Rainwater from your roof: downspouts, extensions & discharge
A roof sheds more water than people expect. One inch on a 1,600 sq ft roof is nearly a thousand gallons. Here is how to size for it and move it away.
Rain that lands on your roof has to go somewhere. Knowing how much tells you whether a rain barrel is worth it, how far to run the discharge, and why getting water away from the wall matters.
How much water a roof sheds
The identity is exact: one inch of rain on one square foot is 144 cubic inches = 0.6234 US gallons. So gallons = footprint × rainfall inches × 0.6234 × runoff coefficient, where runoff (~0.90-0.95) accounts for the little that clings and evaporates.
Worked example. A 1,600 ft² footprint, 1 inch of rain, 0.95 runoff: 1,600 × 1 × 0.6234 × 0.95 = ~948 gallons per inch. A 2-inch storm doubles it to ~1,896 gallons. Run yours in the rainwater volume calculator. Note this uses the footprint, not the pitch-adjusted effective area — the same volume of rain falls whether the roof is flat or steep.
Why the number matters
- Rain barrels. A 50-gallon barrel fills from ~5% of an inch on that roof — it overflows fast. Sizing for harvest means big cisterns or multiple barrels with overflow routing.
- Downspout load. Nearly 1,000 gallons an inch is why undersized downspouts overflow — see how many downspouts you need.
- Foundation risk. That volume, dumped at the base of the wall, is exactly what you do not want near a foundation.
Get it 4-6 feet from the wall
The gutter’s job ends at the discharge point 4-6 feet from the foundation. Extensions carry water past the backfill zone so it does not seep down beside the wall. Extension pieces = ceil(discharge distance ÷ piece length) — to reach 6 feet with 4-foot extensions, ceil(6 ÷ 4) = 2 pieces (the extension calculator does it). Beyond that discharge point — grading, French drains, dry wells, rain gardens — is a foundation and yard-drainage job for a different specialist, not a gutter calculation.
Elbows: getting the water down and out
A downspout needs elbows to reach the wall at the top and kick water out at the bottom. Figure 3 elbows per downspout (two at the top to bring it back to the wall, one at the bottom) plus 2 more per soffit offset. For 3 downspouts with 2 offsets: 3 × 3 + 2 × 2 = 9 + 4 = 13 elbows (the elbows & offsets calculator counts them). A soffit offset (kick-out around the overhang) is a pair of elbows working together.
Putting the drainage path together
The full roof-to-away path: rain hits the roof → gutters collect it (sized to the effective area and rainfall) → downspouts carry it down (counted by area and spacing) → elbows and extensions push it 4-6 feet out → grading takes over from there. Each step has its own calculator, and each is a pure quantity — no prices, no rates, just measurements.
Not a stormwater design
These figures are for rain-barrel and discharge planning, not engineered stormwater management. Detention/retention sizing, storm-sewer connections and lot drainage are set by local code and a professional. The volume math tells you what your roof produces; it does not design a drainage system for it.
What to measure first
You need the roof footprint, a rainfall depth to model (a design storm or a typical downpour), and your intended discharge distance and downspout layout. From those the volume, extensions and elbows all follow.
Bottom line: a roof sheds ~0.62 gallons per square foot per inch of rain — hundreds to thousands of gallons a storm — so size the downspouts for it and run the discharge 4-6 feet from the wall with the right elbows and extensions. Everything past the discharge point is a separate drainage job. These are planning quantities on your own measurements, not a stormwater design.