Should downspouts drain into perforated pipe?

No. Downspouts should drain into smooth solid pipe, not perforated pipe. Perforated pipe is built to let water in and out along its length, so a perforated roof line releases water right beside the foundation, the place it was meant to protect. Rein Irving Drainage Company runs roof water in solid pipe to an outlet and keeps it separate from any French drain.

What each pipe is for

Both pipes look similar in a trench. They do opposite jobs.

Why perforated pipe fails as a downspout line

It releases water at the wrong end

A downspout delivers a lot of water at once. Each inch of rain on 1,000 square feet of roof is about 620 gallons, and the roof runoff guide shows how that adds up to hundreds of gallons per downspout in an ordinary Irving storm. In perforated pipe, that water starts escaping through the first holes it reaches, a few feet from the slab. The line soaks the very soil it was meant to keep dry.

Clay makes it worse

Much of Irving sits on expansive clay. At a USDA soil survey sample point in Las Colinas, Ferris and Heiden clay is about 52 percent clay particles with a linear extensibility near 17 percent: it swells hard when wet and shrinks when dry. A sample point describes one spot, not every lot, but on clay like that, a leaking roof line near the foundation adds water to one side of the house and not the other. That uneven moisture is what the foundation drainage page warns about.

The water also has nowhere to go. Clay at that sample point absorbs water extremely slowly, so the leaked water does not soak away; it saturates the trench and backfill around the pipe and sits there.

The holes let debris in

Roof water carries shingle grit, seeds and leaf fragments, the same debris that fills catch basins. Solid pipe with steady fall moves that material through to the outlet. Perforated pipe lets fine soil in through the holes as well, and tree roots find the moist trench and grow into the openings. The line slowly fills, and when it backs up, water pours out of the downspout connection at the house.

It hides the problem

A perforated line can seem to work for years because water disappears into the ground. It is going somewhere, often under the slab or into a flower bed. A solid line shows its problems: water either comes out at the outlet or backs up where you can see it.

Why downspouts should not tie into a French drain

A related shortcut is connecting downspouts to an existing French drain. In a burst storm, a roof sends far more water than a French drain’s stone and perforated pipe can take. The drain floods, and water can push back out of the trench near the house. Roof water belongs in its own solid line. Where a yard needs both, the two can join in a solid pipe downstream, past the perforated section, on the way to a shared outlet. The downspout drainage page shows how that is laid out.

The narrow exceptions

Perforated pipe at the end of a roof line can make sense in one situation: well away from the house, at the far end of a solid line, discharging into an area designed to let water soak in, such as a gravel infiltration bed or dry well. Even then:

• The solid section has to carry the water well clear of the foundation first.
• The soil has to absorb water. In sandier ground, such as the sandy loam over sandy clay loam mapped at a sample point near Southlake, an infiltration area may work. In Irving’s heavier clay, water fills the bed and stays. The guide to dry wells in clay explains why.
• It needs an overflow. A storm larger than the bed can hold has to go somewhere safe, usually a pop-up emitter or swale.

For most Irving lots, a solid line to a pop-up emitter, a swale or the curb is simpler and more dependable; the permits guide covers curb outlets. University Park requires roof drainage to reach the curb or alley in an underground conduit, which rules out ending in a soakaway there.

Checking an existing line

1. Run a hose into the downspout connection at a steady flow.
2. Walk to the outlet and time how long until water appears.
3. If little or no water reaches the outlet, the line is leaking, clogged or broken. A camera shows which.

If you find perforated pipe close to the house, replacing that section with solid pipe often fixes the problem. Drainage repair covers how lines are traced and repaired.

Next step

As a national budgeting range, buried downspouts run about $150 to $350 each; the downspout cost guide explains what moves that. To have your lines checked or replaced with solid pipe, send photos of each downspout.

Questions people ask

Can I use corrugated black pipe for downspouts?

Solid corrugated pipe is better than perforated, but its ridges catch roof grit and leaves and it can sag or crush under soil and traffic. Smooth-wall solid pipe carries water faster, is easier to clean and holds its fall.

My downspouts already run into perforated pipe. Do I need to dig it all up?

Not always. A camera or a hose test shows whether water reaches the outlet or soaks out near the house. Often only the run closest to the foundation needs replacing with solid pipe.

Is a sock on perforated pipe enough to stop clogging?

No. A fabric sock clogs with fine clay and silt, and once it does the pipe neither takes in nor releases water well. For roof water the better answer is pipe without holes.