Downspout drainage system

Let’s design a layout and calculate the slope and flow capacity for connecting your three downspouts (40', 32', and 60') to a single pop-up emitter using PVC pipe.

Slope Calculation

To ensure proper drainage, aim for a minimum slope of 1% (i.e., 1/8" per foot or 1" drop every 8 feet).

Required Drop

Segment	Length	Minimum Slope	Required Drop
Downspout A	40'	1%	0.4' (4.8")
Downspout B	32'	1%	0.32' (3.84")
Downspout C	60'	1%	0.6' (7.2")

You can increase the slope slightly if your yard allows, especially for longer runs like the 60' section.

Flow Capacity

A 4" PVC pipe can handle:

Approx. 240 gallons per minute (GPM) at full flow
More than enough for typical residential roof runoff

To estimate runoff:

Assume 1" of rain over 1,000 sq ft = ~623 gallons
If each downspout drains ~500 sq ft, total runoff per inch of rain = ~935 gallons

Your system will easily handle this if the emitter is placed in a low area with good discharge potential.

Note

If all three downspouts discharge simultaneously during heavy rain, a standard Tee might not distribute flow evenly. A sanitary Tee or wye configuration could improve flow direction and reduce clog risk.

A wye configuration uses a Y-shaped fitting—called a wye—to connect multiple pipes at a gentle angle, typically 45°, which promotes smooth water flow and reduces turbulence or backups.

You may set up a daylight drain instead of a pop-up emitter—and in many cases, it’s even preferable.

A daylight drain is when your underground drainage pipe simply exits at the surface in a low-lying area, allowing water to flow out naturally—like into a ditch, swale, or slope.

When to Use Daylight Drain

You have a natural downhill slope from the house.
You want low maintenance and fewer moving parts.
You’re in a cold climate where pop-up lids may freeze.
You can discharge water safely into a ditch, swale, or wooded area.