What is a trench drain and where is one required?

A trench drain is a channel, usually built into a concrete slab, that runs along a line and is covered by a grate. A point drain takes water at one spot. A trench takes it along its whole length. The channel slopes or is set so water flows to an outlet, which connects to a pipe.

That shape is useful where water arrives across a wide front instead of at a single spot. Examples include a wide driveway, a dock apron, a wash bay or a kitchen floor.

What a trench drain takes in depends on where it sits and where its pipe goes. See floor drain vs trench drain vs area drain. An outdoor trench catching rain acts like an area drain and belongs on the storm side. An indoor trench catching washdown acts like a floor drain and belongs on the sanitary side. The 2024 International Plumbing Code (IPC) does not define "trench drain" or "floor drain" in Chapter 2, so the use, not the name, decides how the rules apply.

The 2024 IPC speaks about trench drains in two main places.

Product standard. Section 413.1 says trench drains shall comply with ASME A112.6.3. Floor drains may meet that standard, ASME A112.3.1 or CSA B79. So if a trench drain is installed as a plumbing drain, it has to be a listed product to the standard.

Oil separators. Section 1003.4 says oil separators shall be installed at repair garages where floor or trench drains are provided, and at car washing facilities, factories where oily and flammable liquid wastes are produced and hydraulic elevator pits. The oil-bearing waste goes into the separator before it empties into the building drainage system or another point of disposal. So a trench drain in a garage is not just a drain. It triggers a pretreatment rule. See does a loading dock drain need an oil separator.

Trap seal protection. A trench drain connected to the sanitary side has a trap, and a trap needs water to keep its seal. Section 1002.4.1.5, on fixture drain connection for trap priming, says a fixture drain from a lavatory or hand sink can serve as a method of providing trap seal protection for an emergency floor drain, a trench drain or a floor sink located in the same room. A trench that rarely sees water is the kind that dries out, so ask your plumber how yours is protected.

Other sections apply to a trench drain as they would to any drain that serves the same water:

• Storm water shall not be drained into sewers intended for sewage only (Section 1101.3).
• Floor drains shall not be connected to a storm drain (Section 1104.2).
• Storm water from paved areas shall drain to an approved place of disposal (Section 1101.2).

None of these say "put a trench drain here." They say what to do with the water once a drain exists.

Here is where the answer is not a code section. The plumbing code sections cited on this page do not list a building type or location that must have a trench drain. The requirement usually comes from one of three places.

1. 1The approved drainage design. A civil engineer or plumbing designer decides how water leaves a site or a slab. If the plans call for a trench drain, the permit and inspection follow those plans. Maricopa County's commercial drainage plan review requirements are one example of how site drainage decisions are made in the plan set and its review.
2. 2A city or county comment. Plan reviewers can ask for a drain at a garage entrance, a ramp or a wash area, based on local rules.
3. 3A use-specific rule. A car wash, a repair garage with floor or trench drains, or a facility with oily wastes falls under Section 1003.4 and the city's pretreatment rules, and the drain layout has to work with the separator.

The safest way to answer "is one required at my building?" is to check the approved plans and ask the city's plan review office. Do not assume one is or is not needed from the building type alone.

### Grates and accessibility

Where a trench drain crosses a walking route, accessibility rules apply to the grate.

• The ADA Standards, Section 302.3, say openings in floor or ground surfaces shall not allow passage of a sphere more than 1/2 inch (13 mm) in diameter, with a few listed exceptions.
• They also say elongated openings shall be placed so the long dimension is perpendicular to the dominant direction of travel.
• Section 403.3 says the running slope of walking surfaces shall not be steeper than 1:20 and the cross slope shall not be steeper than 1:48. A drain does not change those limits. The surface still has to comply around it.

A grate with wide slots, a missing bolt or a bent bar is a trip hazard as well as a drain problem. Note that ADA rules apply to the surface people walk on and are separate from plumbing code rules. Check both for a trench in a public entrance.

A trench drain fails the same way most drains do: the outlet or channel fills with debris. Outdoor trenches collect leaves, sand and trash. Indoor trenches collect food, grease, shavings or sludge, depending on the use.

• Lift the grates and look at the channel during a routine walk-through.
• Clear debris from the channel and outlet before storm season.
• If the trench feeds a separator or interceptor, follow its pump-out schedule. IPC Section 1003.10 requires access for service and maintenance and periodic removal of accumulated grease, scum, oil or other floating substances and solids.
• Keep wash water out of an outdoor trench unless you know it goes to a sanitary connection. See can you hose a parking lot into the storm drain.
• If an outdoor trench floods in monsoon rain, read why a parking lot floods after monsoon for the order of checks.

When you talk to a plumber or plan reviewer about a trench drain, bring three questions: what water does it take, where does its pipe end, and what does the city require before that water leaves the site. Those three answers settle most disputes about whether a trench is right, wrong or missing.

A trench drain is a tool, not a code requirement by itself. The plumbing code sets product standards and pretreatment rules for trench drains that exist, and accessibility rules cover the grate. Whether your building must have one is answered by your approved plans and your city. Check what your site has, learn where each trench drains to, and keep the channel and outlet clean.