Carry a downspout or bury it
Written 25 Aug 2026. One downspout serving 600 square feet of roof delivers about 374 gallons in a 1-inch storm. That is the whole argument: small in the hand, a flood at the shoe. The question is not “pipe or swale” — it is whether the water can leave on the surface at 2% grade, because surface water you can see is water you can manage, and buried pipe you can't reach is debt.
Written 2026-08-25 · re-checked 2026-09-10
The arithmetic
| Step | Value |
|---|---|
| Roof share per spout | 600 sq ft (plan view) |
| 1-inch storm volume | 600 × 1/12 × 7.48 ≈ 374 gal |
| Carry it 10 ft at 2% grade | Fall = 2.4 in — a shallow swale carries this easily |
| Bury it instead | 4-in pipe at 1% min, cleanout required, forever |
When to carry, when to bury
- Carry when a 2% surface path exists: an extension to a swale, a splash block to a planted bed, a shallow channel to daylight. Visible, cleanable, repairable.
- Bury when the path must cross a walk, a mowing line, or flat ground with no grade to work with — then it is a French drain to a real outlet, with cleanouts, or nothing.
- Never bury into a low spot with no lower exit. That is the buried-pond trap with a downspout attached.
The disagreement sentence
“Bury the downspouts” is the first sentence of most drainage sales pitches and the last sentence of most drainage failures. The pipe that can't be cleaned is a future clog with a warranty.
Where to go next
- Calculate catchment — the roof share, measured not assumed.
- French drain — when burying wins.
- Measure grade — whether the 2% path exists at all.
Sources
- Minnesota Stormwater Manual, roof runoff — stormwater.pca.state.mn.us
- USGS Water Science School — usgs.gov
- US EPA NPDES — epa.gov/npdes