How to Install a Yard Catch Basin in a Soggy Low Spot

There is a spot in the lawn that stays wet for three days after every storm, the grass has turned to moss, and the mower leaves ruts every time you cross it. You have probably already tried topsoil and seed, and the puddle came back with the next heavy rain.

A yard catch basin is often the right fix for a low spot that collects surface water, but only if the water has somewhere lower to go. In this guide I will walk through how to check that before you dig, how to size and set the basin, and how to run the outlet pipe so the system still works in five years. I design residential drainage for a living, and most failed basins I see were set too high, too flat, or with nowhere for the water to go.

When a yard catch basin is the right fix

A catch basin is a box with a grate on top. Surface water runs in through the grate, debris settles in the bottom, and water leaves through a solid pipe to a lower discharge point. It handles surface water. It does not dry out saturated soil, and it does not fix groundwater seeping up from below.

It works best when:

• The puddle forms in one clear low point, not across a broad flat area.
• Water drains away within a day or two once the rain stops. Water that sits for a week often means a high water table or heavy clay, which calls for a different approach.
• You have a discharge point at least 6 to 12 inches lower than the bottom of the basin outlet, within a reasonable pipe run.

If the whole yard slopes toward the house, deal with that first. Our guide to grading a lot so water moves away from the foundation covers the slope you need near the walls.

Check the fall before you buy anything

This is the step people skip, and it decides everything. Water in a pipe needs a continuous downhill slope. The minimum I accept for a 4 inch solid pipe is 1 percent, or about 1/8 inch per foot. I prefer 2 percent, about 1/4 inch per foot, because it moves sediment and gives some room for error.

So a 60 foot run at 1 percent needs 7.2 inches of drop from the basin outlet to the discharge. At 2 percent it needs 14.4 inches. Add the basin depth, typically 10 to 14 inches from grate to outlet, and you can see why a flat yard rarely allows a basin with a pop-up emitter at the curb.

Measure it with a laser level, a builder's level, or a clear water level made from 50 feet of vinyl tubing. Mark the low spot, the planned route and the discharge point. Write down the elevations. If the math does not work, stop and choose a different outlet or method.

Sizing the basin and the pipe

Most residential yard drains are 9 inch or 12 inch square plastic basins with a 4 inch outlet. For a lawn low spot collecting runoff from up to about 3,000 square feet, a 12 inch basin and 4 inch pipe is plenty.

Here is a quick check. A lawn area of 2,000 square feet with a runoff coefficient of 0.35 in a 2 inch per hour storm produces roughly 0.03 cubic feet per second, about 14 gallons per minute. A 4 inch smooth wall pipe at 1 percent carries several times that. If you also tie in a downspout from 1,000 square feet of roof, add about 20 gallons per minute, and you are still fine.

Use smooth wall pipe: SDR 35 or Schedule 40 PVC. Avoid corrugated black pipe for the outlet. It is cheaper, but its ridges catch silt and it sags between supports, creating low spots that hold water and sediment.

Installing the basin step by step

1. Call for utility locates. Wait for marks before digging anything, even a shallow trench.
2. Dig the basin hole 4 to 6 inches deeper than the basin body and a few inches wider on each side.
3. Set a base of 3 to 4 inches of compacted crushed stone so the basin will not settle.
4. Set the basin height so the top of the grate sits about 1/2 inch below finished lawn grade. Grass will grow around it and water will reach the grate. A grate set flush or high turns into a little dam.
5. Dig the trench along your marked route. Check slope every 10 feet with a level and a straight board, or with string lines set to grade.
6. Lay and glue the pipe, bedding it on 2 inches of sand or fine stone. No dips, no humps.
7. Test before backfilling. Run a hose into the basin for 10 minutes and watch the outlet. Water should flow out steadily with nothing ponding in the box.
8. Backfill and compact in 4 to 6 inch layers, then restore topsoil and seed or sod.

Some basins come with a sump below the outlet. Keep it. Those 4 to 6 inches at the bottom catch grit and leaves so they do not wash into the pipe. Clean it out each spring and after big leaf drops in fall.

Why I do not put a basin in every wet spot

A common suggestion online is to drop a catch basin wherever water puddles. I disagree. Without enough fall, a basin just becomes a buried bucket. It fills, sits full, breeds mosquitoes and makes the spot softer than before.

When the fall is not there, I look at other fixes first. Often the low spot is only 2 or 3 inches lower than the ground around it. Filling it with a sandy loam topsoil and blending the grade over 10 to 15 feet fixes the problem for a few hundred dollars. A shallow swale can carry surface water around the area without any pipe at all.

When you truly have no outlet, the water has to go into the ground. A dry well for yard drainage can take the basin's outflow, provided the soil percolates. A basin plus a perforated trench can also help, but read why French drains fail within two years before you go that way.

Frequently asked questions

How deep should a yard catch basin be?

Most residential basins are 10 to 14 inches deep from grate to outlet, set on 3 to 4 inches of compacted stone. The grate should sit about 1/2 inch below lawn grade. The real limit is your outlet elevation, since the pipe must keep falling all the way to discharge.

Where should a catch basin drain to?

It should drain to a point lower than the basin outlet, such as a pop-up emitter in a lower part of the yard, a dry well, or a ditch where local rules allow. Many towns do not allow discharge onto a neighbor's lot or into the sanitary sewer. Check local code before choosing the outlet.

How much slope does catch basin pipe need?

Plan for at least 1 percent, about 1/8 inch per foot, on a 4 inch smooth wall pipe. A slope of 2 percent, about 1/4 inch per foot, works better and helps keep sediment moving. Check the slope every 10 feet while laying the pipe.