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Gutters and downspouts are more than accessories; they’re an essential network that channels water away from your home. But if your downspout shoots water into your landscaping, near your foundation or onto sidewalks, you’ll need to create a better drainage system. You may need to learn how to run a downspout under a sidewalk.

Why Would You Run a Downspout Under a Sidewalk?

You might think that evacuating a downspout onto a paved surface is ideal. The rush of water won’t wash away your garden, and a sloped driveway, path or sidewalk will divert the water into the lawn. 

However, running a downspout over a sidewalk has the following disadvantages:

  • Staining and discoloration where the downspout empties
  • Increased risk of mold and mildew growth, which can become slippery
  • Susceptibility to ice formation in freezing weather
  • Pooling water can crack the surface of the pavement
  • Water erodes the soil adjacent to the sidewalk
  • Runoff increases the load on the sewer system instead of filtering through the soil

Installing a Downspout Before the Sidewalk

If you’re working on a new construction or changing the layout of your landscaping, you might be able to dig a hole for drainage before installing pavers or a sidewalk. It takes planning, but it delivers meticulous results. 

The labor is much easier when you can carve out drainage paths before paving an area. Prepping the spot for a downspout under the future sidewalk will also ensure that the ground is graded properly. 

However, you could run into problems that aren’t evident during the planning stages. Consulting with an experienced landscaper or roofer can help you prevent future problems when designing downspouts under sidewalks.

Installing a Downspout Beneath an Existing Sidewalk

In many cases, homeowners need to install a downspout under a sidewalk that’s already in place. If this is the case for you, you might have noticed that the water doesn’t drain properly in that area. Perhaps the downspout is pushing debris onto your sidewalk, creating an eyesore or fall hazard. 

You don’t want to compromise the sidewalk, but you want to create a tunnel that channels the water beneath the soil. How to run a downspout under a sidewalk is more straightforward than you might expect.

Mark the Trenches

The easiest way to mark the location of the trenches and tunnel is to attach a string to two garden stakes. Make sure that the string is long enough to span the length of both trenches and the tunnel. Place one stake at the downspout exit. Place the other across the sidewalk. Sprinkle white flour along the string line to identify where you’ll need to dig.

Dig the Trenches

You’ll need to dig a trench that travels from the downspout to the sidewalk. A second trench will begin on the other side of the sidewalk and continue to the outlet of the drain.

These trenches should be long enough to contain a corrugated downspout extender or PVC pipe drain. They should extend at least 10 inches below the lower surface of the sidewalk. Because most sidewalks are 4 inches thick, you might have to dig a trench that’s more than 14 inches deep. 

The trenches should be at least 6 inches wide. Because you should leave at least 4 inches of space around all sides of the drainage tube, your trenches will usually be wider than 6 inches, though.

Make sure that the trench isn’t too close to the sidewalk. If there isn’t enough support in a high-traffic area, the pavement could crack or cave in.

Create the Tunnel

There are two primary methods for digging the tunnel beneath the sidewalk:

  • Dry PVC method – Cut one end of a segment of PVC pipe that’s slightly longer than the sidewalk is wide. That end should be at a sharp 45-degree angle. Insert the PVC horizontally, directing the angled end toward the beginning of the tunnel. Place a wooden block on the other end of the pipe to protect it, and hit it with a mallet or sledgehammer. This should break up the dirt below the sidewalk. Use a metal shovel to scoop out the loose debris as you go.
  • Wet PVC method – If it’s difficult to break up the soil using the dry PVC method, try attaching a pressure washer or garden hose to the flat end. Screw a jet nozzle onto the front of the PVC if necessary. The strong stream of water will loosen up dirt and rocks that are in your way.

Insert the Drainage Tube

Place a corrugated drainage tube through the tunnel. Make sure that it is long enough to stretch from the downspout to the exit trench. Replace the dirt that came out of the trench around the tube. Pack it tightly under the sidewalk to fill in all of the voids and avoid weakening the pavement.

What to Consider Before Installing a Downspout Under a Sidewalk

The biggest challenge for how to run a downspout under a sidewalk is the soil type. Hard, rocky or clay soil is harder to remove than soft, sandy dirt. Root-bound soil will also give you more resistance than loose particles.

Unfortunately, dense soils are often prone to collecting standing water. If drainage problems are leading you to research steps for how to run a downspout under a sidewalk, the chances are high that it’s going to take some elbow grease to get rid of the soil in that spot.

Look for an area where the soil is fluffier and less compact. Dirt near a garden is often loose enough to tunnel through.

You’ll also need to think about the depth of the trench and tunnel. While freezing temperatures aren’t a major concern in our area, they can affect drainage in other locations. To prevent the drain from freezing in the winter, you could dig the trench and tunnel below the frost line. However, that’s usually overkill. 

Tips for Running a Downspout Under a Sidewalk

You’ll get the best results from your new drainage system by following these guidelines for how to run a downspout under a sidewalk:

  • Mark underground utility lines in your yard before digging
  • Grade the trench so that it slopes away from the downspout to keep water moving
  • There should be a ¼-inch drop in the slope of the trench for every foot of drainage tubing
  • Take the shortest route to avoid standing water in the drain
  • Avoid creating dips in the drainage tube, which can cause pooling
  • Start the tunnel using a narrow piece of PVC; widen it by inserting a wider segment
  • When filling in the dirt, start at the center of the tunnel and work your way out
  • Use perforated tubing to reduce ice problems and allow for natural drainage
  • Add crushed gravel beneath and around the tube to increase drainage capacity
  • Lay cardboard next to the trench when digging it out so that you can easily slide the dirt back in to fill it in
  • Consider draining the tube into location that can collect rainwater, like a rain garden

Make sure that your new drain allows water to flow freely. During the next rainstorm, inspect the area for pooling or standing water. This project should channel water away from your home effectively while keeping your sidewalks free and clear of moving water and debris.