Why the pet smell comes back a week after you clean it

You found the accident, you scrubbed it, and the carpet looked clean by the evening. Two weeks later the room smells again, always worst on a warm afternoon, and always in the same place. Most people conclude they did not scrub hard enough. That is almost never the problem.

The problem is that you were cleaning the wrong part of the floor. What you can see is a mark on the pile. What is producing the smell is sitting underneath it, and no amount of work on the surface will reach it.

Carpet is the thinnest layer of the problem

A carpet is a fibre face, a backing, a pad underneath, and then the subfloor. Urine passes through the face in a couple of seconds. It is a liquid landing on something designed to let liquid through, so it keeps going until something stops it, which is usually the pad.

The pad is foam or rubber and it holds a remarkable amount. On a bad one it saturates and passes the rest to the boards or the slab below. So by the time you get a cloth to it, perhaps ninety percent of what was spilled is already below the layer you are able to touch.

Blotting the top lifts the visible stain. It leaves the reservoir intact. That is the whole mechanism, and once you see it the rest of the behaviour makes sense.

Why heat and humidity bring it back

Dried urine crystallises. In that state it is relatively quiet, which is why the room can smell fine for a fortnight. Add moisture and warmth and those crystals become active again, and the smell is released back into the room.

In San Diego that means the smell tracks the weather. A cool week and you think it is solved. A humid stretch, or the underfloor heating going on, or a steam clean that adds water without removing the source, and it is back exactly as strong as before.

That last one catches people out. Cleaning a contaminated area with water and not extracting properly can make the smell worse for a while, because you have just rehydrated everything you failed to remove.

Finding it matters more than treating it

The visible mark is usually smaller than the contamination, because liquid spreads outward through the pad in every direction. There are also, in almost every house we go to, spots nobody knew about. Behind a sofa, along a skirting board, in the corner an older dog favoured.

So the first job is working out the real extent rather than treating the patch you already knew about. That is what decides whether this is a treatment, a pad replacement, or in the case of a loose rug, a trip back to the shop.

• How long it has been happening, because old contamination behaves differently from a fresh accident
• Whether it is one animal returning to one spot, which concentrates it badly
• Whether the floor underneath is board or slab, because they absorb differently
• Whether the carpet has been cleaned since, and with what

Enzymes do the work, perfume does not

Odour masking sprays sell extremely well and solve nothing. They add a scent on top of the one you have, and when it fades you are back where you started with a slightly stranger smell.

Organic enzymes break down the material producing the odour. That takes two things a spray bottle cannot give it: dwell time, and depth. The treatment has to reach as far as the urine reached, and it has to be left long enough to work rather than wiped off after a minute.

Sanitiser and disinfectant follow where appropriate. On badly affected areas the honest answer is sometimes that the pad has to come out, because there is a point past which treating it costs more than replacing it and works less well.

Rugs are their own case

On a loose rug there is no pad to catch anything, so urine passes right through and dries on both faces, wicking through the foundation as it goes. Cleaning one face leaves the other, which is why rugs are the worst offenders for a smell that will not go.

Those come back to the shop for a full submersion wash, where the whole textile is worked in water and the contamination is flushed out rather than diluted. It takes longer and it is the only method that genuinely finishes the job.

What to tell us when you call

Roughly how long it has been going on, roughly how often, and whether it is carpet or a loose rug. Those three answers change the treatment more than anything else, and they let us tell you on the phone whether this is a straightforward job or one where the pad is coming up.

We would rather say that at the start than after you have paid for a clean that was never going to fix it.