Floor drain vs trench drain vs area drain: what differs?

• A floor drain is a drain in the floor of a room. It takes washdown water, spills and leaks from inside the building.
• An area drain is a drain in an open outdoor area. The 2024 International Plumbing Code (IPC) defines it as a receptacle designed to collect surface or storm water from an open area.
• A trench drain is a long, narrow drain, usually a channel with a grate on top. It can sit inside a building, where it acts like a long floor drain, or outside across a driveway or dock apron, where it acts like an area drain.

The words tell you three different things. "Floor" and "area" say where the water comes from. "Trench" says what the drain looks like. That is why a trench can be either one, and why the label alone never tells you where the pipe goes.

The IPC is direct about some of this and silent about the rest. Chapter 2 defines "area drain," but it does not define "floor drain" or "trench drain." So the code's meaning has to come from how the rules use the words.

Floor drains. Section 413 covers floor and trench drains together. It says floor drains must meet ASME A112.3.1, ASME A112.6.3 or CSA B79, and trench drains must meet ASME A112.6.3. Floor drains must have removable strainers, be built so they can be cleaned, and have ready access to the inlet. Their outlet must be at least 2 inches in diameter. Public laundries and central washing facilities need floor drains placed to drain the entire floor area, with outlets of at least 3 inches.

Storm rules. Section 1101.2 says storm water from paved areas, yards and courts shall drain to an approved place of disposal. Section 1101.3 says storm water shall not be drained into sewers intended for sewage only. And Section 1104.2 says floor drains shall not be connected to a storm drain.

Oil separators. Section 1003.4 requires oil separators at repair garages where floor or trench drains are provided, and at car washes, factories with oily or flammable liquid wastes and hydraulic elevator pits. So the code treats a trench drain in a shop the same way as a floor drain for oil.

Put together, the rules make a clean split. Rain from open areas goes to storm disposal through area drains. Floor drains go to the sanitary side, and oily wastes go through a separator first. For the dock version of this question, see where does a loading dock drain go.

A trench drain is the one most likely to be mislabeled, because the same product is used everywhere. Here are the common cases.

1. 1Outdoor trench across a driveway or ramp. It catches rain that would otherwise run into a building or garage. It acts as an area drain and normally goes to storm.
2. 2Indoor trench in a kitchen, brewery or wash bay. It takes washdown water and acts as a floor drain. It goes to sanitary, often through a grease interceptor or oil separator depending on the use.
3. 3Trench at a dock door. This one can take rain from the apron and washdown from inside. If it takes both, there is a design problem to solve, since storm water may not go to a sewage-only sewer and a floor drain may not go to storm.
4. 4Trench at the entrance to a car wash or shop. The oil separator rule in Section 1003.4 mentions trench drains by name for repair garages, so this is a pretreatment case.

If you inherit a site and cannot tell which case you have, do not guess from the grate. Trace the pipe. The steps in where does a loading dock drain go work for any drain, not only at docks.

Cleaning and access. Floor drains have strainers and traps you can reach. Area drains and trench drains outdoors fill with sediment and leaves. If an outdoor trench is clogged, water floods the space it was meant to protect. See what a catch basin is and how often to clean it for the same idea on the storm side.

Trap seals. A floor drain has a trap, and a trap needs water to keep its seal. A floor drain that dries out can let sewer gas into the room. The IPC's trap seal protection rules mention floor drains, trench drains and floor sinks, and allow a nearby lavatory or hand sink drain to serve as a method of trap seal protection where the fixtures are in the same room. If a floor drain smells, see floor drain backing up and floor sink vs floor drain vs mop sink.

Compliance. A floor drain tied to storm is a code violation under Section 1104.2 and can send pollutants to the storm system. An area drain tied to the sanitary sewer sends rain into a system that was not sized for it, and Section 1101.3 bars it. Neither problem shows on the surface. That is why sites with unknown drains need a dye or camera test.

Walking surfaces. A grate in a walking route has to be safe to cross. The ADA Standards say openings in floor or ground surfaces shall not allow passage of a sphere more than 1/2 inch in diameter, and that elongated openings should be placed so the long dimension is perpendicular to the dominant direction of travel. That applies to trench grates across a sidewalk or an entrance. What a trench drain is and where one is required covers it in more detail.

Inspections and sales. Buyers, lenders and city inspectors may ask for a drain map. It helps to have each drain labeled by type and destination.

### What to do

1. 1Walk the property and list every drain, marking whether it is indoors or outdoors.
2. 2For each one, decide whether it takes floor water or rain.
3. 3Find where the pipe goes, using plans first and then a dye or camera test.
4. 4Label each drain on a site plan with its type and destination.
5. 5If any drain does not fit the pattern, such as a floor drain that goes to storm, ask a licensed plumber and your city how to correct it.

The name of a drain matters less than its job. Water from inside a building goes to the sanitary sewer, rain from open ground goes to the storm system, and a trench can do either one. Know which job each of your drains does, and check that its pipe matches. That one habit prevents most surprise citations and most flooded doorways. For the bigger picture of how the two systems relate, read do storm drains and sanitary sewers connect.