Commercial Kitchen Drainage and Grease Trap Guide 2026: Floor Gullies, Channel Drains, Falls, Separator Sizing and FOB Prices

Every other utility in a commercial kitchen can be corrected after the floor goes down. Power can be re-run in trunking. Gas can be re-piped along a wall. Water can be teed off almost anywhere. Drainage cannot. It is cast into the slab, it runs on gravity, and once the screed has cured the only way to move a gully is to break concrete in a room that is now full of stainless steel.

That is why drainage is worth an hour of attention at drawing stage rather than a week of jackhammering later. This guide covers what a kitchen drain has to carry, how to choose between gullies and channels, how to size a grease separator under both the European and North American methods, the falls and pipe sizes that belong on your drawing, and what the stainless components cost FOB China in 2026.

What actually goes down a commercial kitchen drain?

Domestic drainage design assumes water with some soap in it. A commercial kitchen discharges something quite different, and each component causes a distinct failure:

• Fats, oils and grease (FOG). Discharged hot and liquid at 60-80 °C from pot sinks and dishwashers, then cooling as it travels. Grease begins to congeal below roughly 40 °C, so it does not block the drain at the kitchen — it blocks it 15 metres downstream, or in the street sewer, where it becomes the local authority’s problem and then your fine.
• Solids. Vegetable trimmings, rice, bones, cutlery, cling film, bottle caps. Anything that reaches the pipe without a basket to catch it will eventually find a joint to lodge in.
• Starch and detergent. Starch from pasta and rice water sets like weak cement in a slow-moving pipe. Detergents temporarily emulsify grease, carrying it past a badly sized separator and letting it re-form further along.
• Heat. Repeated 85 °C dishwasher discharges cause thermal movement at pipe joints, which is one reason high-temperature-rated pipe is still specified on the first few metres of a dishwash branch.

The design consequence is simple: move water fast enough that solids stay in suspension, and strip the grease out before the water leaves the building, while that grease is still warm enough to separate.

Floor gully, channel drain or kerbed bund: which goes where?

These three are not interchangeable, and the most common drawing error is using one where the wet-area behaviour calls for another.

Floor gullies are point drains, typically 200 x 200 mm or 300 x 300 mm in 304 stainless, with a removable silt basket and a 110 mm outlet. They suit locations with a defined, modest discharge: under a single sink, at a hand-wash station, at the entrance to a walk-in chiller. They rely on the floor falling towards them from every direction, which is achievable over two or three metres and increasingly optimistic beyond that.

Channel drains — a linear trough, commonly 150-250 mm wide with a removable slot or mesh grating and an integral fall to a single outlet — suit anywhere water arrives along a line rather than at a point: the front of a dishwash area, the discharge side of a rack conveyor machine, the run in front of a cooking suite where hoods drip and floors are hosed. A channel needs the floor to fall in one direction only, which is far easier to build and far more likely to be built correctly.

Kerbed bunds are raised stainless plinths that contain rather than drain, keeping wash-down water out of zones that must stay dry — electrical panels, a dry store threshold, the base of a combi oven stack. They are the cheapest insurance in the room.

A workable arrangement for a 150-cover hotel kitchen: channel drain along the dishwash line and the cooking suite, floor gullies at prep sinks and chiller doors, kerbed bunds at panels and the dry store threshold. Mark all three on the layout before the slab is poured.

How do you size a grease separator?

Two methods dominate, and which one applies depends on whose code your project is inspected against. Both are worth running, because they cross-check each other.

The European method: EN 1825-2

BS EN 1825-2 sizes a separator by nominal size (NS), expressed in litres per second, using NS = Qs × ft × fd × fr, where Qs is the peak wastewater flow in l/s, ft is a temperature factor (commonly 1.0 for discharge below 60 °C and 1.3 above), fd is a grease density factor (1.0 for densities up to about 0.94 kg/l), and fr accounts for detergents and cleaning agents (commonly 1.3, with some authorities applying 1.5).

Worked example, 150-cover hotel restaurant with a hood-type dishwasher and a three-compartment pot sink. Peak simultaneous flow Qs is assessed at 4.0 l/s. Dishwasher discharge exceeds 60 °C, so ft = 1.3. Standard vegetable and animal fats, fd = 1.0. Detergents present, fr = 1.3.

NS = 4.0 × 1.3 × 1.0 × 1.3 = 6.76, rounded up to the next standard size, NS 7. Rounding down is the classic false economy: a separator one size small does not fail gradually, it passes emulsified grease straight through under peak load, which is exactly when anyone is looking.

The North American method: PDI-G101

PDI-G101, the Plumbing and Drainage Institute’s testing and rating procedure for hydromechanical grease interceptors, sizes on total fixture flow rate in US gallons per minute. Typical published fixture ratings are 20 GPM for a three-compartment sink, 15 GPM for a commercial dishwasher, 10 GPM for a prep sink and 2 GPM for a floor drain.

The same kitchen: pot sink 20 + dishwasher 15 + two prep sinks 20 + four floor drains 8 = 63 GPM. A hydromechanical unit must therefore be rated at 63 GPM or above. Where a gravity interceptor is required instead, the common approach is flow rate multiplied by a 30-minute retention time, giving roughly 1,900 gallons, with codes frequently adding around 25% storage allowance for floating FOG and settled solids on top.

Note the divergence: the European calculation lands on a compact unit that can sit inside the kitchen, while the North American gravity route lands on a buried tank outside the building. Establish which regime applies before the civil works package is priced — retrofitting a 1,900-gallon tank into a finished car park is not a small variation.

Falls, pipe sizes and the numbers that belong on the drawing

• Branch drains, 100 mm: fall of 1:60 to 1:80. Steeper is not better — above roughly 1:40 the water outruns the solids and leaves them behind.
• Main runs, 150 mm: fall of 1:100 to 1:150.
• Floor fall to gully or channel: 1:80 to 1:100, roughly 10-12 mm per metre. Below 1:100 a wet floor stays a wet floor.
• Trap seals: 50 mm minimum, 75 mm where a gully serves an area that runs dry for long periods, otherwise the seal evaporates and the room smells of sewer by the second week of a seasonal closure.
• Distance to the separator: keep it short, ideally under about 10 metres of pipe, and insulate the run if it must be longer. Grease that has cooled below 40 °C before it reaches the separator has already deposited most of itself in your pipe.
• Rodding access: at every change of direction and at least every 15 metres, positioned so a rod can be pushed without moving a 300 kg cooking suite.

Which appliances must never be hard-piped to the drain?

Several appliances require an indirect connection with a physical air gap — typically at least 25 mm, or twice the outlet diameter where that is greater — discharging over a tundish or open funnel. Hard-piping them is one of the most common installation defects found at inspection:

• Ice machines. A hard connection puts the ice storage bin one blockage away from a sewer backflow. This is a potable-water contact surface.
• Combi ovens. Condensate is hot and intermittent, and most models require a vented, gravity-fed drain. Combi drains must not share a branch with a gully that can surcharge, or steam and odour travel back into the cooking chamber. Where hard water is present the same drain carries descale discharge, which is a good reason to read our commercial kitchen water treatment guide alongside this one.
• Food preparation sinks and vegetable washers. Anywhere food contacts water directly, an air gap is normally required.
• Refrigeration and cold room condensate. Small volume, continuous, and a frequent source of blocked lines when run without a gap and without a fall.

Dishwashers vary by jurisdiction and by machine: some codes permit a direct connection with an integral air break, others require a full air gap. Confirm the machine’s requirement against local code before the plumbing package is fixed.

304 or 316 stainless, and what to check on the grating

Use 304 for the great majority of drainage components. Move to 316 where chlorides are aggressive: coastal sites, kitchens using heavy chlorine sanitiser regimes, fish preparation areas and brine handling. The chloride, not the humidity, is what pits the steel.

On gratings, check three things. Load class first — a grating on a route where a 400 kg loaded rack trolley crosses daily needs a different section from one under a hand-wash basin. Removability second: if a grating cannot be lifted by one person wearing wet gloves, the basket underneath will not be emptied. Slip resistance third.

Gullies, channels and bunds are also the components most often needed in non-standard sizes, because they have to land between an existing column and an existing wall. These are fabricated items, not catalogue items. At our own works they are cut, folded and welded to the approved drawing with outlet position, channel width, invert depth and grating type taken from the site survey, on the same 25-45 day production cycle as the main equipment rather than the 60-90 days a separate civils supplier typically quotes, and with no tooling charge for a non-standard length. Buyers comparing sources for fabricated stainless will find the same distinction between manufacturers and traders discussed in our guide to the principales fabricantes de equipos de cocina comercial en China.

What does commercial kitchen drainage cost?

Indicative FOB China prices for 2026, for stainless components at project quantities:

Prices vary with steel gauge, grating class and quantity. Buried gravity interceptors, civil works, screed falls and below-slab pipework are local-contractor scope everywhere and are not included above.

Five drainage mistakes that cost money on site

1. Gullies located from the equipment plan rather than the equipment schedule. The plan shows where a machine sits; the schedule shows where its outlet is. A gully 600 mm from the outlet means a visible surface pipe forever.
2. An under-sink trap standing in for a separator. A 40-litre under-sink trap serving a whole dishwash line is not a grease separator, whatever the invoice calls it.
3. Separator installed where it cannot be serviced. A unit needing a full-height lid lift, positioned under a 900 mm worktop, will be emptied once and then never again.
4. Screed falls set after the gullies are cast. Uncoordinated levels leave water ponding within a metre of a drain in a brand-new kitchen.
5. Drainage left out of the equipment order. Gullies then arrive as a late local purchase, in whatever size the merchant stocks and an unknown steel grade.

What to put on the purchase order

Six lines prevent most of the above. Steel grade (304 or 316) and gauge. Component type, size and quantity, cross-referenced to a drawing number. Outlet diameter and outlet position, left, right or centre. Grating type and load class. Trap seal depth. Whether the separator is supplied loose or pre-piped as a stainless skid. Add these to the equipment order and drainage stops being a variation and starts being a delivered item, which is the same logic that governs the rest of the site services covered in our guides to gas supply, voltage and power y installation and commissioning.

Because drainage has to be fixed before anything else is built, it is the one service best resolved on a drawing rather than in an email. Send us your floor plan and cover count and we will mark up gully and channel positions, floor falls and separator location on a dimensioned layout, free of charge and before any order is placed — [email protected].

Preguntas frecuentes

How big a grease trap does a 150-cover restaurant kitchen need?

Sizing follows flow, not covers. Under EN 1825-2 a kitchen with a peak flow of 4.0 l/s, hot discharge and detergents calculates to NS 7. Under PDI-G101 the same fixture set totals about 63 GPM. Run both methods and check which regime your inspector applies.

Can an under-sink grease trap replace a proper separator?

Only for a single low-flow fixture. Under-sink units in the 25-50 litre range are designed for one sink, not for a dishwash line, and cannot hold the retention volume that lets grease separate at peak load.

What fall do commercial kitchen drains need?

Roughly 1:60 to 1:80 on 100 mm branches, 1:100 to 1:150 on 150 mm mains, and 1:80 to 1:100 across the floor towards gullies or channels. Excessive fall is also a fault, because water outruns solids and leaves them in the pipe.

Do ice machines and combi ovens need an air gap?

Yes. Both require indirect drainage over a tundish or open funnel with a physical air gap, usually at least 25 mm or twice the outlet diameter. Ice machines are a food-contact application, and combi drains must not be able to receive backflow or steam from a shared branch.

Should kitchen drainage be 304 or 316 stainless?

304 covers most kitchens. Specify 316 where chloride exposure is high: coastal sites, chlorine-based sanitiser regimes, fish preparation and brine handling. Chloride attack, not humidity, is what causes pitting.