How Is Auto Shop Plumbing Designed for Drains and Service?

The engineer draws two drainage systems that never meet until the sewer. The oily system collects bay trench drains, wash bays, pit sumps, and equipment drains, routing them through the oil-water separator sized to the tributary area. Pipe sizing follows the plumbing code with the separator manufacturer's inlet requirements, and cleanouts are placed where a jetter can actually reach them. The sanitary system serves restrooms, lounge and break room fixtures, and any domestic use, connecting directly to the building sewer. The engineer verifies the sewer authority's pretreatment requirements for the oily system — sampling ports, discharge limits, permit — and confirms whether the reviewing authority is the city utility or, in unincorporated areas, the county sanitation agency, since submittal requirements differ. Keeping washdown hose bibs on the oily side's containment and domestic fixtures on the sanitary side is detailed fixture by fixture.

Trench drains are engineered for traffic: grates rated for the vehicle loads crossing them, trench bodies sloped to drain, and outlets sized so the trench never ponds during washdown. The engineer places trenches where water actually goes — across bay door lines, in wash bays, around the pit — and keeps them out of lift anchor zones and precision floor areas like alignment bays. Sediment buckets at each outlet catch the grit that would otherwise fill the separator. Water service sizing totals the realistic peak: wash equipment, hose bibs, mop sinks, restrooms, and equipment make-up. The engineer sizes the meter, the backflow assembly, and the distribution piping, then provides hot water where the work needs it — restrooms, lounge, wash operations — with recirculation where the runs are long. Backflow preventers go at every hazard connection per the water provider's rules, and the assemblies are placed where they can be tested and serviced without crawling.

Use this checklist during design and plan check: • Oily and sanitary drainage drawn as separate systems that never cross-connect • Oil-water separator sized to the tributary drainage area with sampling ports • Trench drains traffic-rated, sloped, and placed where water actually flows • Sediment buckets or traps at drains, accessible for cleaning • Water service and meter sized to the realistic simultaneous peak demand • Backflow prevention at every hazard connection per the water provider's rules • Hot water with recirculation where runs are long; restrooms per code occupant load • Sewer pretreatment authority confirmed: city utility vs county sanitation agency