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Trench Drain Systems

Picking the right slot size, channel size, pan, and grate material

Water on stone sidewalk reflects a purple building rimmed in yellow light
Trench drain systems are often used in the middle of active areas to manage water flow.

Trench drains are small channels that move standing water from surfaces.

Outside, trench drain systems often manage stormwater runoff. Small drains are also used to capture splashes and drips around pools and water features.

Trench is often installed when standing water becomes an issue due to landscape, hardscape, land use, or weather pattern change. A system of small capillaries can be installed through active spaces without compromising the look or function of an area.

In residences, trench is often installed around pools or in showers. In industrial or commercial settings, these drains can be used to manage spills, chemicals, or cleaning fluids.

Picking a trench drain system

To choose the best trench drain system for an application, follow these steps:

  1. Calculate peak flow
  2. Choose pan and grate material
  3. Measure slope of trench system
  4. Calculate drain slot width

Calculate peak flow

Indoor drains often are picked in standard sizes, without having to calculate peak flow, since pools and showers all have similar needs. If the drain is for a shower, pool, warehouse, or garage, peak flow is less important than just going with recommended standard width.

However, if the drain is used for cleaning, or emptying vessels, the amount of water it needs to drain must be calculated. Cleaning systems often have specifications noting how many gallons-per-minute of water they use. Similarly, if a trench drain is used to empty a storage vessel, it’s important to figure out the rate liquid will drain.

Rainfall is less predictable than many indoor applications. In order to choose the right size, the area of drainage and the amount of expected rain is important to determine. In many municipalities the formula used for channel drain is “The Rational Equation,” which provides likely gallons-per-minute of stormwater based on location, area to be drained, and the type of surface around the drainage system.

Rain water pours over the edge of a tile roof
Expected rainfall is based on location averages: run-off co-efficient is based on immediate surfaces.
The Rational Equation for managing rainfall


Q = C * I * A / 96.23

  • Q = Quantity of stormwater to manage (gallons per minute)
  • R = Runoff co-efficient of the surfaces served by the drain
    The runoff co-efficient is calculated by looking at the surface and the slope surrounding the channel. Surfaces and soils absorb water at different rates, having different permeability. Steeper inclines cause water to move more quickly, giving them less time to permeate the ground. Find a simple table estimating R here.
  • I = Rainfall intensity (inches per hour)
    It’s worthwhile looking this number up for your location on the National Weather Service’s Precipitation Frequency When planning trench drain systems, it’s common to build for the most intense one-hour period of rainfall likely to happen in 100 years.
  • D = Drainage area in square feet
    Water doesn’t respect property lines, so make sure you’re calculating for the area around you, if you’re likely to get runoff from adjacent properties. Consider slopes and soil, as well as runoff from roofs and hardscape.
  • 96.23 = Conversion factor
    The standard Rational Equation measures area in acres and quantity of water in cubic feet per second. This number converts area to square feet, and quantity of water to gallons per minute.

Choose trench pan and grate material

The right pan and grate materials for a given job are based on installation expertise, aesthetics, cost, and how much load the system must bear.

Installation expertise
Homeowners installing trench often choose pre-formed trench drain systems made of a plastic or PVC pan and grate, for simpler installation. In contrast, many contractors form trench from concrete. Concrete-formed trench is harder to install, but is also stronger and allows for custom options. It will last the lifetime of the concrete slab. Slope and depth can be customized more adroitly when hand formed.

Load bearing
Trench pans and grates are often part of the hardscape, and have to manage the same traffic the pavement must manage. They can be rated from light duty to special duty. Light-duty grates and pans, like the plastic drains near pools or the stainless ones in showers, only need to handle people.

Heavy-duty pans and grates built to handle vehicle traffic are usually metal or reinforced concrete. Special-duty drains in airports or shipping docks are almost always rebar-reinforced concrete.

A person pressure washes a blue car in a driveway
Drains can be installed indoors or out to manage water from cleaning or spills.

Aesthetics and cost
Grates are available in many materials. Trench drain systems are used indoors and outdoors, from warehouses to high-end residences, and so many options are available. Grates can be decorative or purely functional. Find a complete guide to the cost, durability, and aesthetics of trench grate materials here.

Measure drain slot slope

The slope of your drain determines how fast water runs through it. The steeper the slope, the faster water moves. Narrower, steeper drains may move water as quickly as wider drains with less of an incline. Fast-moving water is also less likely to get clogged on debris. Locations with a lot of small debris may choose a greater slope.

Channels can be of any length at surface, but the need for slope may limit their internal length. Very long stretches of trench drain may seem continuous at grade, with grates lined up in a long linear strip. Yet belowground the drain may be made of shorter individual segments, with a series of outlets.

Drains can be installed into existing slopes. This video shows a simple method to determine the slope of the ground with two poles, some string, and a level.

On flat ground, slope has to be created in the drain.

Concrete systems can be customized by the installer. Slope can deviate from the ground conditions or be matched to them. The channel can have an equal depth all along a sloped grade, or be varied to slope as the installer decides.

Pre-formed drains come sloped with a marked direction flow of water. 0.5% is standard, but check your specification to be sure.

When installing, make sure to consider the drain’s outlet and its capacity. Trench drains may empty into planted areas, gravel beds, storage areas, or sewer systems.

A cast iron 6-inch trench grate with slots in the shape of Greek keys.
Aesthetic and heel-proof grates are available to enhance public spaces.

Choosing channel size and grate hole size

The volume of water a trench system can handle is determined by the width of the drain, the material it is made of, the holes in the grate, and the slope. Debris is also a factor. Unscreened debris can clog drains and limit their function.

Smaller grate sizes prevent more debris from getting into the trench, but can also slow how fast water moves through the grating. When putting trench grates near trees, installers sometimes use larger widths then they’ve calculated based on flow sizes, and then use smaller grate hole sizes, to help strain debris without impeding function.

To find the minimum width of outdoor trench, take:

  • Volume calculated by the rational equation
  • Determined trench slope
  • Material used inside the trench

These numbers can be put into a flow calculator. Find the width of the trench that moves the right volume of water as determined with the rational equation.

Indoor applications often come with standard slot sizes and slopes, since the volume of water it needs to manage is fairly standard. Additionally, many indoor applications don’t deal with much debris.

Standard Indoor Trench Systems
Standard size
Liquid Volume
Debris
Grate requirements

Indoor Residential Pool

2-inch drain

Minimal volume

  • Splashes
  • Water carried out by bathers

Minimal

Will be walked on with bare feet. No tripping, slipping, or pinch hazards. No sharp edges or abrasive materials. Light-duty materials acceptable.

Indoor Community Pool

4-inch drain

Medium volume

  • Splashes
  • Water carried out by bathers

Minimal

Will be walked on with bare feet. No tripping, slipping, or pinch hazards. No sharp edges or abrasive materials. Light-duty materials acceptable.

Warehouse

6-inch drain

Medium to large volume

  • Risk management for broken or leaking inventory
  • Precipitation carried in on boots, assets, and vehicles
  • Floor cleaning

Minimal to Medium

Consider likely size of dropped objects.
Must be medium- to heavy-duty depending on vehicles and machines (forklifts, etc.)

Garages and Hangers

6- to 8-inch drain

Medium to large

  • Precipitation carried in on vehicles
  • Possible vehicle wash facilities

Minimal

“Heel-proof” in public garages to prevent high heels from becoming entrapped. Maximum grate hole size will be 5/16 inches or 8 mm.
Heavy-duty to special-duty for large trucks and airplane hangers.

Factory or Commercial Kitchen

6- to 8-inch drain

Medium to large

  • Risk management
  • Precipitation on vehicles
  • Cleaning stations or food preparation spills 

Minimal to Medium

“Heel-proof” may be important in sections of factory not requiring work-boots to prevent high heels from becoming entrapped. Maximum grate hole size will be 5/16 inches or 8 mm.
Medium-duty for small vehicles like forklifts and jacks. Heavy-duty where vehicles enter.

Special cases: car wash, cleaning, or vessel emptying

Variable

Large

  • Wash water through hoses or buckets
  • Larger trench for faster vessel emptying

Variable

Consider what is being washed to determine likely debris.

Flexibility with trench drain systems

Trench drain can be installed unobtrusively and works effectively through the center of hardscape that slopes in only one direction. When compared to a central circle drain, it offers more flexibility in surface design. The huge variety of options available allows trench to work as an unobtrusive and functional grate or as an attractive complement to the architecture and design of a location.

Contact us with any further questions you have, or browse our line of grates to see what might work best for your application.

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