Can a landlord take money off your deposit for the pale strip of carpet in front of the patio doors? No. I give that answer on the phone two or three times a month between October and Christmas, when the low sun starts reaching further into south-facing lounges and check-out season gets going.

The longer answer involves what light does to dye, how to tell fading from dirt in half a minute, and why the cleaning invoice that agents like to attach to a deduction schedule proves nothing about the strip of carpet in front of the patio doors.

What has the sunlight done to the fibre?

It has broken the dye rather than covered it.

Carpet colour comes from dye molecules with a structure that absorbs some wavelengths and reflects others. Ultraviolet light, and to a lesser degree visible light, carries enough energy to break the bonds in those molecules, and the reaction is helped along by oxygen, heat and moisture. Once a dye molecule has come apart it reflects everything, which the eye reads as pale. The process is cumulative and it does not reverse. There is no chemistry in my van, or in anyone’s, that reassembles a broken chromophore.

Fibre type governs how fast it happens. Acid-dyed wool and nylon take their colour on the surface regions of the fibre, where light reaches it, and those fade first. Solution-dyed polypropylene has pigment mixed into the polymer before the fibre is extruded, so the colour runs all the way through and sits protected, which is why outdoor and contract carpets are specified that way. Modern double glazing helps a little, particularly units with a low-emissivity coating, and helps far less than the glazing salesman suggested.

Position matters as much as fibre. A rug left in the same spot for four years protects what is under it, so the floor around it fades and the rectangle underneath keeps its colour, and the owner reads that as a dirty border. Furniture does the same at a smaller scale, which is why the mark left by a sofa arm can be a shadow rather than a dent.

Why a faded strip often goes pink or yellow

Carpet colour is rarely one dye. A mid-brown is built from a blue, a red and a yellow component, and those three degrade at different rates, so the strip in the sun loses one before the others. A grey carpet that has lost its blue goes warm and slightly pink. A beige that has lost its red goes green-yellow. Wool adds its own contribution by yellowing under UV independently of what happens to the dye. Tenants describe this to me as a stain, because a warm patch on a cool carpet reads as something spilled.

How do you tell fading from soiling in thirty seconds?

Part the pile and look at the base of the yarn.

Light reaches the tips of the fibres and very little else. Push your fingers into the pile in the middle of the pale strip and bend the yarn apart, and on a faded carpet you will find the original colour intact at the bottom, sharp and obvious, with a band of paler fibre at the top. Soil does the opposite, sitting through the yarn and often heaviest at the base where vacuuming has never reached. Crushing looks flat under a low light and has no colour difference at all.

Do the same test under the sofa, behind the curtain, inside the shadow line of a bookcase. A fade edge follows a shadow, and shadows have straight edges that no spill or traffic pattern ever produces. I photographed a living room in Greenwich last winter where the fade line ran diagonally across the floor at exactly the angle of the November sun through the door frame, and the agent had written it up as a cleaning issue.

The one case where I am wrong

Sometimes it does clean. Greasy soil in a bay window, where a cat sleeps and sunlight bakes it in, can look like fading and lift beautifully. So can a residue mark left by a previous over-wet DIY clean. I test a square foot before I give an opinion, because a technician who diagnoses from the doorway is guessing, and I have been the one guessing wrong in a flat in Kilburn where what I took for UV damage came out with a solvent and twenty minutes.

Where does fading sit in the fair wear and tear argument?

Squarely inside it, on any reading I have seen from the deposit schemes.

Fair wear and tear is generally defined in this context as deterioration through reasonable use, and the legal formulation that adjudicators fall back on describes the reasonable operation of natural forces. Sunlight through a window is the textbook example of natural forces. The tenant did not install the patio doors, choose the orientation of the building, or select a carpet with poor lightfastness for a south-facing room. Living in a room with the curtains open is reasonable use of a home.

Two further principles sit alongside it. Adjudicators cannot make an award that leaves a landlord better off than at the start, which is betterment, so a five-year-old carpet cannot be replaced at a tenant’s expense with a new one. And where a deduction is justified, the cost gets apportioned according to the age, quality and remaining life expectancy of what was there. A carpet with a notional ten-year life that has done seven has three years of value left, and that is the most that could ever be in play.

Where a tenant does become liable is anything that accelerated the damage beyond reasonable use, and in practice that is a short list. A spill that was never mentioned and then sat in sunlight for a year. Curtains taken down and never rehung after a decorating job the tenant did without consent. Furniture dragged rather than lifted. A landlord arguing that a tenant should have kept the blinds closed for two years is arguing that the property should not have been lived in.

What decides these disputes

Evidence from the start of the tenancy. A check-in inventory with dated photographs showing the carpet as it was, and a check-out showing it now, is what an adjudicator works from. Without a check-in report the landlord is asking somebody to assume the property was perfect on day one, and the schemes say plainly that an adjudicator cannot assume that. Tenants moving into a south-facing flat should photograph the floor in front of the glazing on the day they get the keys, with the pile parted, and send it to themselves by email so the date is fixed.

What does a cleaning invoice prove at check-out?

That somebody cleaned a carpet. Nothing whatever about why it looks the way it does.

I am asked with some regularity to produce quotes and invoices that will support a deduction, and once or twice a year to word a report in a way that implies tenant damage where I saw natural forces. I decline both. What I will write is what I found, with photographs, including the parted-pile test and the shadow line, and landlords who want that document are welcome to it because it is often the thing that stops a dispute going further and costing them weeks.

There is also a point about compulsory cleaning clauses. Since the Tenant Fees Act 2019 came into force in England, a clause requiring a tenant to pay for professional cleaning as a condition of the tenancy is a prohibited payment, and the deposit schemes are explicit that a tenant who cleans a property to the required standard themselves has met the obligation. The standard is the condition at check-in, allowing for fair wear and tear, and no invoice changes what that condition was.

The job I turn down

Somebody rings and says the living room carpet in a let property looks patchy and could I clean it before the new tenant moves in, and describes a pale area along the window wall. I ask which way the room faces and whether the pale area has a straight edge. If the answers are south and yes, I say the clean will make no difference to that strip and offer to come and confirm it in writing instead, at a fraction of the price of the clean. Some take that. Some book the clean anyway and are disappointed on the day, which teaches me nothing new and costs them eighty pounds.

What can a landlord do before the next tenancy?

Accept the fade, and then make the next carpet last.

Specify for the room rather than for the price list. Solution-dyed polypropylene in a bright south-facing lounge will hold its colour for years longer than an acid-dyed wool twist at the same money, and it takes a lot of the argument out of future check-outs. Where wool is wanted for how it feels, ask the retailer for the lightfastness rating.

Glazing film is the other intervention and it is cheap relative to a carpet. A clear UV-blocking film on patio doors cuts the fade rate substantially without changing how the room looks, and it protects the sofa, the flooring and whatever hangs on the wall opposite at the same time. Voiles do a decent job for less. Blinds that nobody ever closes do nothing at all.

Lightfastness comes with a number

The blue wool scale runs from one to eight, eight being the most resistant, and manufacturers of contract-grade carpet publish the rating. Anything specified for a room with large south-facing glazing should be at the upper end of that scale, and a supplier who cannot tell you the number is telling you something by not knowing it.

In Sutton I metered nothing, tested nothing and charged nothing on a visit that took eleven minutes, because the tenant had already parted the pile herself, photographed the base of the yarn against the tips, and only wanted somebody in the trade to put it on headed paper before her deposit went to adjudication.