Direct answer: If old cat urine smell keeps coming back even after surface cleaning, the odor is usually soaked below the finished flooring or into the subfloor itself. Start by figuring out whether the smell is only in the top flooring, in the underlayment, or deep in the subfloor before you tear anything out.
Most likely: The most common real-world cause is urine that got past carpet, pad, seams, or trim edges and dried into wood subfloor around a litter box, doorway, or wall line.
Old pet odor is stubborn because dried urine salts react again when humidity rises. Reality check: if the smell gets stronger on damp days, that usually means the contamination is still in the floor assembly. Common wrong move: soaking the area over and over just drives moisture and odor deeper into the wood.
Don’t start with: Do not start by painting over the floor, flooding it with cleaners, or replacing random flooring pieces before you know how deep the contamination goes.
The odor is concentrated near a baseboard, closet corner, doorway, or old litter box location.
Start here: Check trim edges and the nearest flooring seam first. Localized odor usually means a soaked path, not whole-room contamination.
The room has a broad stale urine smell, especially when closed up or humid.
Start here: Figure out whether the finish flooring, pad, or underlayment is holding odor over a wide area before assuming the subfloor is ruined.
You cleaned the floor or sealed the surface, but the smell returned within days or during damp weather.
Start here: Look for contamination below the top layer. Surface treatment rarely fixes urine that reached wood subfloor or wall bottom plates.
There are dark stains, lifted seams, crumbly particleboard, or soft spots underfoot.
Start here: Treat this as damage, not just odor. Once the wood fibers are broken down, patching out the bad section is usually the right path.
This is the usual reason odor survives normal cleaning. Wood holds dried urine salts deep in the grain and releases smell again with humidity.
Quick check: Open a floor edge, threshold, vent opening, or small hidden section and compare the smell at the surface versus the exposed subfloor.
If the room smells strong but the wood below is only lightly affected, the pad or underlayment may be the main source.
Quick check: Lift one corner or edge and smell the underside of the flooring and the pad or underlayment separately.
Dark staining, swelling, delamination, or soft spots mean the urine sat long enough to break down the panel.
Quick check: Press with a screwdriver handle or your hand near the stain. Spongy, flaky, or swollen material points to patching, not just sealing.
Urine often wicks sideways at wall lines and under baseboard, so the floor may not be the only source left.
Quick check: Remove a short piece of baseboard in the worst area and smell the back side and the wall bottom edge.
You want to separate a small contaminated spot from a room-wide problem. That keeps the repair smaller and tells you where to inspect first.
Next move: You have a clear target area instead of guessing at the whole room. If the smell seems everywhere, start at the most likely original pet area and the nearest wall edge or threshold.
What to conclude: A tight odor zone usually means a localized soak-in. A broad odor can still be localized below the surface, but you need an inspection opening to prove it.
This is the big split. If the finish flooring or pad is the main source, you may not need subfloor patching. If the subfloor itself is hot, surface work will not hold.
Next move: You can tell whether the top layer is the main problem or just the lid over a deeper odor source. If you still cannot separate the layers, open a slightly larger hidden edge rather than guessing and buying materials.
What to conclude: Strong odor in pad or underlayment points to removal of that layer first. Strong odor in the wood below points to subfloor cleaning, sealing, or patching.
A sound wood panel can sometimes be cleaned and sealed. A swollen or softened panel usually needs to be cut out and patched.
Next move: You now know whether you are dealing with odor treatment on sound wood or a true subfloor repair. If the damage line is hard to read, assume the repair area needs to extend past any stain or softness into clean, solid material.
Once you know the depth, you can stop wasting time on surface treatments that will never last.
Next move: The smell drops sharply and stays down after the area dries and the room is closed up again. If odor still pushes through after the contaminated layer is removed or sealed, the contamination extends farther than the opening and more material needs to come out.
Pet odor can seem fixed while the area is open and ventilated, then come right back once flooring goes back down.
A good result: You fixed the source instead of just covering it up.
If not: Reopen the strongest area and extend the repair to the next clean, odor-free boundary, or bring in a flooring contractor if the contamination runs under walls or built-ins.
What to conclude: Stable results after the room is closed up are the real test. If it still smells then, more contaminated material is still in place.
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Yes. That is very common. The visible flooring may look fine while urine has already passed through seams, carpet pad, or edges and dried in the wood below.
Not usually once the odor is in the wood itself. Baking soda can help with light surface odor, but it will not remove contamination that soaked into subfloor panels.
Seal solid, dry subfloor that still smells but has not broken down. Replace subfloor that is swollen, soft, delaminated, crumbly, or structurally weakened.
Old urine salts react to moisture in the air. That is why a floor can seem fine in dry weather and then smell strong again on damp days.
Sometimes, yes. If the odor is strongest at the wall line, urine may have wicked under the baseboard or into the drywall bottom edge. Leaving contaminated trim in place can make it seem like the floor repair failed.
That usually backfires. New flooring can trap the odor for a while, but if the contaminated layer stays in place, the smell often comes back through seams, edges, or humid air.