# Catch Basin [CB]

> A catch basin is a storm drain inlet with a sump that traps sediment before water enters the pipe. How it works, types, vs area drains, and site tips.

- URL: https://essential.construction/academy/glossary/catch-basin/
- Author: Kyle Parry (https://essential.construction/about-us/kyle-parry/, https://www.linkedin.com/in/kyleparry/) - Kyle Parry is the founder of Essential Construction. He has worked in construction for more than 20 years, leading projects across Canada for the country's largest contractors and developers.
- Publisher: Essential Construction (https://essential.construction/)
- Last updated: 2026-11-03

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A **catch basin** is a storm drainage inlet that collects surface water through a grate at the top and directs it into the storm sewer. Below the outlet pipe is a sump that traps sand, debris and sediment so they settle out instead of clogging the pipes, which means catch basins need regular cleaning.

You will see catch basins in parking lots, roads, loading docks, landscaped areas and around buildings. On civil and site drawings they are usually labelled **CB**.

## How a catch basin works

Runoff flows across the paving or ground, which is graded to fall toward the basin. Water drops through the grate into the box. Because the outlet pipe sits some distance above the floor of the box, the space below it (the **sump**) holds water and lets heavier solids settle. Cleaner water then leaves through the outlet to the storm main, a ditch, a pond or another structure.

Many basins also have a **hood or trap** over the outlet. It keeps floating trash, leaves and some oil inside the basin, and in some designs it limits odours from the sewer.

## Parts of a catch basin

- **Grate and frame:** usually cast or ductile iron, sized and load-rated for pedestrian, vehicle or heavy traffic areas.
- **Box or riser:** precast concrete sections, cast-in-place concrete, brick or block in older work, or plastic for small yard basins.
- **Sump:** the depth below the outlet invert where sediment collects.
- **Outlet pipe:** connects to the storm system at the invert elevation shown on the drawings.

## Types of catch basins

| Type | Typical use | Notes |
| --- | --- | --- |
| Precast concrete | Parking lots, roads, commercial sites | Fast to set; riser sections adjust height |
| Cast-in-place concrete | Odd shapes or deep structures | Formed on site to suit the design |
| Plastic (yard) basin | Residential lawns, downspouts, patios | Small square units connected to drain pipe |
| Catch basin manhole (CBMH) | Where an inlet and pipe junction meet | Larger structure with room for access and several pipes |

Names vary by region. In the UK a similar roadside inlet is usually called a **gully**, and in Australia a **stormwater pit**.

## Catch basin vs area drain, trench drain and manhole

- **Area drain:** also has a grate, but usually connects straight to a pipe with little or no sump.
- **Trench drain:** a long, narrow channel with a linear grate, used across driveways or at door thresholds.
- **Manhole:** an access structure for inspecting and cleaning pipes, normally with a solid cover rather than an inlet grate.

## Installation and inspection tips

- Confirm rim and invert elevations against the civil drawings before the basin is backfilled.
- Set the grate slightly below the surrounding finished surface where the detail calls for it, so water actually reaches it.
- Check the grate load rating suits the traffic, especially in drive aisles and loading areas.
- Install inlet protection (filter fabric or silt sacks) during construction so site sediment does not fill the sump and pipes.
- Clean out the sump and flush the lines before handover, and record it in your closeout documents.

## Related templates and guides

- [Drains AutoCAD blocks](https://essential.construction/files/design/autocad/drains-autocad-blocks/)
- [Quality checklist: road and drain works](https://essential.construction/files/construction/quality/quality-checklist-road-and-drain-works/)
- [QA/QC checklist: site works (underground plumbing)](https://essential.construction/files/construction/quality/qaqc-checklist-site-works-underground-plumbing/)
- [Shallow masonry manholes drawing](https://essential.construction/files/design/autocad/shallow-masonry-manholes-drawing/)

## Frequently asked questions

### What is the purpose of a catch basin?

A catch basin collects surface runoff from paving, lawns or roofs and sends it into the storm sewer or another outlet. Its sump also traps sand, grit and debris so they settle out in the basin instead of building up in the pipe network, where they are much harder to remove.

### Is a catch basin the same as an area drain?

Not quite. Both collect surface water through a grate, but a catch basin has a sump below the outlet to trap sediment, while an area drain usually connects straight to the pipe with little or no sump. On site the names are often used loosely, so check the drawing detail.

### How often should a catch basin be cleaned?

It depends on how much sediment reaches it. Many owners and municipalities inspect at least once a year and clean when the sump is getting full. On a construction site, check basins after storms, because exposed soil can fill a sump very quickly.

### What does CB mean on a drawing?

CB is the usual abbreviation for catch basin on civil and site plans. You may also see CBMH for a catch basin manhole, a larger structure that combines an inlet grate with manhole access. The drawing legend confirms what each abbreviation means on that project.