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Roof runoff route

Downspout Drainage: Calculate the Water and the Fall

A buried pipe is not the first question. First estimate how much roof runoff reaches this downspout, then verify that the proposed route actually falls to a safe outlet.

Downspout runoff + route grade

Enter the horizontal roof plan area draining to this downspout, a storm depth, the horizontal route length and the measured vertical fall to the candidate outlet.

Use horizontal projected area, not sloped roof-surface area.
Positive value means the outlet is lower.

Why roof area matters

A downspout only receives the roof area connected to its gutter branch. For volume planning, use the horizontal footprint that contributes to that branch. A steep roof does not create extra rainfall volume simply because its roofing surface is longer.

The runoff estimate uses the same transparent volume relationship as Wrivel's runoff calculator: rainfall depth × contributing plan area × an explicit runoff coefficient. It does not calculate peak flow rate or pipe capacity.

Why outlet elevation matters just as much

Extending a downspout underground can hide a bad route. A pipe with no reliable downhill endpoint can hold water, back up, freeze in cold climates, leak at joints or simply move the wet spot farther away. Measure the candidate outlet elevation rather than judging fall by eye.

End-to-end percent grade is simply vertical fall divided by horizontal run. The calculation does not prove every point along the route has continuous fall; a high spot in the middle can still trap water. Verify intermediate grade breaks before digging.

Surface extension, solid pipe or infiltration?

Short visible route to a safe lower areaA surface extension can be easy to inspect and clean.
Longer route with verified continuous fallA solid conveyance pipe can move concentrated roof water without intentionally releasing it along the trench.
No lawful gravity outletDo not assume perforated pipe fixes the endpoint problem. Evaluate soil, groundwater, setbacks, storage and overflow separately.
Water is already pooling beside the foundationPrioritize immediate roof discharge and near-building grade; use the pooling diagnostic for the broader yard problem.

Do not confuse a French drain with a downspout pipe

A French drain is a collection feature for water entering through gravel and perforated pipe. A downspout carries already-concentrated roof runoff. When the goal is to convey that concentrated flow to an outlet, a solid route is conceptually different from a perforated collector that releases water along its length.

If you want storage instead of immediate discharge

Use the rain barrel size calculator to compare roof runoff with a chosen barrel or cistern capacity. For underground infiltration storage, measure the proposed soil location first and keep a safe overflow route for storms larger than the storage target.

Field checklist before burying a route

  • Confirm the contributing gutter/downspout area.
  • Measure route length and vertical fall, including intermediate high points.
  • Verify the outlet is lawful and will not direct water to a neighbor or foundation.
  • Locate utilities before digging.
  • Plan access for cleaning and a visible overflow path if the route clogs.
  • Check local requirements for discharge, setbacks, frost depth and stormwater controls.
Planning scope: this tool estimates storm volume and end-to-end grade. It does not size a pipe, inlet or overflow for peak flow, establish code compliance, or verify that the outlet is permitted.