The letting agent was annoyed before I’d got my shoes off. A two-bedroom flat off Clapham Common, tenants gone after five years, and a pale oblong in the middle of the living room carpet, two metres by three, with crisp edges where a rug had sat for the whole tenancy. The landlord wanted it cleaned out before the next viewing on Thursday. Failing that, he wanted the tenants to pay for a new carpet.

“Can you get rid of it or not? I need to tell him today.”

Not entirely. That wasn’t what either of them wanted to hear, and nine times out of ten it’s the answer I end up giving about a rug-shaped rectangle.

Why Does a Rug Leave a Rectangle on the Carpet?

Because the carpet around the rug has spent years living a different life from the carpet under it.

Two things happen to the exposed part of a room. It gets walked on, so soil builds up in the pile, and that greys the colour and dulls it. And it gets light – daylight through the windows, with its ultraviolet component slowly breaking down the dyes in the fibre. The patch under the rug gets neither. It stays close to the colour the carpet was on the day it was fitted.

So the rectangle looks lighter or a slightly different shade, depending on which of those two effects has been stronger. In a north-facing room with a busy family, soil usually dominates. In a south-facing room with a big bay window and not much traffic, fading does.

In most rooms it’s a mixture, and the mixture decides everything about whether cleaning helps.

Crushed Pile Plays Tricks With the Light

There’s a third effect that has nothing to do with colour. A rug presses the pile beneath it flat in one direction for years. When the rug comes up, those tufts lie at a different angle from the rest of the room and reflect light differently, so the rectangle can look lighter from the doorway and darker from the window. Walk round it. If the rectangle changes as you move, at least part of it is pile, and that part responds well to cleaning and grooming.

How Can You Tell Soiling From Sun Fading?

Get down on the floor with a torch and a white cloth.

Soil sits on and around the fibres. Rub a clean, dry white cloth firmly across the carpet just outside the rectangle, then across the carpet just inside it. If the outside cloth comes up noticeably greyer, a good share of the difference is dirt. Soiled carpet also tends to feel slightly harsh or gritty to the hand compared with the protected area, which usually feels softer.

Fading is in the fibre itself, and no cloth will pick it up. It shows most on the tips of the tufts, which are what the light hits, and on the side of the room nearest the window. Check the carpet along the skirting boards and behind the sofa as well. Those areas get little traffic and less light, so they give you a rough idea of the carpet’s original colour.

Wool carpets and nylon carpets dyed after the yarn was made are the most likely to fade. Many polypropylene and some nylon carpets are solution-dyed, meaning the colour went into the fibre while it was being made, and those resist fading far better. A mostly grey-beige polypropylene in a rental flat will nine times out of ten turn out to be soiled.

The Base of the Tuft Test

This is the most useful check of all, and it takes a minute.

Part the pile with your fingers in the exposed area, somewhere near the window, and look down into the base of the tufts. The base of a tuft gets very little light even in a sunny room. If the tips are clearly paler than the base, the carpet has faded. If the tips and base are the same colour but the whole tuft looks grey and dull, you’re mostly looking at soil.

In the Clapham flat, the tips of a sage-green wool twist nearest the bay window were almost oatmeal. The base of the same tufts was still green.

What Does a Professional Clean Do to the Rectangle?

It removes the soil, and only the soil.

A thorough hot water extraction clean, with proper pre-vacuuming and a pre-spray worked into the pile, will lift most of the greyness from the exposed carpet. The protected rectangle gets cleaned as well, of course, but it has much less to give up. On a carpet where soil was the main difference, the two areas come much closer together, and in some rooms the rectangle disappears almost entirely. The pile gets groomed upright at the end, which deals with the crush.

That’s the good outcome. It happens in north-facing rooms and in basement flats where the sun barely reaches the floor.

The less welcome outcome happens where the sun has been doing the work.

When the Rectangle Changes Colour After Cleaning

Before cleaning, soil on a faded carpet can hide some of the fading. Grey soil on pale, sun-bleached tips and the deeper original colour under a rug can end up looking closer together than they are. Clean the soil out and you reveal the true difference: faded carpet all around, and a rectangle of rich, unfaded colour in the middle.

People expect the rectangle to go. Instead it can look darker and sharper than it did before we arrived.

In a Georgian terrace in Islington, a customer had a claret wool carpet in a first-floor drawing room with two tall south-facing sash windows. Her rug had been down for about eight years. Before cleaning, the rectangle looked slightly lighter than the rest of the room, because the rest of the room was dirty. After cleaning, the rest of the room was a clean, faded dusty pink, and the rectangle was deep claret. She took it well. I’d warned her twice.

When a customer isn’t sure, we clean a test strip first. A strip about a metre long and a hand’s width across, running from well outside the rectangle to well inside it, cleaned and dried with an air mover while we wait. Twenty minutes later you can see exactly what a full clean will do to the edge. If the strip shows the rectangle fading away, we carry on with the room. If it shows the rectangle sharpening, the customer decides whether to go ahead knowing that, and plenty of them still do, because a clean faded carpet with a crisp dark rectangle in it is still a clean carpet, and the rug is going back down on top anyway. A few decide to leave the carpet as it is until they’ve bought the next rug. The strip is small enough to blend back in once the room is cleaned, whenever that happens.

This is why I won’t take on a rectangle job without looking first, and why the Clapham agent got a phone call before anyone booked anything.

Can Faded Carpet Be Re-Dyed to Match?

Small areas, yes. A whole room, not realistically.

Colour restoration on carpet is a real skill. A trained technician can spot dye a small bleach mark, or a few faded tufts, using acid dyes on wool or nylon, building the colour up gradually until it blends. It works because the area is small and the eye has a clear reference colour right next to it.

Tinting a whole faded room back towards its original colour is another matter. The fading is uneven – stronger by the windows, weaker by the door – so the dye has to be graded across the floor by eye. Polypropylene won’t take dye at all. On wool, the result can look good on the day and patchy in a week once the customer has seen it in morning light. I think colour-matching a whole faded room is a fool’s errand, and I’ve turned down that job more than once when the customer was willing to pay for it.

What About Yellow Marks From the Rug Pad?

Sometimes the rectangle isn’t lighter but faintly yellow or brown, often in a pattern of grid lines or dots. That’s usually the underlay the rug sat on. Some rubber and PVC rug grippers release plasticisers over time, and those can stain the carpet beneath them. Those marks sometimes lift partly with solvent spotting, sometimes not at all, so tell the cleaner about the rug pad before they start.

What Should You Do About the Rectangle Instead?

Nine times out of ten, the cheapest fix is to put a rug back.

It doesn’t have to be the same rug or even the same size, as long as it covers the rectangle’s edges. A slightly larger rug hides the whole thing. Rearranging furniture so a sofa or a bed sits across the rectangle also works.

Time helps too. Once the rectangle is exposed, it starts fading at the same rate as the rest of the room, and in a sunny room the difference narrows noticeably over a couple of years, though it never quite catches up.

In a rental property, fading from daylight is generally treated as fair wear and tear, and a landlord would struggle to charge a tenant for a sun-faded carpet. Soil is different. A carpet left very dirty at the end of a tenancy is a fair cleaning charge. The Clapham flat got cleaned for its soil, and the landlord bought a large jute rug for the viewing.

Blinds on the South-Facing Window

For anyone fitting a new carpet in a sunny room, a UV-filtering blind or window film does more to prevent a future rectangle than any cleaning ever will. So does choosing a solution-dyed fibre for that room.

A customer in Enfield did both after her first carpet faded. Her new one still has a small rug in front of the fire. Every spring she turns it by a quarter.