Rug wash pit, 14 × 20 ftRaised, sloped, fiberglass-lined platform you can build from Lowe's and Home Depot

The floor of the pit is a wood platform that sits on concrete-block piers on your existing slab. Three beam lines are set at stepped heights, so every joist that rests on them is automatically tilted ¼ inch per foot toward the front. All wash water runs to a trench drain along the front edge, through a sediment and lint trap, and into the sanitary sewer.

The platform is skinned with two layers of plywood, then laminated with fiberglass and finished in gelcoat with non-skid grit. That makes one seamless, boat-hull-style tub, including the 6-inch curbs, that you can scrub with a rotary machine and hose down every day. No slab cutting or jackhammering is needed.

Why 14 × 20: it takes a 12 × 18 rug flat with about a foot of working border, and two 9 × 12s side by side. Why 18 inches high: it leaves room under the deck for the beams, joists, and a drain line that can fall by gravity into the trap without a pit in the floor.

1. Where the water goes

This is the decision that drives everything else. Find your nearest floor drain, sewer cleanout, or sanitary line and put the pit's front-left corner (the trench outlet) as close to it as you can.

2. Waterproof skin

3. Size

If you regularly wash rugs larger than 12 × 18, grow the pit in 4-ft steps along the 20-ft side (more joists, longer beams, more plywood). Don't make it deeper front-to-back without adding a fourth beam line.

4. Tearing out an existing pit

If you're replacing an old pit, plan one extra day for demo, disposal, and patching. Check the slab where the old pit sat for cracks or soft spots before you set any piers on it.

Quantities are for the 14 × 20 plan with about 10% extra. Prices are rough October 2026 ranges; check your cart before you commit.

Framing

Deck and curbs

Fiberglass and finish

Drainage

Walls, water, power

Budget summary

Licensed plumber and electrician labor, permits, and tankless venting typically add $2,000–5,000 depending on how far the gas, water, and power runs are.

• Circular saw, miter saw, impact driver, drill, two caulk guns
• Self-leveling line laser or rotary laser (rent one); 4-ft and 8-ft levels; chalk line; tape
• Random orbit sander with 80 grit; angle grinder with flap disc for trimming cured glass
• Fiberglass laminating rollers (aluminum fin type), plenty of chip brushes
• Organic-vapor respirator, nitrile gloves (boxes), disposable coveralls, safety glasses, knee pads
• Two box fans for cross-ventilation, ABC fire extinguisher within reach

1. Before day 1

   Plan, permits, trades

   • Mark the spot: trench outlet near your drain, 4 ft of walkway on open sides, short carry to your wringer and drying room.
   • Call your city or county about a plumbing permit and commercial wastewater rules (if you're in Chamblee, that's DeKalb County Watershed Management). Ask whether they require a specific sand or lint interceptor.
   • Book a licensed plumber (drain tie-in, hot water, gas) and electrician (GFCI circuits, lights) for days 9–11.
   • Order fiberglass mat and resin now; online delivery can take a week.
2. Day 1

   Demo and layout

   • Remove any old pit, scrape and patch the slab.
   • Snap the 20′6″ × 14′6″ rectangle. Square it: diagonals should both measure 25′1⅜″.
   • Snap three beam lines parallel to the long side: 4″ in from the front edge, on the centerline, and 4″ in from the back edge.
   • Shoot the slab with the laser at each pier spot and write the high/low reading on the floor. Slabs are rarely flat; you'll shim to make up the difference.
   • Lay the 6-mil poly over the footprint.
3. Day 2

   Piers and beams

   • Set 6 solid blocks per beam line, about every 4 ft, with the end ones flush to the short edges.
   • Build each beam from two 2×8s glued and screwed, splices centered over a pier.
   • Shim so beam tops land at these heights above a flat slab: front 11″, middle 12¾″, back 14⅜″. Adjust for your slab readings.
   • Check each beam is dead level along its length. The slope comes from the beam steps, not from tilting the beams.
4. Day 3

   Joists and rims

   • Cut 21 joists to 14′6″. Set them front-to-back across all three beams, 12″ on center.
   • Tie each joist to each beam with a hurricane tie.
   • Add front and back rim joists. Add a row of blocking over the middle beam.
   • Check slope: a 4-ft level laid front-to-back should show 1″ of drop.
   • Frame a pocket for the trench drain along the front, between the front rim and a header about 8″ back, set low enough that the channel's top sits ⅛″ below the finished deck.
5. Day 4

   Decking

   • Layer 1: pressure-treated plywood, glued and screwed every 6″ on edges and 8″ in the field, ⅛″ gaps.
   • Layer 2: untreated exterior BC plywood, seams offset at least 2 ft from layer 1, glued and screwed every 6″, screws countersunk. Treated plywood is wetter and fiberglass bonds poorly to it, which is why the top layer is untreated.
   • Cut the trench opening through both layers.
6. Day 5

   Curbs, trench, prep

   • Stand 2×6s on edge around the perimeter, glued and screwed down into the rims. Clad inside, top, and outside with ½″ plywood. Round all outside edges to about ¼″; fiberglass won't wrap a sharp corner.
   • Bed the channel drain in adhesive, outlet at the front-left corner. Dry-fit the 4″ outlet pipe through the skirt.
   • Confirm the plywood reads under 12% moisture. Fill screw holes, sand everything with 80 grit, vacuum, wipe with acetone.
   • Run a ¾″-radius cove of thickened resin into every inside corner where floor meets curb.
7. Days 6–7

   Fiberglass

   • Work between 65 and 85°F. Mix small batches; follow the hardener ratio on the can for your temperature.
   • Roll a thin seal coat of resin onto the bare plywood.
   • Layer 1: lay mat in sections, wet it out, and roll until every white fiber turns clear and no bubbles remain. Overlap seams 2″. Run mat up the curbs, over the tops, and down the outside, and lap it onto the drain flange.
   • Layer 2 the next day, seams offset from layer 1. Lightly sand any high fibers first.
8. Day 8

   Gelcoat and non-skid

   • Scuff with 80 grit and wipe clean.
   • Roll two coats of waxed gelcoat. Broadcast non-skid grit into the floor coat only; keep the curbs smooth so they wipe clean.
9. Day 9

   Drain hookup and flood test

   • Plumber connects outlet → basket strainer → trap → sewer (or pump).
   • Plug the outlet, fill until there's 1″ of water at the front, and leave it 24 hours. Check underneath with a flashlight. Grind out and patch any weep spot.
10. Days 10–11

    Walls, water, power

    • Glue FRP panels to the back and side walls, 8 ft up. Bring the bottom edge down over the outside of the curb and seal with silicone so splash runs into the pit, not behind it.
    • Plumber: tankless heater, thermostatic mixing valve, two hose reels on the back wall.
    • Electrician: 20A GFCI receptacles mounted at least 4 ft above the deck, vapor-tight LED fixtures overhead.
    • Hang the curtain track and curtain on the open side.
11. Days 12–18

    Cure and commission

    • Keep machines off it for 5–7 days while the resin reaches full hardness. Light hose rinses are fine after day 3.
    • Wash a cheap rug first. Watch for puddles, check the trench keeps up with both hoses running, and empty the trap.

With two people working full days. Add 1–3 weeks before day 1 for permits and getting trades scheduled.

Yellow: your crew. Solid blue: licensed trades. Hatched: cure time, no heavy use.

With an EPDM liner instead of fiberglass, days 6–8 collapse into one day and the cure wait mostly disappears, so you're looking at about 8 work days.

• Wash water carries soil, sand, fiber, detergent, and sometimes dye. It goes to the sanitary sewer only, never a storm drain, parking lot, or ditch. Commercial discharge is regulated, so confirm the requirements with your county before you connect.
• The trap does the real work: the first chamber settles sand and grit, the baffle holds back fiber, and only clearer water passes on. Empty it daily on busy days.
• The basket strainer at the trench outlet catches fringe and lint before it gets into the pipe.
• Include a cleanout fitting in the 4″ line right after the trap so you can snake it without cutting pipe.

• Hot water: a storage tank runs out mid-rug. A gas tankless unit gives continuous flow; it needs a gas line and venting, so it's a plumber job. An electric tankless big enough for two hoses usually needs more amperage than a warehouse panel has spare.
• Temperature: the thermostatic mixing valve lets you cap the water. Wool and dyed rugs are safer lukewarm (around 100°F or lower) to limit dye bleed and shrinkage; synthetics tolerate hotter water.
• Pressure: use a rug-rinse wand or fan tip and keep a pressure washer well back from the pile. High pressure can blow out foundation threads and fringe.
• Power: dedicated 20A GFCI circuits for the rotary scrubber and the pressure washer, outlets high on the wall with in-use covers, cords never lying in standing water.
• Air: a wash pit puts a lot of moisture into the building. Run an exhaust fan near the pit and keep the drying room separate.

Daily and monthly

• Pull the trench grates and rinse the channel at the end of each wash day; empty the basket and trap.
• Look for chips in the gelcoat monthly. Fill them with a gelcoat repair kit before water reaches the wood.
• Use nylon brushes only; no wire brushes or metal scrapers on the floor.
• Plan to scuff and roll a fresh gelcoat coat every few years as the non-skid wears.

Common ways these builds fail

• Building it flat. Without the ¼″ per foot slope, water puddles and rugs sit in dirty water.
• Putting resin straight onto treated plywood, or onto plywood that isn't dry. The glass peels.
• Sharp inside corners with no cove. The mat bridges, leaves an air pocket, and cracks.
• Skipping the 24-hour flood test. A pinhole you find on day 9 is a 10-minute fix; one you find in six months is rotten plywood.
• Outlets mounted low or non-GFCI.

Checked against the plan's own numbers on 5 October 2026. Two things to change before you buy, then the parts that hold up.

1. The resin is 20–25% short

1,000 sq ft of 1.5 oz chopped strand mat is about 94 lb of glass. The 18–20 gallons in the list only works at a 2:1 ratio, and chopped strand soaks up far more resin than cloth does — hand layup with a roller lands nearer 2.5:1.

2. The flood test only wets the front third

Day 9 says fill to 1″ at the front and leave it 24 hours. Because the floor is sloped on purpose, water 1″ deep at the front runs out 4 ft back — the rear 10 ft and all four curb coves never get wet. The coves are exactly where mat bridges and leaves a void.

3. What checks out

• Slope maths is internally consistent. 18″ front and 21½″ back is exactly 3.5″ over 14 ft, and the stepped beam tops (11″, 12¾″, 14⅜″) round correctly to the eighth.
• 21 joists at 12″ on centre is right for a 20 ft run — 21 positions including both ends.
• Plywood and mat quantities are sensible with waste: 280 sq ft of floor is 8.75 sheets per layer, so 11 covers offcuts.
• Untreated top layer is correct and commonly got wrong. Resin bonds poorly to treated plywood, and the under-12% moisture gate before laminating is the other half of that.
• Building the slope into the beam heights rather than tilting beams means it cannot be accidentally levelled flat later.

4. This page needs a connection

The 3D model and the fonts load from CDNs at runtime (three.js, OrbitControls, Google Fonts). On a warehouse connection that drops, the model falls back to a plain message and the build guide below it still works. If it needs to run offline on site, those three files can be bundled into the page.