Can You Put a Floor Drain in a Shop or Garage in Spokane? Slope, Trench Drains and Washington's Floor Drain Rules (2026)

Short answer: you can put a floor drain in a Spokane-area shop or garage, but where it drains is the hard part. In Washington, a floor drain that sends oil, antifreeze or other vehicle fluids into a drywell, septic system or the ground is a prohibited "motor vehicle waste disposal well," and Spokane County says it usually won't let a shop floor drain connect to the sanitary sewer either. For most private shops, the practical options are a floor sloped to the door, a trench drain at the door with a safe outlet for snowmelt only, or a drain to a sealed holding tank. Here's what the rules say and how we pour the slab around each choice.

What the building code requires

The residential code doesn't require a floor drain in a garage. It requires the floor to move liquids somewhere:

• IRC R309.1: garage floors must be an approved noncombustible material, and the area used for parking vehicles "shall be sloped to facilitate the movement of liquids to a drain or toward the main vehicle entry doorway"
• No minimum slope: the code doesn't set a number. Builders commonly use about 1/8 inch per foot, which works out to about 4-1/2 inches over a 36-foot-deep floor
• Commercial buildings: garages built under the IBC follow IBC 406, which uses the same "slope to a drain or toward the main vehicle entry doorway" language

So the simplest code-compliant shop floor has no drain at all. It just falls toward the overhead door.

Why Washington is strict about shop floor drains

The rules come from groundwater protection, and the Spokane area has more reason than most to care:

• Sole source aquifer: in 1978 the EPA designated the Spokane Valley-Rathdrum Prairie aquifer a sole source aquifer, meaning it supplies at least half of the area's drinking water with no practical alternative
• Fast-draining ground: the aquifer sits in thick gravels, cobbles and boulders left by the Ice Age Missoula floods, so whatever goes into the ground gets to the water quickly
• Ecology's rule: under WAC 173-218-040, a "motor vehicle waste disposal well" that receives fluids from vehicle repair or maintenance is a prohibited Class V well in Washington and must be decommissioned immediately. Ecology says these are typically found in repair bays at service stations, garages, dealerships and shops that repair or maintain vehicles
• The federal rule: the EPA banned new motor vehicle waste disposal wells nationwide in April 2000. It says the typical one is a floor drain or sink in a service bay connected to a septic system or dry well, and that any underground system receiving vehicle waste counts
• Stormwater wells: a drywell that takes only stormwater at a site where vehicles are worked on isn't treated as a waste well, but only if it's protected from receiving vehicle waste

The state rule is written around repair and maintenance activity, and it classifies the well by what goes into it, not by how it's built. If you change your own oil or work on equipment in your shop, plan as if the rule applies to you.

Can a shop floor drain go to the sewer?

If you're on Spokane County sewer, the County's answer is "usually not":

• Case-by-case review: Spokane County's Wastewater System Division reviews shop and garage floor drain requests one at a time
• If allowed: you'll probably have to install a sand trap and an oil-water separator to keep grit and oil out of the sewer
• No stormwater in the sewer: County code prohibits discharging storm water into the sanitary sewer, which includes snowmelt off a truck
• Permits: connecting or altering a side sewer takes a County sewer connection permit and inspection

Inside the City of Spokane, Spokane Valley or other cities, the sewer utility sets its own rules, so we check with the right office before anything goes in the slab.

Can it go to a septic system?

Treat this as a no. The EPA lists floor drains tied to septic systems as the most common kind of motor vehicle waste disposal well, and a septic system is designed for household sewage, not oil, solvents or antifreeze. The Spokane Regional Health District permits septic systems in Spokane County, so ask them before connecting anything other than household plumbing.

Shop floor drain options that work

These are the setups we see work for private shops and garages:

• Slope to the door, no drain: the cheapest, simplest and lowest-risk option. Snowmelt runs out the door. Plan the apron so the water doesn't pond and freeze right at the threshold
• Trench drain at the door: a trench drain across the opening catches snowmelt before it runs onto the apron. Where it drains has to be legal, so it isn't a shortcut around the rules above
• Central drain to a holding tank: the EPA lists a sealed holding tank, pumped and disposed of properly, as an alternative for shops that can't use a drywell. The floor can then slope to the middle instead of the door
• Sewer with an oil-water separator: possible where the sewer utility approves it, with the separator and sand trap they require
• Wash bays: washing vehicles inside means a lot more water and more contaminants. That's a design question for the sewer utility or Ecology before the slab is formed, not after

Whatever drain you pick, it's cast into the slab, so the decision has to be made before we pour.

How the drain changes the slab

• Slope sets the forms: a floor that falls to the door is formed on a plane, while a floor that falls to a center drain is formed in sections that slope toward it. That changes where the control joints go
• Thickness at the low point: slope can't come out of the slab's thickness. The base is graded to the slope so the concrete stays full depth. See how thick a shop slab should be
• Vapor retarder: a heated shop gets a vapor retarder under the slab, and drain bodies and pipe penetrations have to be sealed through it. See our vapor barrier guide
• Frost: drain lines under an unheated shop or apron sit in frozen ground for much of the winter, so depth and routing matter. Spokane's frost line is 24 inches

What it costs

We don't publish set prices for shop slabs, drains or separators yet, because they depend on the size of the slab, the drain type and what the sewer utility or health district requires. Raven's current installed price for a PEMB metal building or a pole building is about $60 to $75 per square foot, with electrical, plumbing and utility hookups extra. Drains, separators and holding tanks fall under plumbing, so we quote them per project.

Get a shop slab quote

Raven Buildings pours shop and garage slabs and builds the shops that go on them with our own crews, so the slope, drains and doors are planned together. Call 509-350-2016 or request a free quote with your address, building size and how you plan to use the floor.

Sources

• PEMB and pole building pricing: Raven Buildings' current rates
• IRC R309.1 garage floor slope requirement, quoted by Building Code Trainer
• Hansen Buildings, Sloping Concrete Floors (1/8 inch per foot practice, IBC garage floor language)
• Washington Department of Ecology, Underground Injection Control Program
• WAC 173-218-040, UIC well classification including allowed and prohibited wells
• WAC 173-218-120, Decommissioning a UIC well (Cornell LII)
• US EPA, Motor Vehicle Waste Disposal Wells
• eCFR, 40 CFR 144.89, closing a Class V well (alternatives such as holding tanks)
• USGS, Spokane Valley-Rathdrum Prairie Aquifer (geology and 1978 sole source designation)
• Spokane County Sewer Connection FAQs (shop and garage floor drains)
• Spokane County Side Sewer Installation Handbook
• Spokane Regional Health District, Septic Systems
• City of Spokane / Spokane County Joint Policy BUILD-07, Official Frost Depth (PDF)