Catchment
Catchment is the area that sends water to one point: roof planes, driveway, walks, and the uphill yard that visibly drains toward the wet spot. It is measured in square feet, in plan view. It is the third of the three numbers, and the one most often inflated by people selling bigger drains.
Counting rules
| Surface | Count it? | Rule |
|---|---|---|
| Roof plane draining to the spot | Yes | Plan-view footprint (length × width of the house area it covers), not the pitched surface area |
| Driveway sloping toward the spot | Yes | Only the part whose water actually reaches the spot |
| Uphill lawn | Partly | Lawn absorbs much of what lands; count it but discount — the desk uses 25–40% of lawn area depending on slope, and says so |
| Neighbor's yard | If it drains to you | You don't own it, but it owns your wet spot; count it honestly |
| Lawn sloping away | No | Water that leaves your lot is not your catchment |
| Pitched roof surface area | Never | The vendor's favorite number; rain falls on the footprint, not the slope |
Why plan view
Rain falls vertically. A roof pitched at 8:12 has about 1.2 times the surface area of its footprint, but every drop that lands on it still came from the same sky column. Catchment math uses the footprint. Any sizing number built on the larger figure is a fifth too big — not catastrophic, but the desk will not hand you an inflated number and call it conservative. It is the wrong kind of conservative.
Catchment changes the verdict
The same 200-square-foot puddle means different things under 800 square feet of roof versus 3,000. Small catchment, bad soil, standing water: the water is a gift of the soil, not the roof. Big catchment, decent soil, wet basement corner: the roof is the donor and the downspouts are the tap. Count first; the verdict follows the count — the counting job does it in half an hour.
Where to go next
- Calculate roof and yard catchment — the 30-minute arithmetic.
- Grade — how to know which way “toward the spot” is.
- Size a swale — where the catchment meets the channel.
Sources
- Minnesota Stormwater Manual, runoff computation — stormwater.pca.state.mn.us
- USGS Water Science School, runoff — usgs.gov
- US EPA NPDES — epa.gov/npdes