A commercial kitchen waste pipe needs enough continuous fall to carry solids and grease without outrunning the water, but there is no single gradient that suits every diameter, appliance and route.
Installation boundary: This article explains design and coordination principles. Drainage, ventilation, fire, electrical and fabrication work must be designed and carried out by competent people using current project documents, manufacturer instructions and the requirements that apply in the relevant UK nation.
The short answer
Do not specify every commercial kitchen waste at 1:40 by habit. Approved Document H gives different limits and design routes for different branch pipes, while larger or heavily loaded systems need calculation to BS EN 12056 principles. The working design must consider pipe diameter, discharge rate, length, ventilation, temperature, grease loading, changes of direction and access for clearing. A pipe that looks steep on a drawing can still hold water if the slab, brackets or final connection force a backfall.
Why the number on the drawing is only part of the answer
Fall is the vertical drop divided by the horizontal run. A nominal 1:40 fall means 25 mm of drop per metre; 1:80 means 12.5 mm per metre. Those figures are useful for setting out, but they do not replace hydraulic design. A small branch from a sink behaves differently from a combined line receiving a pre-rinse sink, dishwasher discharge and pot-wash sink. Pumped discharge behaves differently again because the appliance may deliver a short, high-rate slug rather than a steady gravity flow.
The fall must also survive construction tolerances. Long plastic runs can sag between supports, suspended pipework can be lifted by other trades, and a floor outlet may be installed higher than the level used by the kitchen designer. The useful check is not simply the start and finish level: survey the invert at intermediate points and prove there is no belly or reverse fall. Where the route changes gradient, Approved Document H expects access to be considered because that is where solids and grease are more likely to collect.
Design the whole route from appliance to foul drain
Start with the appliance manufacturer's waste connection, maximum hose length and permitted outlet height. Then identify the trapped receptor, branch size, grease-management arrangement, venting method, stack or drain connection and every access point. The waste route should remain as straight as practicable, use large-radius changes where appropriate and avoid hidden joints under fixed equipment or inaccessible plinths.
On refurbishment projects, do not assume the visible floor connection falls correctly beyond the room. Record invert levels, pipe material and the direction of flow during the commercial kitchen site survey. If the connection cannot be proved, allow for testing or a drainage survey before the equipment order becomes irreversible.
What common failures reveal
| Site symptom | Likely installation issue | Practical consequence |
|---|---|---|
| Water remains in an open pipe after testing | Backfall, sagging support or a high downstream connection | Odour, grease settlement and repeated blockage |
| Sink drains slowly only at busy periods | Combined flow exceeds the branch or downstream capacity | Overflow and loss of usable sink capacity |
| Drain gurgles or trap seal moves | Pressure fluctuation, poor venting or an unsuitable branch arrangement | Foul air can enter the kitchen |
| Pipe blocks at the same elbow | Tight change of direction with no cleaning access | Costly strip-out each time the line blocks |
| Waste leaks after equipment is moved | Hose used to correct levels or tensioned connection | Water damage and hygiene failure behind the appliance |
Installation and coordination checklist
- Record finished-floor level and the invert level at every connection.
- Confirm each appliance discharge type: gravity, pumped, hot, greasy or chemical-bearing.
- Size the branch and combined drain for simultaneous use rather than one appliance in isolation.
- Set a continuous design gradient and show intermediate support or slab levels.
- Keep bends near accessible points and use a route that can be mechanically cleared.
- Coordinate the grease separator before fixing the final downstream level.
- Check trap seals and ventilation so discharge does not siphon adjacent traps.
- Water-test the completed run and inspect it before enclosing floors, walls or plinths.
Commissioning and handover
The handover drawing should show pipe sizes, materials, gradients, invert levels, traps, vents, rodding points, grease-management equipment and the final connection. Photograph concealed pipework with a scale before it is covered. During commissioning, release water from the actual appliances, not only a bucket at the nearest sink, and inspect the whole accessible route for standing water or leakage.
Questions to ask before approving the work
- What design standard and loading calculation has been used?
- Where can every section be rodded or cleaned?
- What tolerance is allowed before a nominal fall becomes a backfall?
- Has the existing downstream drain been surveyed?
- Who signs off the final levels before the floor closes?
Capture the installation detail before it becomes a site problem
Capture the service positions, levels, route constraints and access risks before equipment or fabrication is ordered.
Technical sources and further reading
- Approved Document H: Drainage and waste disposal
- Food Standards Agency: Getting ready to start a food business
- RATIONAL: Documents and appliance instructions
Last technical review: 14 August 2026. Always check the latest manufacturer instructions, statutory guidance, standards and project approvals before installation.