Phoenix Journal · Kitchens
Drainage is the part of a commercial kitchen nobody notices until it floods or smells. Plan the falls, channels and grease separator properly at the start and you avoid breaking out a finished floor later.
Kitchen planning
Drainage is the part of a commercial kitchen that nobody notices until it fails - and by then you are mopping the floor, chasing a blocked gully and explaining a bad smell to an environmental health officer.
Getting drainage right at the planning stage costs a fraction of putting it right later. Falls have to be cut into the screed, channels have to sit where the water actually lands, and grease has to be intercepted before it reaches the public sewer. Change your mind after the floor is laid and you are into breaking out concrete, not moving a bracket. So it pays to think this through properly, in the right order, before the first tile goes down.
This guide walks you through how to plan drainage for a working commercial kitchen in the UK - the falls and channels, the grease separator that the Building Regulations expect, the food-waste rules that changed in 2025, and the maintenance regime that keeps the whole thing flowing. It is written for operators and their designers rather than for drainage engineers, so the aim is to give you enough to ask the right questions and spot the wrong answers.
Good drainage design follows the work, not the wall. Before anyone draws a channel, map where water actually hits the floor: under the pot-wash, at the combi ovens that get pulled out and hosed, beside the tilting bratt pan, at the ice machine, below the pass. Those are your drainage points, and the floor has to fall towards them.
For a commercial kitchen floor, a fall of roughly 1:80 - about one and a quarter per cent - moves water briskly without making the floor feel like a slope underfoot. Anything flatter and you get standing puddles that breed bacteria and smell; anything much steeper and trolleys roll and staff notice. The fall is formed in the screed, so it has to be decided before the floor finish is specified, and it has to be coordinated with the position of every gully and channel so the water has somewhere to go once it gets there.
Then there is the choice between floor gullies and channel drainage. A gully is a single point - simple, cheap, ideal under a single appliance or a pot-wash. A channel, or slot drain, is a long linear run that collects water across a wider area and is the better answer in front of a cook line or a row of appliances that all shed water. Both should be stainless steel for a commercial kitchen: it is the most hygienic surface, it copes with near-boiling water and caustic cleaning chemicals, and it takes the weight of loaded trolleys and mobile combi ovens without deforming. Each should carry a removable basket or sieve to catch solids, and a trap to seal foul air out of the room, in line with the principle in BS EN 12056 that indoor drainage must not vent sewer gases back into the building.
One more layout point that is easy to miss: leave access. Every gully needs to be liftable for cleaning, every channel needs its grating to come out, and the run to the grease separator needs rodding access. Box something in behind a fixed counter and you have designed in a future problem. This is exactly where drainage planning has to talk to the rest of the kitchen plan - the same discipline you bring to planning the layout, extraction and gas runs so that services do not fight each other for the same slice of floor and ceiling.
The rules that shape the design
Three numbers do more to shape a UK kitchen drainage scheme than any others. They come from the Building Regulations, from the falls you cut into the floor, and from the food-waste law that changed in 2025.
Approved Document H, section 2.21, is explicit: drainage serving a commercial hot-food kitchen should be fitted with a grease separator complying with BS EN 1825-1 and designed in line with BS EN 1825-2, or another effective means of grease removal. That is not guidance you can wave away - it sits behind the sign-off on the building work. Sizing under Part 2 works from your peak flow, water temperature and the density of the grease, giving a nominal size for the separator; get it wrong and either it overflows at the busy service or you pay for a unit far bigger than you need.
Behind Part H sits harder law. The Water Industry Act 1991 makes it an offence to discharge into a public sewer anything likely to interfere with the free flow of its contents, and fats, oils and grease are exactly that. Water companies do inspect hot-food premises, and enforcement is real: a prosecution can bring a fine of up to £50,000 in the magistrates’ court, and an unlimited fine in the Crown Court. A grease separator is not a nice-to-have - it is how you stay on the right side of that line.
Sizing and siting the separator is the design decision that repays the most thought. It can be a large below-ground interceptor outside the building or a smaller under-sink or in-line unit, and the right answer depends on your throughput and your available space. Whatever the type, plan for it to be emptied: the working guideline is that a separator should be de-sludged before it is around a quarter full of grease, which in a busy kitchen typically means at least monthly, and often more. Site it where a tanker or an operative can reach it, not buried under a delivery bay.
Food waste is the other half of the story, and the rules have moved. From 31 March 2025, England effectively banned the disposal of food waste through sink macerators for commercial premises, bringing it into line with Scotland, which did so back in 2014, and Wales in 2023. Under the Environment Act 2021 and the Simpler Recycling regime, solid food waste has to be separated and sent for licensed collection and treatment, not ground up and flushed. In practice that means your drainage plan should assume solids are kept out of the system - caught in baskets and strainers at every gully and sink - and that you have bin space and a collection contract for separated food waste designed into the kitchen from the start.
Volume matters too. The busier the kitchen, the more water, grease and covers per hour your drainage has to swallow, so the scheme has to be sized for where the business is heading, not just where it is today. If you are scaling up service or adding delivery, factor that into the falls, the channel capacity and above all the separator size - the same forward view you take when planning capacity for a growing delivery business. It is far cheaper to size once for the peak you expect than to dig the floor up in two years.
Finally, none of this survives on its own. Falls silt up, baskets fill, separators cake, and a channel that ran clear on day one will back up if it is never lifted and cleaned. Build a planned regime around the system from the day it is commissioned - scheduled emptying, regular flushing of the runs, and a proper deep clean of the channels and gullies - and keep the records to prove it. That paperwork is what you show an inspector, and it is what keeps a small problem from becoming a closed kitchen.
Questions
In practice, yes. Approved Document H of the Building Regulations expects drainage serving a commercial hot-food kitchen to have a grease separator complying with BS EN 1825-1 and sized to BS EN 1825-2, or another effective means of grease removal. On top of that, the Water Industry Act 1991 makes it an offence to discharge grease that interferes with the flow of a public sewer, with fines up to £50,000 in the magistrates’ court and unlimited fines in the Crown Court. A separator is how you meet both.
A fall of around 1:80 - roughly one and a quarter per cent - towards the gullies and channels works well. It moves water away briskly enough to avoid standing puddles, which cause smells and hygiene problems, without making the floor feel like a slope or letting trolleys roll. The fall is formed in the screed, so it has to be decided and set out before the floor finish is laid.
Not in a commercial kitchen in England from 31 March 2025, when their use for food-waste disposal to the sewer was effectively banned. Scotland introduced the same ban in 2014 and Wales in 2023. Under the Environment Act 2021 and Simpler Recycling, solid food waste must be separated and sent for licensed collection, so your drainage should be planned to keep solids out with baskets and strainers, with bin space and a collection contract designed in.
The working guideline is to have it de-sludged before it reaches around a quarter full of grease. In a busy kitchen that usually means at least once a month, and often more frequently depending on your volume and menu. Site the unit where a tanker or operative can actually reach it, and keep records of every empty and clean - that paperwork is what you show a water company or environmental health inspector.
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