How to Clean a Cat Litter Tray to Avoid Bad Smells If You Live in a Small Flat
The complaint is almost never “the litter tray smells”. It’s “my bedroom smells and I can’t work out why”, usually from someone in a one-bed conversion who has already washed the duvet twice. The tray is in the hallway or wedged beside the bath, nowhere near the bed, and the smell has still ended up in the last place anybody wants it. That’s not a coincidence and it isn’t the cat’s fault. Almost everything about how a flat is laid out works against you here, and the fix is mostly a matter of understanding which part of the setup is doing the smelling – because in my experience it usually isn’t the litter.
Why does the smell end up in the bedroom rather than by the tray?
Fresh cat urine barely smells of anything. What you’re smelling is what happens to it afterwards. Urine carries urea, and the bacteria living on the tray surface produce urease, an enzyme that splits urea into carbon dioxide and ammonia. That reaction needs warmth and a few hours. In a heated flat in February it gets all three, and the ammonia clock starts running the moment the cat covers up and walks away.
Ammonia gas has a molecular weight of 17 against air’s 29, so it rises. In a house that means it disperses up a stairwell and out. In a fifth-floor flat off Chatsworth Road in Clapton, with a mezzanine bed platform and no through draught, it means the ammonia goes straight up to head height at the one point in the day when your head is horizontal and still for eight hours.
Then there’s the second compound, which nobody warns you about. Cat urine contains felinine, a sulphur-bearing amino acid that degrades into a thiol – 3-mercapto-3-methylbutanol, if you want the name for it. Thiols are the family of molecules that put the warning smell into mains gas. Human noses detect them at parts per billion, and unlike ammonia they are heavy and readily absorbed by fabric. Curtains take them. Mattresses take them. A wool rug will hold onto that smell for months after the tray has been sorted.
What that means for your bedding
If the smell has been going for weeks, cleaning the tray alone won’t clear it, because the thiols have already moved into the soft furnishings. Duvet and pillows on a 60°C wash with biological powder. Curtains down. Mattress vacuumed with the upholstery head, then dampened lightly with an enzyme cleaner and left to dry with a window open, which takes most of a day and is worth losing the day for. Skip that step and you’ll fix the tray and swear blind it hasn’t worked.
Is the litter itself the problem, or the tray underneath it?
Here’s what I find in most flats. The litter is fine – the plastic under it is the problem.
Urine only clumps if it meets enough litter to absorb it before it reaches the bottom of the tray. With sodium bentonite clumping clay that means a bed of seven to ten centimetres, which looks like a lot more litter than most people pour in. At three centimetres, which is what I usually see, the urine goes straight through, spreads across the base and sits there in a thin film. You then scoop out a clump that isn’t a clump and leave the bulk of the urine exactly where it landed. The tray gets emptied and refilled around it for weeks. That film is where the urease bacteria live, and it is the single most common source of persistent flat smell I come across. You can smell it if you tip the tray out and put your face near the empty base, which is an unpleasant test and a conclusive one. The ammonia clock never resets on a tray like that, because the reaction is running on the plastic rather than in the litter, and the litter is the only part getting changed.
The other half of it is the plastic itself. Litter is abrasive and cats dig hard, so a polypropylene tray develops a fine web of scratches across the base within about a year. Urine wicks into those scratches and no amount of scrubbing gets it out, because you can’t reach it. A tray that smells within an hour of being washed and dried is a tray that’s finished. They cost eight pounds. Replace it annually and stop trying to save it.
A client on Bellenden Road in Peckham had a studio flat, one very tidy cat, and a smell she was convinced had got into the mattress. She’d bought a new mattress. The tray was four years old, the base had gone slightly cloudy and rough the way old plastic does, and there was a two-millimetre skin of set litter dust and urine salts welded into the corners that had been there long enough to have its own texture. New tray, deeper litter, and the mat outside it washed. The flat was clear in three days and she’d spent six hundred pounds on a mattress she didn’t need.
Which litter is worth the money
Clumping bentonite, for a flat, more or less always. It traps the urine as a solid you can lift out whole, which is the only method that removes urine from the flat rather than storing it. Silica crystals absorb well for a fortnight and then saturate quietly without any visible sign. Wood pellets and paper work if you’re changing them completely twice a week, and most people don’t. As for bicarbonate of soda sprinkled in the litter, which every forum recommends – bicarb neutralises acids. Ammonia is alkaline. It cannot do the job people think it’s doing, and cats with any sensitivity will just stop using the tray.
How often does it need scooping in a small flat?
Twice a day, and the timing matters more than the frequency. The ammonia clock runs on a scale of hours, so a scoop in the morning and one when you get in from work catches almost everything before it converts. Once a day at 11pm gets you the smell all afternoon and none of the benefit.
Full change depends on the litter. Clumping clay in a single-cat flat: strip it out, wash the tray and start again once a month, topping up between times to keep the depth right. Non-clumping: weekly, without argument.
Then the bin, which is where a lot of otherwise diligent people undo the work. A carrier bag of clumps knotted and left under the sink is a small ammonia generator sitting in a warm cupboard. It goes out of the flat the same day, or into a nappy bin with a proper twist seal on the landing. I know people who keep a bag in the freezer between bin days. It works, it’s completely reasonable, and I’d still rather not open your freezer.
When more than one tray is a mistake
The received wisdom is one tray per cat plus one spare. That’s sound advice for a house. In a studio in Northfields with one cat, I’d rather you had a single large tray scooped twice a day than two trays each scooped when you remember, and I’ll defend that against every rescue leaflet that says otherwise. Two trays in a small flat means double the surface area and a second tray that nobody quite takes ownership of. Depth and timing beat quantity. Two cats is a different conversation.
What should you clean the tray with, and what should never touch it?
Hot water and washing-up liquid, a stiff brush, and proper drying. That’s the routine. The drying is the part people skip – a tray refilled while still damp gives the bacteria a head start and the smell arrives days early.
For mineral crust in the corners, white vinegar at roughly one part to one with hot water, left to sit twenty minutes. Those deposits are urine salts and they are alkaline, so an acid shifts them where detergent won’t.
Two things stay away from the tray. The first is any household cleaner containing ammonia, including a lot of glass sprays, because to a cat that reads as another animal’s territory marker and can put it off using the tray entirely. The second is bleach. Sodium hypochlorite and ammonia produce chloramine gas, and a bathroom in a flat off Muswell Hill Broadway with the door shut is a small enough space for that to matter. For the floor around the tray and for any accidents on carpet, use a bacterial or enzyme cleaner and give it the full contact time on the label – ten minutes, not a wipe.
The mat, and the tracking problem
Bentonite dust travels on paws and settles wherever the cat next sits, which in a small flat is frequently the bed. A rubber-fingered mat directly outside the tray catches most of it. Vacuum the mat, don’t shake it out of the window over the courtyard in Barking that I once watched somebody do.
What if the smell is worse than the routine explains?
Sometimes the setup is right and the smell is still wrong. A sudden change in strength, urine outside the tray, or a cat that’s started going more often is a veterinary matter before it’s a cleaning one – urinary tract infection, diabetes and kidney disease all announce themselves this way, and an intact male produces far more felinine than a neutered one.
That’s also the honest limit of what I can do. I clean flats for a living and I’d be glad to take the job, but this particular problem doesn’t need me. It needs a new tray, a deeper bag of clumping clay and nothing else from me.
The one thing to change first
Depth. Before you buy anything, before you move the tray, pour in twice the litter you currently use and give it a week.
Most of the trays I meet in London flats are on their third year and their owner’s fourth air freshener.
