How to Bury a Downspout Extension That Drains Away

Every heavy rain, your downspout dumps water right next to the foundation, and the plastic splash block you bought keeps getting kicked aside or run over by the mower. If you bury a downspout extension and run it 10 feet or more from the house, that water stops soaking the soil against your basement wall. Done right, it takes one weekend and around $150 in materials. Done wrong, it clogs, freezes or backs water up the gutter within a year.

I have dug a few hundred of these lines over thirty years of grading work. This is the method that keeps working.

Why one downspout matters so much

A single downspout often drains 500 to 800 square feet of roof. One inch of rain on 600 square feet is about 370 gallons, all of it concentrated at one point on the ground. Spread that across a typical summer storm and you are pouring several bathtubs of water into a two foot patch of soil next to your foundation.

If your lot already slopes away from the house, a buried line finishes the job. If it does not, start with grading the lot so water moves away from the foundation, because no pipe fixes ground that tilts toward the wall.

How to bury a downspout extension step by step

Plan the route and outlet

Walk the yard during or right after rain and find where water naturally wants to go. Your outlet should be at least 10 feet from the foundation, and farther is better on clay soil. Good outlets include:

• A pop up emitter in the lawn on ground lower than the downspout
• A daylight outlet on a slope or into a drainage swale built to carry water
• A dry well, if you have no downhill spot and your soil drains reasonably

Do not discharge onto a neighbor's lot, a sidewalk or a driveway that freezes. Many towns prohibit it, and it creates a liability you do not want.

Before you dig, call 811 at least three business days ahead so utilities get marked. Shallow cable and gas lines run exactly where downspout trenches go.

Choose the pipe

Use 4 inch solid PVC, either SDR 35 or Schedule 40, for the buried run. Skip the black corrugated pipe for this job. Its ridges catch shingle grit and leaves, it crushes under a loaded mower wheel, and you cannot snake it cleanly when it clogs. Solid PVC costs a bit more, about $25 to $40 per 10 foot length depending on grade, but it lasts decades.

You also need a downspout adapter that converts the rectangular 2x3 or 3x4 spout to a round 4 inch pipe, one or two 45 degree fittings for direction changes, PVC primer and cement, and your outlet fitting.

Dig the trench with real slope

Your trench needs at least 1/8 inch of fall per foot, and I aim for 1/4 inch whenever the yard allows it. Over a 20 foot run, that is 2.5 to 5 inches of total drop. Check it with a 4 foot level and a shim, or stretch a string line with a line level from start to finish.

Dig 8 to 12 inches deep at the house end. Deeper only helps if your outlet can go deeper too, so let the outlet elevation decide the trench depth. A trenching shovel moves this much dirt in two or three hours. Cut sod in strips first and set it on a tarp so it goes back down cleanly.

Lay, glue and test

1. Lay 1 to 2 inches of sand or fine gravel in the trench bottom so the pipe sits evenly without high spots.
2. Dry fit everything from the downspout to the outlet.
3. Prime and cement each joint. Use 45 degree bends instead of 90s; tight turns are where clogs start.
4. Before backfilling, run a garden hose into the gutter for five minutes and watch the outlet. Water should flow steadily with no backing up at the adapter.
5. Backfill in layers, tamping lightly, then replace the sod and water it in.

Add a cleanout or a leaf trap

Most buried lines fail from debris, not from bad pipe. Install a cleanout tee or a leaf filter where the downspout meets the buried line. A leaf trap with a removable screen costs $20 to $35 and catches seed pods, shingle grit and leaves before they reach the pipe. Empty it twice a year, in late spring and after the leaves drop.

When a pop up emitter is the wrong choice

Pop up emitters are the default advice on most home improvement sites, and on sloped, well drained yards they work fine. On flat clay lots, I usually talk people out of them.

Here is why. An emitter only opens when the pipe fills and pressure builds. On a flat yard, water sits in the line between storms. In northern climates that water freezes, and the ice blocks the next rain from draining, which sends it right back out the gutter seams beside your foundation. Even in mild areas, standing water lets silt settle in the low point of the pipe until it clogs.

On those lots, a daylight outlet to a swale or a lower part of the yard is better, even if the run is longer. If you truly have no low spot, a gravel dry well with an overflow is the next choice. And if the emitter is your only option, drill two 1/4 inch weep holes in the bottom of the pipe just upstream of it so the line drains out between storms.

Signs your buried extension is failing

• Water spills over the gutter at the downspout during heavy rain
• The soil above the pipe stays soggy for days after a storm
• The emitter never pops up, even in a downpour
• Ice forms at the downspout base in winter

Any of these usually means a clog or a belly in the line. Run a drain snake from the cleanout first. If water still backs up, dig at the soggy spot; that is almost always where the pipe sagged.

Frequently asked questions

How far should a downspout extension go from the house?

At least 10 feet is the common target, and many grading pros prefer 15 to 20 feet on clay soil or near a basement. The outlet must also be lower than the downspout so water keeps moving. Make sure the discharge point does not flow back toward the house or onto a neighbor's property.

How deep should you bury a downspout extension?

Most buried downspout lines sit 8 to 12 inches deep at the house end and get shallower or deeper based on the outlet. Slope matters more than depth, so keep at least 1/8 inch of fall per foot. In freezing climates, make sure the line can drain fully between storms.

Should I use corrugated or PVC pipe for a buried downspout?

Solid PVC is the better choice for buried downspout lines. Its smooth walls resist clogs, it holds slope without sagging, and it can be cleaned with a snake. Corrugated pipe is cheaper but tends to collect debris and crush over time.