downspout drainage system

Downspout drainage is important. Where should the rain gutter water go? Designing proper drainage to prevent flooding and erosion.

A client of mine recently retired from the real estate investment rental business. Sold all his properties in Hawaii and moved back into his original home in Carlsbad. While most people dream of retiring and then moving to Hawaii, this gentleman wanted to come back home to North San Diego County.

So while he was living in Hawaii acting as property manager to his 20+ rentals, he was also renting out his personal home in Carlsbad. In the many months since moving back he gutted the house for a major complete overhaul home renovation.

After the first rain storm his thoughts quickly turned to the gutter… haha I mean his RAIN gutters! You see, his back yard flooded and he began wondering if he needs to dig up and replace the underground drainage system he installed over 25 years ago.

He also took a good long look at his gutters. They were new back in 1978, now they are ready for replacement. He’s also concerned with his downspouts as they tie into his underground drainage system. Water needs someplace to go. Into the house is not that place! Rain water must divert away from your home.

Downspout Drainage System Underground

downspout flows into catch basin

While I don’t install underground downspout drainage systems I can tie into existing drainage systems. His home renovation got me to thinking this is a good subject to write about.

As far as I know, there are no mandatory regulations to underground rain gutter drainage systems. Although please be sure to contact you utility companies before digging. They’ll let you know where their stuff is buried so then you’ll know where it’s safe to begin digging.

Most common is digging a trench about 12 to 14 inches deep for downspout drain lines. If the lot is flat you’ll need to allow 1/8” inch extra depth for every foot of drainage pipe run. For example, for proper water flow, if running 8’ of pipe the outlet should be 1” deeper than the inlet.

Which pipe is best for underground drainage?

Plastic pipes are the best drainage pipe due to its versatility, availability, and their resistance to corrosion while underground. But make sure to use solid PVC pipes instead of cheap thin plastic flexible water pipes. Those break down too fast and are also prone to clogging too easily. PVC pipes buried underground last about 100 years. Corrugated drain pipes will last about 20 years under best conditions. So just avoid those if possible.


Calculate the amount of water you think might be flowing through the pipes and choose whatever diameter you feel is large enough to accommodate that flow rate.

Bigger is not always better. A larger pipe will flow water more slowly due to a reduction of back pressure and weight of the water. Conversely a pipe that is too small will not flow enough water fast enough and may back up. You don’t want that! If you’re not sure what size is best then I can help you calculate flow and decide on the right diameter.

Try to keep bends to a minimum because it slows flow rate and is a potential clog junction.

Where should the drainage water empty out?

It depends on local regulation. Some common drain points are the street or maybe into a garden or a pond. Another option is PVC pipes with holes in it so water drains out underground as it travels along its length. There are many options. Just make sure wherever it drains that it is legal, safe, and avoids causing ground erosion or flooding damage somewhere else.


Call me for a free onsite no B.S. quote on your next rain gutter project. New installs, repairs, gutter maintenance, downspouts, on your home or commercial buildings, whatever you need, when you need it. Call or Text 760-445-8762.

Owner: Greg Martin
Location: San Marcos, CA. 92069
Phone: 760-445-8762

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