Car Wash Drainage

One property, four waters that must never mix

Stand on a wash site and count what you are draining. The wash envelope — wand bays, an in-bay automatic, or a 70- to 130-foot express tunnel — where water is applied deliberately and leaves on the vehicle carrying grit, brake dust, and salt. The entry and exit aprons, the transition zone and hardest part of the job. The clean side: roof, stacking lane, and at an express wash a vacuum court of 10,000 to 15,000 sq ft. And the below-grade equipment room and reclaim pit — someone else's scope, but it sets your elevations.

Those four have opposite jobs. The clean side wants water gone: basin, pipe, storm connection. The wash envelope wants it captured and routed: caught on a defined line, kept off storm, handed to treatment. Nearly every wash-drainage failure we get called about is a site where the two met, usually at a doorway apron nobody drew a line across.

Separate this from the sites people compare it to. On a fueling forecourt the containment line exists because of fuel and vapor; in a service department, because of oils and solvents. Here the contaminant that destroys your system is not a hydrocarbon — it is sand: heavy, abrasive, relentless, free. Design around it and the rest gets easy. That also sets the regulatory frame: wash water is process water, so it goes to sanitary sewer under a local pretreatment permit behind an approved sand/oil interceptor, essentially never to storm. The storm network stays separate under MS4 illicit-discharge rules.

What a wash site needs, including what we don't sell

The water path, in order

Downstream of the slab the sequence rarely changes. Say it out loud on the first call with the contractor, so everyone knows which pieces are ours:

1. Bay or tunnel trench drain — a line intercept, because water arrives as a wide sheet off a flat pour and off the vehicle. Ours.
2. Grit trap — an in-line catch basin with a removable bucket, or a basin with its outlet raised to leave a sump. Also ours, and the highest-return item on the list.
3. Sand/oil interceptor, a three-chamber structure sized in gpm — downstream, by others, as are sampling manholes.
4. Reclaim tank, settling pit, filtration, odor control, pumps — all by others. We sell no pumps, no treatment, no recirculation hardware.
5. Permitted discharge to sanitary, with reclaimed water returning to the equipment room on its own loop.

Keep rain off the dirty side while you are at it: every gallon you let into the wash envelope is interceptor capacity you buy once and sewer volume you pay for forever. Tie canopy and building-edge runoff to the storm side, and grade so nothing sheets toward the door.

Subsurface and french drains — real here, and not ours

Subsurface drainage genuinely shows up on wash sites, so here is the straight answer. A french drain is perforated pipe bedded in washed stone and wrapped in geotextile, set below grade to pull water out of the soil, not off the surface. At a wash it appears as perimeter and underslab drains around a below-grade equipment room or pit — an empty tank in a high water table will genuinely try to float — and as underdrains beneath a detention basin. We sell no part of that system: no perforated pipe, no fabric, no aggregate. That is contractor-installed civil work, and nothing we sell is a french drain. We sell the layer above it: surface collection that keeps water out of the soil.

Sizing off the equipment schedule, not the sky

Here is the inversion that catches people: on nearly every other site rainfall sizes the drain, but inside a wash envelope the equipment schedule does — and that number comes from the supplier, not from car counts.

• Self-serve wand bays — a high-pressure wand runs 3 to 5 gpm, so four bays firing at once is 12 to 20 gpm. That is nothing.
• In-bay automatic — 35 to 70 gallons per vehicle over a 3 to 5 minute cycle: 10 to 25 gpm average, spiking during high-pressure rinse.
• Express tunnel — 60 to 120 gallons per car, and with arches over six or eight vehicles at once the schedules we see total 60 to 150 gpm at peak — the number that sizes the spine.

Notice what that does to a self-serve bay. At 15 gpm the trench is not flow-limited — it is grit-limited. The failure mode is not backup but sand parked in the invert two feet short of the outlet, and a silted wash trench stays that way until somebody digs it out. The criterion in a low-flow bay is velocity, not capacity, which is why built-in slope matters more here than width. NDS Dura-Slope carries 0.7% inside each 48" section across a 3.998" to 12.062" depth range; ACO channels step 5 mm deeper per 1 m section, about 0.5%, so a 33-ft run gains roughly 2" of fall without touching the slab. Pitch the floor 1/8" to 1/4" per foot toward the trench on top of that and the run sweeps itself.

Outside the building line, rainfall math takes back over. One inch of rain on 1,000 sq ft of pavement is about 623 gallons, so a 12,000 sq ft vacuum court in a 2-inch storm sheds roughly 14,950 gallons. With a coefficient near 0.9, that court against a 6 in./hr peak intensity works out to about 1.49 cfs, or 668 gpm — more per minute than the tunnel makes. Run your areas through the Runoff Calculator; Sizing for Storms explains the method. Then check the system can carry the result, because the channel is rarely the constraint — the outlet is.

A tunnel at 120 gpm clears a 4" outlet on paper and fails in practice, because that pipe runs at capacity with no margin for the grit it carries. Give it a 6", or split to two outlets. Then check the two constraints usually behind a real bottleneck here: the interceptor's rated gpm and the reclaim system's pump rate and pit turnover. Both are by others, and both will happily throttle a perfectly sized trench.

Load class, told in the vehicles that actually cross the grate

A wash tempts you to spec light, because the traffic is cars. Then a chemical delivery backs to the equipment room door, a customer arrives with a dually and a boat trailer, and the vacuum court gets a striping truck.

• Class B 125 (125 kN, about 28,100 lbf) — the floor inside a car-only bay or tunnel where guide rails keep everything else out.
• Class C 250 (250 kN, about 56,200 lbf) — our default outside the building line: aprons, stacking lanes, the vacuum court, the equipment room door.
• Class D 400 (400 kN, about 89,900 lbf) — truck and fleet washes, and any wash sharing an apron with a fuel island where tractor-trailers turn.
• Class A 15 (15 kN, about 3,370 lbf) — pedestrian only: the pay-station walk and curbed islands between vacuum stalls.

Your engineer may work in AASHTO shorthand: H-20 corresponds to a 16,000-lb wheel load, HS-25 to 20,000 lb. Those come from a different standard, so treat any mapping onto EN 1433 as a trade heuristic, never equivalence — table in the Load Class Recommendation Guide.

Two realities decide whether the rating survives. This is not a static-load problem: a conveyor pushes a vehicle with the driver's foot off the brake, and every grate gets lifted for cleaning, so locking grates are not optional — and pick one a single person can lift, because a grate that takes two never comes out, and a wash trench that never gets cleaned is the failure mode of this vertical. Second, every class assumes full concrete encasement per the manufacturer's instructions, grate set 1/8" to 1/4" below the finished surface so wheels load the encasement, not the grate edge. A channel set proud of a bay slab fails faster here than anywhere, because acid chemistry is already working that pour.

Where each piece lands on a wash site

Trench drains do four jobs here: the bay or tunnel line, the aprons that divide clean from dirty, the vacuum court and stacking lanes, and the ribbons nobody drew — the puddle at the pay station, the dog-wash bay added later.

The mistake we see most often on tunnels is a trench at the entrance and nothing at the exit — but the exit is where water leaves the building, riding on the vehicle. Even a strong blower package gives up a gallon or two per car; at 120 cars an hour that is 120 to 240 gallons on roughly 30 feet of apron every hour, onto a lot that drains to storm. Intercept at the door line, pitch the apron back toward the building, and tie the run to the wash side rather than the lot inlet twelve feet away.

Catch basins do a job here they do nowhere else in our catalog: they are the grit trap. Sand near 0.5 mm drops out of moving water in seconds while silt near 0.01 mm stays suspended for hours, so a basin that buys the flow a minute of quiet detention captures the heavy fraction and passes fines to the interceptor. Size it by the run feeding it: 12" on a single bay, 18" where two runs converge, 24" for a tunnel spine.

Grates get chosen on chemistry here as much as on load, since galvanized coating is the first casualty at pH 12–13 presoak. The material guide lives in our grates section — ductile iron, cast iron, steel, and channel grates by system. Add basin outlets, fittings, and a valve box over any isolation valve. On the clean side only, pop-up emitters and dry wells discharge where infiltration is permitted.

What we'd spec, bay by bay

Related sites we spec the same way

• Fleet & truck wash — the same path at Class D loads, several times the grit
• Gas stations & fueling — containment when the driver is fuel, not sand
• Auto repair & dealerships — dealer wash bays and service-drive collection
• Parking lots & retail — the storm side, plus our Drainage FAQs hub