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How to Extend a Downspout

Need to extend your downspout? Connecting downspouts to buried drainpipes can help dry out a wet basement and soggy lawn.

Downspout on brick house. iStock

Rain gutters and downspouts provide an efficient first step in controlling roof runoff. But if the rain or melted snow channeled off the roof isn't carried far enough away from the house, it will collect against the foundation wall or footing and seep into your basement or crawl space. One surefire way to avoid such water-related problems is to connect the downspouts to a series of buried drainpipes that lead far from the house.

The drainage pipe used for extending downspouts is non-perforated Schedule 40 PVC pipe with 4-inches-diameter. Some jobs will also require flexible drainpipe, which is invaluable for getting around obstacles or following uneven terrain. For our installation, we used it to snake around a few large boulders we couldn't pry out.

What You Need for a Downspout Extension

Along with the drainage pipe, you'll need an assortment of PVC elbows, T-fittings, couplings, adapters and other specialty fittings that allow you to connect together the pipe sections. Also, purchase a 16-ounce can of PVC primer and PVC cement for gluing together all the parts.

How to Extend Your Downspout

Installing drainage pipe is a straightforward job that any homeowner can handle. It’s typically done in three phases: digging the trench, laying the pipe and backfilling with soil. Read on to learn how to expend your downspout in 10 steps.

Laying out the trench for the downspout.
Lay out the trench.

Step 1: Lay out the Trench

Use a string and 1x2 stakes to lay out the trench. Pound in two stakes 4 feet from the house and directly in front of each downspout.

Place other stakes spaced 8 feet apart in between. Tie the string to the downspout, then stretch it out and around all the stakes.

Step 2: Mark the Trench Line

Sprinkle flour from a can along the string to mark the trench line. Remove the string and lay cardboard along the edge of the white line.

Marking the trench line.
Mark the trench line.

Step 3: Dig the Trench

Begin digging the trench and dumping the dirt onto the cardboard. This is a trick that can make backfilling go much more smoothly later on.

The trench should be at least 6-inches-wide and slope down slightly (1 to 2 inches per length of pipe) so water drains away from the downspouts. Continue until you have completed the trench.

Depending on the trench length and, more important, the type of soil, digging the trench with a shovel can range from relatively easy to downright impossible. For our installation, we dug a 70-foot-long trench by hand through rocky soil, a job that took three weekends.

Digging the trench.
Dig the trench.

A quicker, easier alternative to hand digging is to rent a trenching machine or hire a backhoe operator. The average job can usually be done in under two hours. To minimize damage to the landscape, have the operator use a 12-inch trenching bucket.

The trench must be at least 10-inches-deep, but be aware that in cold-weather regions the pipe occasionally will clog with ice during periods of heavy precipitation. This condition most often occurs when the air temperature is mild during the day but drops below freezing at night.

Tip: The only guaranteed way to keep the pipe from freezing is to bury it below the frost line, which ranges between 32 and 48 inches in most areas of the snow belt. Digging a trench that deep is only practical when the end of the pipe drains into a dry well or is connected to an existing drain line.

Step 4: Lay out the Pipe Lengths and Fittings

Laying out the pipe lengths and fittings.
Lay out the pipe lengths and fittings.

Lay out all the pipe lengths and fittings beside the trench to make sure you've got all the necessary parts. Press a downspout adapter into a 3x4-inch reducer coupling and slip it onto the bottom end of the downspout.

Step 5: Install the Starter Elbow

Place a 90-degree elbow in the trench directly below the coupling and measure the distance between the two fittings.

Cut a piece of pipe to match with a handsaw, hacksaw or 12-in. power miter saw.

Swab the two mating surfaces with purple primer, then apply the cement to the same surfaces and immediately press the pipe onto the elbow.

Installing a starter elbow.
Install a starter elbow.

Give them a slight twist, then hold them. The cement cures in just a few seconds.

Step 6: Insert Adapter and Reducer

Gently insert the downspout adapter and reducer assembly into the pipe protruding from the elbow after slipping the adapter onto the downspout; don’t glue this joint.

Then proceed down the trench to the first turn, lay in a 90-degree elbow and cut a length of pipe to fit between it and the assembly at the downspout.

Glue the parts together, then check with a level to make sure the pipe slopes down slightly.

Inserting the adapter and reducer.
Insert the adapter and reducer.

Step 7: Continue Installation

Continue installing pipes along the trench until you reach the second downspout. Use standard couplings to connect the pipe sections.

Step 8: Begin Second Downspout

Make another adapter-and-reducer assembly for the second downspout. This time, glue a sanitary T-fitting to the pipe end and the other end of its trench.

Be sure the curved elbow of the T-fitting faces “downstream.”

Glueing a sanitary T-fitting to the pipe end and the other end of its trench.
Glue a sanitary T-fitting to the pipe and the other end of its trench.

Then glue the drainage pipe from the first downspout to the sanitary T-fitting.

Step 9: Finish Pipe Installation

Continue running pipe from the T-fitting along the trench. Again, use couplings to join together lengths of pipe.

Step 10: Finish the Job and Backfill

If you’re going to run flexible pipe, first glue a 3x4-inch reducer coupling to the end of the rigid pipe in the trench.

Then, slip the flexible pipe over the reducer and secure it with a stainless-steel pipe clamp.

Placing flexible pipe over the reducer.
Place flexible pipe over the reducer.

At the very end of the trench, dig a wide, shallow hole (approximately 16-inches-deep x 36 inch-diameter) and line it with flat rocks or a 4-inch layer of gravel.

Lay down the pipe end on this rocky bed and cover it with more rocks or several inches of gravel.

In Step 3, you put down wide pieces of cardboard beside the proposed trench. Finish by backfilling the trenches, using a garden hoe to pull dirt off the cardboard and into the trench.