Calculate roof and yard catchment
Timebox: 30 minutes with a tape, a pencil, and this page. Output: the third number, in square feet, of everything that sends water to your wet spot. The rules are few and the traps are real — a pitched-roof area counts as its plan-view footprint, and half a roof belongs to half a downspout.
Checked 2026-09-10 · procedure re-verified against sources below
Steps
- Measure the roof in plan view. For each roof plane, multiply the ground length by the ground width it covers. A 30 × 40-ft gable is 1,200 sq ft of plan area, full stop. Do not climb; measure the building footprint and split by plane.
- Split by downspout. Each spout takes a share. Two spouts on a 1,200-sq-ft roof usually mean 600 each; a valley or a gutter slope changes the split. When in doubt, watch the gutters in rain.
- Add hard surfaces. Driveway, walk, and patio sections whose water visibly runs toward the spot. Only the part that reaches the spot — a 400-sq-ft driveway sloped to the street contributes zero to the backyard.
- Add uphill lawn, discounted. Lawn absorbs much of what lands on it. The desk counts 25–40% of the uphill lawn area that drains to the spot — 25% on gentle slope, 40% when the lawn is steep or the grass thin. State your discount; don't hide it.
- Total and date it. Write: “catchment ≈ 1,480 sq ft, 10 Sep 2026.” The number feeds the sizing job directly.
Worked example
| Surface | Plan area | Counted |
|---|---|---|
| Roof, rear plane (one spout) | 600 sq ft | 600 |
| Driveway edge draining to yard | 180 sq ft | 180 |
| Uphill lawn, gentle slope | 2,800 sq ft | 700 (25%) |
| Total catchment | 1,480 sq ft |
The 1-inch storm over 1,480 sq ft is about 920 gallons — the number a swale or dry well must do something with in a day. The catchment page holds the counting rules in full.
Where people stop
- Counting the pitched area. The vendor's favorite inflation: a 10:12 roof is 1.3× its footprint. The desk counts footprints, and says so on the catchment page.
- Counting the whole roof for one spout. The downspout that floods the corner serves a share, not the house.
- Forgetting the neighbor. If their yard drains onto yours, their roof is on your wet spot. Count it, even though you don't own it.
Where to go next
- Catchment — the counting rules, defended.
- Size a swale — the number goes to work.
- Downspout math — carry it or bury it, with numbers.
Sources
- Minnesota Stormwater Manual, runoff computation — stormwater.pca.state.mn.us
- USGS Water Science School, rainfall and runoff — usgs.gov
- US EPA NPDES — epa.gov/npdes