Scratches that catch your fingernail but don't cut into the wood fiber are finish damage, not wood damage. A screen-and-recoat — where the existing finish is lightly abraded and a new topcoat is applied — will restore the sheen and protect the wood. This costs $1 to $2 per square foot and takes one day.

Scratches that cut visibly into the wood itself require sanding, which removes a thin layer of material. This is still a repair, not a replacement — as long as the floor has enough thickness remaining (see Sign 6 below).

Cupping means the edges of each board are higher than the center, giving the floor a washboard appearance. It's almost always caused by moisture: either excess humidity in the air, a slow plumbing leak, or moisture wicking up from a crawl space or slab.

The key question is whether the moisture source has been fixed. If yes, mild cupping often self-corrects over one to two heating seasons as the wood dries and flattens. Once flat, the floor can be sanded and refinished. If the cupping is severe (boards raised more than 1/8 inch at the edges) or if the subfloor was also wetted, replacement is usually the better call — the boards may have permanently deformed.

Never sand a cupped floor before it has fully dried and flattened. Sanding a cupped floor creates a flat surface that will crown (reverse-cup) when the wood dries, leaving you with a worse problem than you started with.

Crowning is the opposite of cupping: the center of each board is higher than the edges. It often happens when a cupped floor was sanded before it fully dried — the sander removed the raised edges, and when the wood dried, the center became the high point.

Mild crowning can be sanded flat. Severe crowning, or crowning that returns after sanding, usually indicates an ongoing moisture problem that needs to be resolved before the floor can be saved.

Walk your floor slowly and press down firmly with your heel at each step. A floor that feels solid and firm throughout is structurally sound. A floor with soft spots — areas that compress noticeably when you step on them — has subfloor damage.

Soft spots are almost always caused by moisture that has rotted the subfloor sheathing or the joists below. This is a structural issue that refinishing will not fix. The damaged subfloor must be repaired or replaced before new flooring goes down. In severe cases, a structural engineer should assess the joists.

If you find soft spots, stop walking that area and call a flooring contractor promptly. Continued foot traffic on a compromised subfloor can cause the floor to fail suddenly.

Pet urine is the most common odor problem in hardwood floors. The question is how deep the contamination goes. Surface contamination — urine that soaked into the finish but not the wood — can often be neutralized with an enzyme-based cleaner and sealed with a stain-blocking primer before refinishing.

But urine that has soaked into the wood fiber itself is a different problem. Sanding will open the wood grain and release the odor rather than eliminate it. If the smell intensifies when you sand a test area, the contamination is in the wood. If it has also reached the subfloor — which is common in areas where pets repeatedly urinated — the subfloor boards will need to be replaced and sealed with a shellac-based primer before new flooring goes down.

A UV light (blacklight) will reveal urine stains that aren't visible in normal light. Rent one from a hardware store and inspect the floor in a darkened room before deciding on a course of action.

Solid 3/4-inch hardwood can be sanded and refinished 5 to 7 times over its lifetime. Each sanding removes roughly 1/32 inch of material. After multiple refinishes, the boards become too thin to sand again without cutting through to the tongue-and-groove joint.

How to check: Remove a floor vent cover and look at the edge of a board from the side. On a new floor, the tongue (the protruding lip that fits into the groove of the adjacent board) sits well below the top surface. On a floor that's been sanded many times, the tongue will be nearly flush with the top surface. If the tongue is within 1/16 inch of the top surface, the floor is at or near the end of its refinishing life.

Engineered hardwood with a 2mm veneer layer can typically be refinished once, maybe twice. If the veneer is worn through to the core in high-traffic areas, replacement is the only option.

Small gaps between boards in winter are normal — hardwood contracts when indoor humidity drops during heating season. Gaps that close in summer and reopen in winter are a sign of a healthy floor responding to normal seasonal humidity changes. These do not require repair.

Gaps that don't close in summer, or gaps wider than 1/8 inch, indicate either a floor installed without proper acclimation, a home with extreme humidity swings (below 30% RH in winter), or boards that have permanently shrunk from repeated moisture cycles. A whole-house humidifier can prevent further gapping. Existing wide gaps can sometimes be filled with a color-matched wood filler, but if the gaps are widespread and the boards have permanently deformed, replacement is the more durable solution.

Squeaks are almost never a reason to replace a floor. They're caused by boards rubbing against each other or against the subfloor — usually from loose fasteners or normal seasonal movement. A flooring contractor can silence most squeaks by:

• Driving screws through the subfloor up into the flooring from below (if there's basement access)
• Injecting construction adhesive between the subfloor and joists
• Applying a dry lubricant between squeaking boards
• Driving finish nails at an angle through the face of the boards into the subfloor

If squeaks are accompanied by soft spots or visible movement in the floor, that's a subfloor issue — but squeaks alone are a maintenance issue, not a replacement trigger.

Black staining in hardwood is almost always iron tannate — a reaction between iron (from water pipes, nails, or soil) and the tannins in the wood. Mild black staining can sometimes be bleached out with oxalic acid before refinishing. Widespread black staining that goes deep into the wood fiber is usually permanent.

Buckling — where boards have physically lifted off the subfloor — is a sign of severe moisture intrusion. Unlike cupping (which is a shape change), buckling means the boards have separated from their fasteners. Buckled boards cannot be re-nailed flat; they need to be replaced.

Visible mold on or under the floor is a health concern that requires immediate action. The mold source must be identified and eliminated, affected materials removed, and the area treated before new flooring is installed. Do not sand a floor with active mold — sanding aerosolizes mold spores.