Why are there dark lines along the edges of my carpet?
A hard-edged dark band against the skirting, under a door or around a fitted unit is filtration soiling — the carpet acting as an air filter — and cleaning alone will not hold it.
- Difficulty
- beginner
- Time
- 1 hr
- Read
- 3 min
Short answer
Air is being drawn from the room down through the carpet pile at the perimeter, and the fine soot and grease it carries stays in the fibres. The mark comes back after cleaning because the mechanism is a leak. Seal the perimeter gap and the pipe penetrations, reduce the airborne particles, and then clean.
The giveaway is the edge. Ordinary soiling fades gradually; filtration soiling stops dead in a straight line, exactly where air enters the pile. Once you know what it is, the pattern makes sense — it appears against skirtings, under doors, along stair risers, around the base of fitted wardrobes and over the joist lines of a suspended floor.
Step by step
- Confirm the diagnosis from the shape.A hard-edged band, usually 25 to 75mm wide, following a skirting, a door line, a stair riser or the edge of a fitted unit. Dirt has soft edges; this does not.
- Work out where the air is going.There is a pressure difference between the heated room and a cooler space beyond — a suspended floor void, a wall cavity, a stairwell, or a cold room behind a door. The mark shows where the air is leaving.
- Seal the perimeter gap.Lift the carpet edge off the gripper along the affected run, or work from the skirting. Seal the gap between the floor deck and the wall with a flexible sealant or expanding foam trimmed back, and seal the gap under the skirting itself.
- Seal the penetrations.Pipes, cables and any hole through the floor near the affected line. These are often larger than they look under the carpet.
- Reduce the particles.The deposit is combustion and cooking soot, candle soot, fireplace and stove particles, and ordinary household dust. Better extraction over the hob, fewer candles, a serviced stove and good filtration on the vacuum all reduce the supply.
- Improve the ventilation balance.The airflow exists because the room is being pressurised or depressurised relative to the space beyond. Trickle vents, an extractor that is not fighting a sealed house, and doors that are not creating strong pressure differences all reduce the driving force.
- Then clean, and expect it to be hard.The deposit is very fine and it is deep in the fibre. Professional hot water extraction with a pre-treatment aimed at oily soiling is the method that works. Domestic machines rarely clear it completely.
- Judge the result after a season.If the line comes back within weeks, a path is still open. If it takes years, you have fixed the mechanism and this is now ordinary maintenance.
Questions people ask
Is it mould?
No. Mould on a carpet edge is usually associated with damp, a musty smell and a patchier, irregular shape. Filtration soiling is dry, sharply defined and follows air paths rather than damp ones.
Why is it worst in the room with the fire?
Because that room produces the most fine particles and has the strongest air movement. Open fires, stoves and candles are the classic contributors.
Will a new carpet solve it?
Not unless the airflow is sealed first. A new carpet in the same room with the same gaps will develop the same lines, usually within a year or two.
It follows lines across the middle of the room, not just the edges. Why?
Those are the joist lines or floorboard joints, where air is passing through the deck. The same treatment applies — the gaps have to be sealed, which usually means lifting the carpet.