The complaint arrives in the same words every time. It was perfect when we moved in, we take our shoes off, nobody comes round, and it looks filthy.
I hear it most from the Royal Docks and the Nine Elms end of Vauxhall, in flats that were handed over eighteen months ago with a warranty folder still shrink-wrapped in a kitchen drawer. The carpet is nearly always a mid-grey or oatmeal polypropylene loop pile, the client is near enough always doing everything right, and the carpet is two shades darker than it was.
There is no single cause. There are four, they compound, and the first three are not the owner’s fault in any way.
Why does a new-build carpet soil faster than an old one?
Start with the fibre, because in a new-build the fibre is almost never wool. Developers specify polypropylene at scale for good reasons – it takes bleach, it does not fade, it costs a third of a wool-rich equivalent per square metre and it survives being walked on by trades before handover.
Polypropylene is oleophilic. It attracts oil. Where wool and nylon soil mainly by physical entrapment, polypropylene holds greasy soil chemically at the fibre surface, and greasy soil is what a London flat generates most: cooking aerosols from an open-plan kitchen, skin lipids, road film tracked in from the Barking Road, plasticiser drifting off vinyl and painted MDF. A carpet that resists a red wine spill perfectly can still go grey from kitchen air.
Loop construction makes it worse. A cut pile presents a tip to the world; a loop presents a curved shoulder that catches light differently and shows soil at a lower loading. Solution-dyed grey shows it earliest of all – you have a mid-tone that has nowhere to go except darker.
The bit the sales office never mentions
Contract-grade polypropylene often arrives with a fluorochemical soil-resist finish applied at the mill. Those treatments have a service life. Foot traffic abrades them, and so does every alkaline cleaner anyone puts on the carpet in year one.
So the flat that felt miraculous for four months is not degrading unusually. It has spent its protector, on schedule, and nobody told the owner the protector was a consumable.
What is the black line along the skirting?
Filtration soiling. It is the single most common thing I am called out to in a new-build and the single most misdiagnosed.
A dark, sharp-edged band appears where the carpet meets the skirting, under doors, along the edge of a fitted wardrobe, sometimes in a neat rectangle around the perimeter of every room. Clients assume mould, damp, or that the fitter left something under there.
What it is: air being pulled through the carpet pile as a filter. New flats are built airtight to hit Part L targets, so the small pressure differences that used to equalise through gaps in the fabric now push air through whatever route is left. The gap between skirting and floor is one such route. Soot, cooking soil and ultra-fine particulate ride along with the air, the pile catches them, and after six months you have a line.
Modern London air makes it faster. Fine particulate off the South Circular, brake dust, candle soot, incense, a gas hob in an open-plan room with the extractor recirculating rather than ducted. Every one of those loads the air with sub-micron carbon.
Can it be removed?
Partly, and I will not pretend otherwise on a quote. The particles are tiny, they are lodged deep in the yarn, and they are held there by an electrostatic charge and a film of oil.
The method that works is a solvent-based filtration soil remover, dwelling properly, worked in with a horsehair brush along the line, then extracted hot with an acid rinse. Two passes on a bad edge. Expect 70 to 90 per cent improvement on a six-month line, less on a three-year one, and expect it to come back, because the airflow that caused it has not gone anywhere.
The edges nobody thinks about are the ones under fitted furniture. Pull a wardrobe base off in a flat at Wembley Park, HA9, and you will often find the line is heavier there than along the skirting, because the void behind the unit is where the pressure difference is largest. It costs twenty minutes to move the furniture and it is near enough the only way to leave a bedroom looking even.
Anyone who quotes you a full removal on filtration soiling has either not seen it before or is not telling you the truth.
Is construction dust still in there?
Yes. Almost certainly yes, and this is the part that surprises people.
Carpet in a new block goes down late but not last. Second fix carries on around it, snagging crews come through, gypsum dust from plasterboard sanding gets everywhere and settles into a pile that has been protected by nothing more than a roll of self-adhesive film. That film traps as much as it excludes.
Gypsum is calcium sulphate. Fine, faintly alkaline, abrasive at a microscopic scale – it grinds at the fibre every time somebody walks across it, which dulls the surface and makes the carpet reflect less light. Half of the greying in a year-one flat is not soil sitting on the fibre at all, it is fibre that has been scuffed matt.
A domestic vacuum gets nowhere near it. Even a decent one is running a single motor and a plastic beater, and gypsum has worked its way to the base of the yarn where the airflow does not reach.
The pre-clean that matters more than the clean
Before any water goes down in a flat under two years old, I dry vacuum it properly. Twin-motor upright, eight passes minimum in alternating directions on the traffic lanes, slow. It takes forty minutes in a two-bedroom flat off Hackney Wick and it is the most valuable forty minutes of the job.
The reason is simple. Wet a carpet full of dry gypsum and cement dust and you make a slurry that redeposits as the pile dries. I have taken over remedial jobs at a block near Deal Porters Way in Rotherhithe where the previous cleaner went straight to the wand and left the flat looking worse, and it was not their chemistry that failed.
Does professional cleaning make it worse?
It can, and here is where I will annoy a large part of my own trade.
The default London approach to a grey polypropylene loop is a strong alkaline prespray, hot water, and a rinse of whatever comes out of the tap. It looks superb on the day. Polypropylene is non-absorbent, so the surface responds fast and dramatically, and the client is delighted.
Then the residue stays. Detergent left in a non-absorbent fibre has nowhere to soak away to, so it sits on the surface as a sticky film that grabs soil at speed. The carpet re-soils faster than it did before, the client concludes that professional cleaning does not last, and books somebody cheaper next time who does exactly the same thing harder.
My position, and I have argued it with people who have been doing this longer than me: for polypropylene in a new-build, low-moisture encapsulation with a proper acid rinse afterwards beats aggressive hot water extraction on almost every measure that matters at the twelve-month mark. It looks slightly less spectacular on the day of the clean. The flat is cleaner in March.
The exception is gypsum-loaded year-one carpet, which needs the water. That is a one-off, and it should be the last aggressive clean the carpet ever gets.
What about protector reapplication
Worth it, on polypropylene, and I say that knowing it sounds like an upsell. The mill finish is gone by month twelve and reapplying a fluorochemical after a clean restores most of the oil resistance for another year or two of normal use.
It has to go onto a properly rinsed carpet or it bonds to detergent film instead of fibre. Applied over residue it is money burnt, which is why I will decline to apply it on a carpet somebody else cleaned last week.
What should a new-build owner do about it?
Vacuum twice a week and slowly, because slow is the whole technique. Most people push a vacuum at walking pace and recover perhaps a third of what a slow pass would.
Duct the cooker extractor if the block allows it. Almost every London flat gets a recirculating hood, and a recirculating hood returns the grease to the room, which returns it to the carpet. Where ducting is impossible, replace the charcoal filter on schedule rather than when it smells.
Barrier matting inside the front door, 1.2 metres of it minimum, which is near enough always longer than anybody wants it to be. Two strides of contact is the point, and a 60cm mat gives you one.
On the skirting line specifically
Do not seal the gap yourself with mastic. It is tempting and it is the wrong fix – the air will find the next route and put the line somewhere less convenient, usually under the wardrobe.
Better to reduce what is in the air. Candles are the one people resist hearing about. A regularly lit candle in a small sealed flat is a soot source, and the line under the door of the room it lives in will tell you so.
Get the block’s MVHR filters checked, if there is a system. Half the new blocks I go into have never had them changed, and the flats with the worst filtration lines are the ones where the management company has an MVHR maintenance clause nobody has read.
The Nine Elms flat I did last spring came back well – two shades lighter, protector on, the skirting line at maybe eighty per cent. The owner asked whether it would stay like that. I said no. She books it every ten months now and it never gets as bad again.