The basic idea
An underground downspout system connects the bottom of each downspout to a buried pipe that carries water away from the house and releases it somewhere safe: at a lower point of the yard (a daylight outlet), through a pop-up emitter in the lawn, into a dry well, a rain garden, or a storm sewer where allowed. The goal is the same as with any extension: get roof water well away from the foundation. See downspouts and your foundation.
Pipe choices
- Smooth-wall PVC (often 4-inch, sometimes 3-inch sewer and drain pipe): smooth inside, resists crushing, easy to clean. The usual recommendation.
- Corrugated flexible pipe: cheap and easy to lay, but the ridges catch debris and it crushes more easily. Many problems with buried downspouts come from corrugated pipe that has clogged or collapsed.
- Solid vs. perforated: use solid pipe to carry water away from the house. Perforated pipe leaks water along its length, which is the opposite of what you want next to a foundation.
Slope and depth
The pipe needs a steady downhill slope all the way, commonly at least 1/8 inch per foot and preferably 1/4 inch per foot. Low spots hold water, collect sediment and freeze. Depth depends on climate and the route; the pipe should be protected from mowers and foot traffic and bedded on firm soil or gravel so it does not sag.
Where the water goes
Daylight outlet
If the yard slopes, the simplest and most reliable option is to run the pipe to a lower point and let it exit at the surface, with a grate or rock splash pad. Water flows by gravity and the outlet is easy to check.
Pop-up emitter
On flat lots, the pipe can end at a pop-up emitter: a lid that lifts when water flows and closes when it stops. Guidance such as This Old House’s runoff how-to recommends setting the emitter in a bed of crushed stone so water disperses. The emitter must be lower than the downspout connection, and the pipe will hold some water between storms, which can freeze in cold climates.
Dry well
A dry well is a buried chamber or pit filled with stone that stores roof water and lets it soak into the soil. State guidance, such as Vermont’s LakeWise and New Hampshire’s Soak Up the Rain programs, sizes dry wells by the roof area they serve; a larger roof area needs a larger well. Dry wells work in sandy or gravelly soil well above the water table. They do not work in clay, where water cannot soak away, or where groundwater is high, and they should not be used in karst areas without an engineer. They need an overflow route for large storms.
Storm sewer or street
Some towns allow downspouts to connect to storm sewers or to discharge at the curb; others prohibit it. Cities with combined sewers often require downspouts to be disconnected. See disconnecting downspouts.
Keeping it working
- Gutter guards or downspout strainers keep leaves out of the pipe. See are gutter guards worth it?
- A cleanout at each downspout connection lets you flush or snake the line.
- A removable connection or a debris trap at the downspout, so you can check for clogs.
- Test annually with a hose: water should appear at the outlet quickly and without backing up.
- Mark the route so future digging does not damage it.
Freezing in cold climates
In snow country, water that sits in a buried line or emitter can freeze and block it, so meltwater backs up in the downspout. A daylight outlet with continuous slope drains completely and is less prone to freezing. Some homeowners add a winter diverter at the downspout that sends water onto the surface when the line is frozen. See snow load and gutters.
Planning the route
Before digging, call 811 to have buried utilities marked. Walk the route from each downspout to the outlet and note obstacles: walkways, driveways, patios, trees and roots, septic fields and property lines. Several downspouts can join into one larger main line, but the main must be large enough for the combined flow. Keep the outlet on your own property and away from neighbors, septic systems and wells.
Common mistakes
- Corrugated pipe that clogs or crushes
- Perforated pipe next to the foundation
- Low spots and flat sections that hold water
- Connecting downspouts to the footing drain, which floods the drain around the foundation
- Dry wells in clay, high groundwater or karst
- Outlets that discharge onto a neighbor’s property
- No way to check or clean the line
Older homes: find out where the old lines go
Many older homes have downspouts that disappear into the ground, sometimes into clay tile lines installed decades ago. They may connect to a sewer, a dry well, the footing drain or nowhere at all. Broken old lines are a common cause of wet basements. Before new gutters are connected to old buried pipes, have them tested. See wet basement? check your gutters first.
Surface options first
Underground drainage is not always necessary. On many lots, a simple above-ground extension, a flip-up extension where people walk, a splash block onto a gravel bed or a shallow swale that leads water across the lawn does the job with nothing to clog or freeze. Buried systems are worth it where extensions are in the way, where water must cross a walkway or driveway, or where the yard slopes toward the house and water has to be carried farther. Try the simple option first and watch it in a heavy rain.
Signs a buried line has failed
- Water overflowing at the top of the downspout or at the connection during rain
- Gutters overflowing near the downspout even when clean
- A wet or sinking spot in the lawn along the pipe route
- No water at the outlet during heavy rain
- Wet basement walls near the downspout
Camera inspections, used by plumbers and drain cleaners, can show where a line is broken or blocked before you dig.
Cost and who does it
Underground drainage is often done by landscapers, drainage contractors or gutter companies that offer it. Cost depends on length, depth, soil, obstacles such as walkways and driveways, and the outlet type. Ask for the pipe type, slope, outlet and cleanout locations in writing.