Why Are There Black Lines Along My Skirting Boards and Under the Doors?

I get asked about this two or three times a week and the conversation always starts the same way. Someone has just had a spring clean, or they have pulled the sofa out for the first time in four years, and there it is: a dark grey band running the full length of the skirting, perfectly straight, about two centimetres wide, with an edge as clean as if it had been drawn with a ruler. Under the bedroom door there is a matching line. Around the fireplace, a soft dark halo.

Nobody spilled anything. The hoover does nothing to it. And nine times out of ten the owner has convinced themselves it is mould.

It is not mould. It is filtration soiling, and the reason it looks so deliberate is that it was produced by a very consistent physical process over a very long time.

What Am I Actually Looking At?

Air moves through your house whether you notice it or not, and it always takes the easiest route. In a room where the doors are shut and the windows are closed, the easiest routes are the gap under the door and the junction where the carpet meets the skirting board. Air pushes through those points, and the carpet pile sits in the way like the filter in an extractor hood.

The air carries things. In London that means diesel and brake dust off the road, soot from candles, carbon from a gas hob, cooking grease that has drifted out of an open-plan kitchen, and in older houses whatever comes up through the floor void from the crawl space. The airborne particles are microscopic and slightly oily, and when they hit the fibre they stick. Not to the carpet backing, not into the pile base – to the very tips of the fibres, in a band exactly as wide as the airflow.

Give it three to five years and you have a black line.

How I tell it apart from a worn edge or a pet problem

There are three things that get confused with each other along a skirting, and the difference matters because the treatment is completely different.

Filtration soiling has a hard boundary. The line is crisp on the room side, follows the wall precisely, goes round the door architrave, and continues at exactly the same width. Rub the tips with a damp white cloth and you get grey on the cloth. Part the pile with your fingers and the base of the fibre is still the colour the carpet is meant to be.

A worn or crushed edge is not a discolouration at all – it is the pile lying flat and reflecting light differently, and it will show a change of shade when you brush against the direction of the pile. Cat and dog marking along a skirting is patchy rather than linear, usually stronger at corners and near furniture legs, and it smells. Cheap trick from the van: get down on your knees with a torch held almost flat against the carpet. Filtration soiling stays uniform under raking light. Everything else does not.

Why Does It Show in Some Rooms and Not Others?

Because the airflow is not the same everywhere. In a Victorian terrace in Camberwell with suspended timber floors, the ground floor perimeter will show it and the first floor often will not, because there is a ventilated void beneath the boards feeding air upward through every gap. In a converted flat where the hallway has been sealed and the bathroom extractor runs on an overrun timer, that fan pulls air from the whole flat and every internal door gets a line under it.

Fireplaces are the other reliable one. An open chimney draws air continuously, so a hearth surround in an Islington front room picks up a dark halo in the carpet even when the fire is never lit. Blocked-up chimneys with a vented cover do the same thing at a slower rate, and stairs are worth checking too – the nosing of each tread catches its own faint line where air is dragged up the stairwell.

I have also seen it appear inside eighteen months in a new-build. Two-bed flat out at Barking Riverside, mechanical ventilation, sealed unit windows, brand new mid-grey twist pile. Perimeter lines in the bedroom already. Everyone assumes this is an old-house problem and it is not – a tight modern envelope with mechanical air handling concentrates the flow paths and can produce it faster.

Candles, and the conversation people do not enjoy

Here is my unpopular opinion, and I will happily defend it in any front room in London. Scented jar candles are the single most damaging thing people voluntarily bring into a house that has carpets. A big three-wick jar burning in a corner produces a genuinely surprising amount of unburnt carbon, and that soot has to go somewhere. It goes to the walls above radiators, to the ceiling above the candle, to the inside of lampshades, and into the carpet at the perimeter.

If somebody shows me heavy filtration lines in a flat with no fireplace, no open floor and no obvious draught, I ask about candles before I ask about anything else. The answer is almost always a shelf full of them. People do not like hearing it, because they have spent a lot of money on those candles and they are usually a gift from someone. I have learned to mention it late in the visit rather than early.

Can It Be Removed?

Partly, usually. Not entirely, sometimes.

This is slow, unglamorous work and it is nothing like cleaning a room. The soiling sits on the fibre tips, bonded to something oily, and it has been there for years. It needs a strong prespray applied to the line only, real dwell time, physical agitation with a hand brush working into the base of the skirting, and then hand extraction with a small tool, one short pass at a time. Then you do the whole thing again. Sometimes a third time on the worst sections.

A hallway perimeter and four internal doorways in a three-bed terrace is comfortably two hours of somebody’s day on their knees, and that is before the rooms are cleaned. You also want to be careful around freshly painted skirting, because a strong prespray and a stiff brush will happily take the sheen off eggshell that went on last month, and then you have a different conversation entirely.

What a realistic result actually looks like

Sixty to eighty per cent lighter is a good outcome. Ninety is a great one. Complete removal happens on newer lines that have not had time to set, and on light-coloured nylon more readily than on a wool blend.

I quote it as a separate item, with a number attached to it, and I say out loud before I start that the aim is to reduce it substantially rather than erase it. Cleaners who fold this in as part of the room price do two bad things at once – they lose an hour they were not paid for, and they set an expectation they cannot meet. The customer then remembers the faint remaining shadow rather than the eighty per cent that went.

There is one more honest caveat. On a pale carpet with heavy long-standing filtration, aggressive treatment can leave the treated band slightly brighter than the surrounding carpet, because you have effectively deep-cleaned a two-centimetre strip in the middle of a floor that has five years of normal soiling on it. I warn people about that too.

Why Does It Come Straight Back?

Because cleaning it addresses the deposit and not the airflow. The gap is still there, the fan still runs, the road outside is still the road outside. Left alone, a treated line will begin to reappear within a couple of years in a busy household.

Reducing the recurrence is a building job rather than a cleaning job. Sealing the junction between the skirting and the subfloor with a flexible sealant before the carpet goes down is the proper fix, and it is the sort of thing that costs almost nothing during a refit and is a nuisance afterwards. Taping the underlay joints and running it right up to the skirting helps. So does fitting brush strips to internal doors, and easing the bottom of doors that have been trimmed too generously by a previous fitter.

The bit a cleaner cannot do for you

I can lighten the line. I cannot seal your floor void, rebalance your extractor, or persuade your neighbour’s wood burner in a Hackney terrace to stop putting particulate into the street. When I am asked what will stop it coming back, I say the same thing every time: talk to whoever does your decorating, and get the skirting-to-floor gap closed before the next carpet goes in.

Weekly vacuuming with a crevice tool along the perimeter genuinely slows it down, because you are removing the deposit before it has time to bond. It is dull advice and almost nobody does it.

Who Pays for It at the End of a Tenancy?

This one comes up constantly and it winds me up. Inventory clerks routinely log filtration lines as staining or as damage, deposits get held against them, and tenants get charged for something that is a characteristic of the building rather than anything they did.

Filtration soiling is fair wear and tear in almost every case. It is a product of airflow, construction and time. Unless the tenant has been burning candles continuously in a way that goes well beyond normal use, it is not a chargeable item, and a decent report from a cleaner saying so is worth having when a deposit is being argued over through one of the protection schemes.

If you are a landlord reading this: get the perimeter treated between tenancies, price it properly, and seal the gap the next time the carpet is replaced. It stops being an argument after that.