes before any French drain: intercepting water you could simply have routed away is building a drain to solve a plumbing problem.
How much water you are moving
A 1,500 sq ft roof in a 1-inch storm sheds about 935 gallons. Split across four downspouts, each one delivers roughly 230 gallons, a bathtub and a half, to one spot. In a 3-inch storm, triple it.
The solutions, cheapest first
| Solution | Cost | Best for | Weakness |
|---|---|---|---|
| Splash block | $10-$25 | Good grade already falling away | Only moves water 2-3 ft |
| Hinged/roll-out extension | $15-$40 | Renters, quick fixes, mowable yards | Ugly; gets moved and lost |
| Buried solid pipe to daylight | $50-$150 DIY per run | Sloped lots with somewhere lower | Needs continuous fall to an exit |
| Buried pipe to pop-up emitter | $75-$200 DIY per run | Flat-ish yards; invisible result | Emitter needs seasonal checking |
| Buried pipe to dry well | $300-$1,500 | No downhill exit anywhere | Sized to soil; see the dry well guide |
Building a buried line right
- Call 811, then trench 12-18 inches deep from the downspout, falling continuously at 1-2% (check with the slope calculator; 2% is 1/4 inch per foot).
- Use 4-inch solid pipe. Smooth-wall PVC beats corrugated: fewer clogs, jettable, mower-proof fittings. One downspout per 4-inch line is clean practice; two small ones can share.
- Adapt the downspout with a rectangular-to-round adapter and a debris strainer or cleanout tee at the top. This is where clogs start; make it serviceable.
- Terminate properly. Daylight with a rodent grate, a pop-up emitter set in a small gravel pocket, or a dry well. Discharge onto your own property, away from walks and the neighbor's line; many towns regulate this.
- Backfill and mark the run. Photos before backfill save future-you an afternoon of probing.
Common mistakes
Perforated pipe at the house. It waters the foundation along the whole run. Solid pipe for transport, always.
Connecting downspouts into a French drain. Roof surges overwhelm and silt up a drain built for groundwater. Run roof water in its own solid line, even if it shares the trench.
No slope discipline. A belly in the line holds water, collects grit, and freezes in winter. String-line the trench like any drain.
Fixing the downspout, ignoring the grade. If ground still tilts toward the house, surface water remains; pair this work with the checks in the standing water guide and the 5% foundation grade rule.
Sources & standards: quantities and factors in this guide follow the references documented in our methodology, including ACI 302/318 (concrete practice), OSHA 29 CFR 1926 Subpart P (excavation safety), ASTM aggregate designations, and the International Residential Code. Local codes, climate, and site conditions govern.
Frequently Asked Questions
How far should downspouts discharge from the house?
Minimum 4-6 feet for splash blocks and extensions; 10 feet is better on clay soils or where grading is marginal. Buried pipe should carry water at least 10 feet out or to daylight, whichever is farther.
Should downspout drain pipe be perforated or solid?
Solid. The job is transport, not collection: perforated pipe leaks roof water into the soil along the run, including right beside the foundation. Save perforated pipe for French drains.
How much water comes off a roof?
About 0.62 gallons per square foot per inch of rain. A 1,500 sq ft roof sheds roughly 935 gallons in a 1-inch storm, concentrated at 3-4 downspouts. That is why one bad downspout can flood a basement corner.
Do buried downspout lines clog?
They can: leaves, shingle grit, and roots are the usual causes. Install gutter guards or a leaf strainer at the inlet, use smooth-wall PVC over corrugated where possible, keep 1-2% slope, and add a cleanout at the first bend.
What about winter freezing?
Buried lines hold some water at fittings and low spots and can freeze shut in cold climates. Keep 1-2% continuous slope so lines drain empty, use frost-tolerant pop-up emitters, and in severe climates add a winter diverter so water can exit above ground when the buried line is iced.