The short answer
- It is filtration soiling, not mould and not dirt walked in
- Air is being drawn through the carpet at the edges and the pile filters it
- Common with draughty floorboards, and under doors
- It usually improves a great deal, but it is not always fully reversible
What is filtration soiling?
Those dark grey or black lines that appear tight against the skirting board, under doors, and sometimes along the edge of a staircase are not dirt that has been trodden in. If they were, they would be in the middle of the room where people actually walk.
They are there because air is moving through the gap between the floor and the skirting, and the carpet pile is acting as a filter. Airborne dust, soot, cooking particles and candle smoke get caught in the fibre as the air passes, and over months they build into a line that follows exactly where the draught is.
Why does it happen in some houses and not others?
- Suspended timber floors. Gaps between boards let air move up from the void below. Older terraces and semis are the usual candidates.
- Pressure differences between rooms. Heating, extractor fans and closed doors all push air from one room to another, and it finds the gaps.
- Anything that puts particles in the air. Candles, open fires, gas hobs and smoking all speed it up considerably.
It is worth saying plainly: filtration soiling is not a sign that the carpet is dirty or that anyone has neglected it. It is a sign that air is moving through a gap, and it happens in very clean houses.
Can it be removed?
Usually improved substantially. Sometimes removed completely. Occasionally not much at all, and it depends on how long it has been building.
The particles are extremely fine and they lodge deep in the base of the pile rather than sitting on top, which is why ordinary vacuuming does nothing and why a standard clean often leaves the lines still visible. It needs the edges worked specifically, with the pile agitated so the soil is released before extraction, rather than a machine passed over the middle of the room.
Where it has been building for years on a light-coloured carpet, expect a real improvement rather than perfection. Anyone who promises to remove it entirely without looking at it first is guessing.
How do I stop it coming back?
You are dealing with the airflow, not the carpet. Sealing the gap between the floorboards and the skirting is the single most effective thing, and it is a job most people can do with a tube of decorator’s caulk. Beyond that, cutting down on candles and making sure extractor fans actually vent outside will slow it.
It will come back eventually if the draught is still there. That is not a failure of the clean, it is the same mechanism starting again.
Common questions
Is it mould?
No. Mould does not form in neat straight lines along a skirting board, and it usually comes with a smell and a damp patch. Filtration soiling is dry and follows the draught exactly.
Will it damage the carpet?
Not structurally, but the particles are abrasive and left long enough they wear the fibre at the edges. It is worth dealing with rather than ignoring.
Does it mean my house is dirty?
No, and this is the question we get asked most. It is about airflow through a gap, and we see it in spotless houses regularly.
We treat filtration soiling across Liverpool and Merseyside, and we will tell you honestly what we expect to achieve before starting. Get a price.