Catch Basins &Drainage Inlets

A catch basin is an underground drainage structure that collects surface water runoff through a grated opening. Water enters through the top grate, debris settles in the sump at the bottom, and cleaner water exits through an outlet pipe connected to the drainage system. The sump traps sediment, leaves, and debris to prevent clogging downstream pipes.

Catch basin sizing depends on the drainage area, rainfall intensity, and flow rate requirements. Small yard drains (6"-9") handle residential applications up to 2,000 sq ft. Medium basins (12"-18") suit larger residential or light commercial areas. Large basins (24"+) are used for parking lots, commercial properties, and municipal applications. Calculate the required flow capacity using local rainfall data and drainage area to select the appropriate size.

Square plastic catch basins come in nominal 9"x9", 12"x12", 18"x18", and 24"x24" sizes, while round basins run 6" to 24" in diameter. Nominal size refers to the grate opening, not the body footprint, so verify actual body dimensions before excavating. Depth is adjustable on most plastic basins with stackable riser sections, and outlet knockouts typically range from 3"-4" on small basins to 6"-8" on larger units. Precast concrete basins for streets and parking lots start around 24" and run to 48" or larger.

The sump is the space below the lowest outlet pipe where sediment settles. Residential plastic catch basins typically provide 6"-12" of sump depending on which outlet knockout you pipe from—using a higher knockout leaves more sediment storage below it. Municipal standard details for precast concrete basins commonly call for a 24" (2 ft) minimum sump. A deeper sump does not drain water faster; it extends the time between cleanings.

An area drain is a shallow grated fitting that connects almost directly to the drain pipe with little or no sump—whatever passes the grate goes straight downstream. A catch basin adds a body with a sump below the outlet, so sediment and debris settle out before entering the pipe, and the structure doubles as a maintenance access point. Use area drains on clean hardscapes like pool decks and patios; use catch basins at low points that collect leaves, mulch, or eroding soil.

A catch basin is a point collection system—water enters through a single grated opening and is piped away. A French drain is a linear collection system—a perforated pipe surrounded by gravel that collects water along its entire length through infiltration. Catch basins are better for collecting surface runoff at low points, while French drains excel at intercepting subsurface water or collecting runoff along edges.

Yes, catch basins are excellent for collecting downspout discharge. You can connect multiple downspouts to a single catch basin using underground pipes, then route the combined flow to a single outlet. This consolidates multiple drainage points and allows for easier debris removal. Use a catch basin with adequate capacity for your total roof drainage area.

Inspect catch basins at least twice per year—before and after the wet season. Clean them when sediment accumulates to within 6 inches of the outlet pipe, or sooner if debris is visible at the surface. High-debris environments (near trees, construction sites) may require quarterly or monthly cleaning. Regular maintenance prevents backups and extends system life.

Catch basins are manufactured in several materials: High-density polyethylene (HDPE) and polypropylene are lightweight, corrosion-resistant, and easy to install. Concrete offers maximum durability and load capacity for heavy traffic areas. Cast iron and ductile iron frames and grates provide high strength for vehicle traffic. Each material suits different applications based on load requirements, soil conditions, and budget.

Catch basin grates come in multiple styles: Flat grates sit flush with the surface for pedestrian and vehicle traffic. Atrium grates have a domed profile to prevent clogging from leaves and debris. Slotted grates have narrow openings that are heel-safe and ADA compliant. Perforated grates offer maximum water intake. Decorative grates add aesthetic appeal for visible installations. Select based on traffic type, debris load, and accessibility requirements.

Match the inlet style to the debris the site sheds. For planting beds and mulched landscapes, a domed atrium grate keeps taking water even when leaves mat over its base. For driveways, patios, and parking areas, use a flat grate rated for the traffic crossing it. Along curbed streets, curb-opening and combination inlets accept flow through the curb face, so the inlet keeps working if the grate clogs. After choosing the style, confirm the grate open area passes your design flow.

Every true catch basin already has a built-in sediment trap: the sump below the outlet where grit settles out. Add-ons make it work harder—debris baskets hang under the grate to catch leaves before they reach the sump, fabric inlet inserts protect new systems from construction-phase silt, and outlet hoods cover the outlet pipe to hold back floating trash and oil. Some municipal stormwater (MS4) programs require hoods on basins that discharge toward streams or coastal waters, so check local requirements.

P-traps are required when connecting catch basins to sanitary sewer systems to prevent sewer gases from escaping. For storm sewer connections, traps are typically not required but may be specified for odor control in certain applications. Check local codes—some jurisdictions require trap primers or water seal maintenance for trapped installations.

Match the grate and frame to the heaviest wheel load using EN 1433 classes and their AASHTO equivalents: A15 is pedestrian only; B125 covers residential driveways and light vehicles; C250 (roughly AASHTO H-20) suits parking lots and light trucks; D400 (roughly HS-20/H-25) handles roadways, forklifts, and loading docks; E600 (HS-25) and F900 are for heavy industrial, airport, and port duty. A residential yard basin typically needs A15–B125, a commercial parking lot needs C250, and public roads or loading docks require D400 or higher.

DIY installation is feasible for small residential catch basins in simple soil conditions. The process involves excavating the hole, creating a gravel base, setting the basin level, connecting inlet and outlet pipes, backfilling carefully, and installing the grate. However, proper slope, pipe connections, and load-bearing capacity require expertise. For larger basins, vehicle traffic areas, or tie-ins to municipal systems, professional installation is recommended.

For most homes, a 12" square HDPE or polypropylene catch basin is the sweet spot—large enough to serve a typical yard low point or collect two or three downspouts, with 3"-4" outlets that match common drain pipe. Choose a 9" basin for a small isolated area, and step up to 18" where a large lawn, long driveway, or several downspouts converge on one point. Use a plastic or light-duty grate for foot traffic, and a B125-rated grate anywhere cars cross, such as a driveway edge.