Wood floors: solid, engineered, finishes and refinishing

Tell solid from engineered, find out what finish is on the floor, keep the room inside the humidity range wood wants, and judge a recoat or a full refinish by how much wood is left.

Solid or engineered, and what hardness tells you Understand

A wood floor is one of two constructions, and almost every later decision depends on which one you have. The National Wood Flooring Association (NWFA), the trade association for the industry, defines them on its consumer site (NWFA, Types of Real Wood Flooring):

• Solid wood flooring is one piece of wood from top to bottom. NWFA says it can go in any room on or above ground and can be sanded many times.
• Engineered wood flooring is several layers glued together, with real wood on top. NWFA says it moves less with humidity than solid wood, which is why it is often chosen for basements, and that it can be sanded, but not as many times as solid.
• Composite engineered flooring has real wood only on the wear surface and a composite core underneath.

NWFA also separates strip flooring (boards under 3 in. wide) from plank (3 in. or wider). Width matters to the installer: NWFA's acclimation rules set different subfloor moisture limits for strip and for plank.

The layer that matters for refinishing is the wear layer: the wood above the tongue on a solid board, or the top layer of real wood on an engineered one. Sanding removes wood from that layer and nowhere else. NWFA's magazine puts a typical solid floor's wear layer at about 1/4 in., and engineered wear layers anywhere from 1/16 in. to 1/4 in. (Hardwood Floors, How Many Times Can a Wood Floor Be Sanded?). That range is why "engineered wood can't be refinished" is wrong as a rule: some engineered floors have room for a sand and some do not, depending on that one number. The recoat or full refinish section says how it is checked.

How to tell which one you have

Start with paper. A box end, a receipt, the installer's invoice, a previous refinisher's note or a listing sheet from the home's sale usually names the product. The installed register has a row for it. If nothing names it, the edge of a board tells you: at a floor vent register, a threshold or a stair nosing, an edge may show one solid piece or visible layers. Lifting a vent register is how our fictional refinisher checks in the examples below; ask whoever installed or services the floor before you lift or pry anything. Ask the authority

If you still cannot tell, write "unknown" on the register. A blank that says unknown is more useful than a guess, because it tells the next person (or you, in five years) that the question is open.

Janka hardness: dents, not scratches

Janka hardness is the force needed to press a steel ball 0.444 in. across halfway into a piece of wood, measured across the grain. The USDA Forest Products Laboratory's Wood Handbook, the federal reference for wood properties, lists side hardness for clear, defect-free samples of solid wood at 12 percent moisture content (Wood Handbook, FPL-GTR-282, Table 5-3b). Physical principle

Read the table for what it measures and nothing more. The handbook's figures are for solid, clear wood. NWFA says Janka is a good indication of dent resistance, says the cut and the finish also affect durability, and says none of these values apply to engineered wood flooring (NWFA, Durability). Neither source says Janka predicts scratches. Scratches land in the finish first, so a hard species under a soft or worn finish still scratches. Physical principle

The labels and ratings decoder carries Janka with its limits, and Samples, ratings, labels and warranties reads it beside the other ratings.

If you are buying

Quantity comes from the room's measured area plus an allowance for cutting and grading. Bruce's installation instructions for its 3/4 in. solid hardwood add 5 percent, 10 percent for diagonal layouts and 10 to 15 percent for glue-down installations (AHF Products, Bruce installation instructions). Those are one maker's figures for its own product. Where your maker gives none, the quantity calculator starts at 10 percent, which is this manual's default. This manual's convention The same Bruce instructions put final inspection of grade and factory finish on the owner or installer before installation, so open boxes and look before the boards go down.

For life figures, NAHB's 2007 survey lists wood floors as "Lifetime" and calls its own table a general guideline only (NAHB, Study of Life Expectancy of Home Components); Fannie Mae's tables for multifamily properties, reprinted by the Port of Seattle in 2025, assume 15 to 20 years for wood floors (Port of Seattle, Appendix C). Neither number counts the recoats a finish needs along the way. How to compare finishes explains why you enter your own figure and name its source.

Wood and water: humidity, gaps and cupping

Wood takes on and gives off moisture from the air until it reaches a balance with it. That balance point is the wood's equilibrium moisture content (EMC): the moisture content, as a percentage of the wood's dry weight, at which it neither gains nor loses water at a given temperature and humidity. As moisture content rises, boards swell; as it falls, they shrink. Physical principle

Relative humidity (RH) is how much water vapor the air holds compared with the most it could hold at that temperature, as a percentage. A hygrometer is the small gauge that reads it. NWFA's relative humidity sheet gives the link between the two at 70 °F: air at 30 percent RH brings wood to about 6.2 percent moisture content, and air at 50 percent RH brings it to about 9.2 percent (NWFA, Wood Floor Facts: Relative Humidity).

The range NWFA gives

NWFA's consumer care sheet and its problem prevention page both give the same range: keep the home at 60 to 80 °F and 30 to 50 percent relative humidity year-round (NWFA care sheet; NWFA, Problem Prevention). The relative humidity sheet adds "with regional exceptions." This is the trade's default; your own floor maker's sheet may give a different range, and its range governs its warranty. Convention The Go deeper section compares one maker's range with NWFA's.

A hygrometer reading in January and another in July tell you more than any single reading. Winter heating dries indoor air; summer humidity wets it. Write both on the care schedule, which carries a winter and a summer humidity check.

Gaps, cupping and what is normal

NWFA's problem prevention page describes the two movements. A floor that loses moisture shrinks, and gaps open between boards, or boards split. A floor that gains moisture swells, and boards cup: their edges rise above their centers. Physical principle

• Seasonal gaps that appear in the dry months and disappear in the humid months are, in NWFA's words, a normal reaction. They are the floor following the air.
• Large gaps, or gaps that stay open through the humid months, are a different matter. NWFA says to call a professional.
• Cupping means the boards are taking on moisture, usually from below or from a leak, a spill left standing or very humid air. The source has to be found before anything else is done to the floor.

A humidifier in winter or a dehumidifier in summer is one way to keep the air in range. Whether one suits your house, and what kind, is a question for a heating and cooling (HVAC) contractor. Ask the authority

Acclimation means matching moisture, not counting days

Many people hear that new wood flooring should sit in the house for a set number of days before it goes down. NWFA's installation guidelines describe acclimation, which it says is more accurately called conditioning, differently: it is bringing the flooring's moisture content into line with the conditions it will live in. If the boards already match the space and the subfloor, NWFA says the flooring may be installed immediately; if they do not, they wait until they do (NWFA, Acclimation/Conditioning). The installer measures moisture content with a meter. The Wood Handbook's targets for interior flooring are an average of 8 percent for most of the US, 6 percent for the dry Southwest and 11 percent for damp, warm coastal areas (Table 13-2 of the Wood Handbook). For engineered flooring, NWFA defers to the maker's own moisture testing and acclimation instructions to keep the warranty.

For you, that means one question for the installer: "What moisture content did you measure in the boards and the subfloor, and what range does the maker require?" Write the answer on the comparison sheet's questions part.

Finishes, protection and cleaning

What finish is on the floor

The finish is the coating that takes the wear, and it decides how the floor is cleaned and whether it can be recoated. NWFA sorts finishes into two families (NWFA, Wood Floor Finishes):

A site-finished floor was sanded and coated in the house; a factory-finished (prefinished) floor arrived coated. NWFA says factory-finished floors can be walked on right away and site finishes allow custom color but need drying time. Conversion varnish is one of the surface finishes NWFA lists; it is applied on site by a contractor. NWFA adds one point that helps when choosing: the less sheen, the less you notice small scratches. Preference

To find out which finish you have, use the same paper trail as for the floor itself: the maker's product page for a prefinished floor, or the refinisher's invoice for a site finish. We found no free source for a reliable home test that identifies a finish. If nothing names it, a flooring contractor can usually tell, and the answer goes on the installed register beside the floor. Ask the authority Wax matters most of all: a practitioner writing in the Journal of Light Construction says any contemporary finish can be recoated as long as paste or acrylic wax has not been applied over it (JLC, Recoating Wood Floors). So if anyone has ever waxed the floor, tell the refinisher.

Protection: grit, feet, pets and light

Most wear on a finished floor is abrasion. NWFA's care sheet lists the protections (NWFA care sheet):

• Breathable throw rugs at entrances, to catch grit before it reaches the floor.
• Floor protector pads under furniture legs. Check them on a schedule: a pad that has picked up grit or fallen off is worse than none.
• Avoid sports cleats and high heels on the floor.
• Trimmed pet nails. NWFA says plainly that pets will scratch a wood floor.

Shaw's hardwood care page adds rugs at pivot points, where people turn in the same spot, and asks for protection from direct sunlight (Shaw Floors, Hardwood Care). Product-specific We found no source on whether a given rug pad discolors a wood finish; if you put a pad under a rug, ask the floor maker or check its sheet.

Light changes the color of wood. The Wood Handbook says indoors, dark wood gets lighter and light wood gets darker, finished or not, and that the change is caused mostly by visible light rather than ultraviolet, so blocking ultraviolet alone does not prevent it. Most of the change happens in the first two to three months; shadow lines usually even out within several months, though cherry takes exceptionally long. The handbook's advice for a new floor is to check under rugs after a few weeks and to move rugs from time to time so they do not leave sharp lines (Wood Handbook, Chapter 16). Physical principle

Cleaning

NWFA's schedule for a finished floor (NWFA care sheet; NWFA, Maintenance):

• Daily or as needed: sweep or dust-mop.
• Weekly: vacuum on the bare-floor setting.
• Monthly: clean with a cleaner made for the finish on the floor, recommended by the installer, the flooring maker or the finish maker. NWFA warns that the wrong product can damage the finish.
• Spills: a dry or slightly damp cloth.

One rule holds across every wood source we read: no wet mops and no steam mops. NWFA says they damage the finish, and the wood itself over long periods; Shaw's hardwood page says the same, and Bona, a finish and cleaner maker, says to keep steam cleaners off wood floors (Bona, streak-free hardwood floors). Physical principle Never use a steam mop on a wood floor.

Beyond that, follow the finish maker. Shaw's page says to use a neutral-pH cleaner and no oil-based wax or polish. Bona says its cleaners are for sealed, finished floors and not for waxed or oiled ones, which need other products (Bona, Floor Cleaner FAQs). An oiled or waxed floor follows its own maker's sheet; the Go deeper section covers them. Where you do not know the maker, use the manual's gentle default: dry soil off first, a pH-neutral cleaner made for finished wood, a cloth or mop damp rather than wet, dry the floor after, and test on a hidden spot first. That default is ours, built from the makers' sheets. This manual's convention Cleaning by surface carries it, with the products to keep off wood.

Recoat or full refinish: how much wood is left

A worn wood floor gets one of two treatments, and they are very different jobs (NWFA, Refinishing Your Floors):

• A recoat (also called a maintenance coat, or screen and recoat) cleans the existing finish, scuffs it lightly so a new coat will bond, and adds a fresh coat. No bare wood is exposed, and almost no wood is removed.
• A full sand and refinish sands through the finish into bare wood, then applies new stain (if any) and finish. NWFA says it is the treatment for deep scratches, dents, or finish worn through to bare wood, and the time to change color.

Sources: the intervals from NWFA's care sheet; the recoat's limits from JLC; the depth of a sand from Hardwood Floors. The intervals are NWFA's convention; a floor in a busy entry wears faster than one in a guest room. Convention

The order matters. A recoat removes almost no wood, so trying it first costs little of the floor. If the result is not enough, a full sand is still possible afterward. The reverse is not true.

How many sandings a floor has left

Every full sand takes wood off the wear layer, so a floor has a limited number of them. NWFA's magazine says most solid floors can be sanded six to eight times over their service life, and that the real number depends on the contractor's skill, the equipment, the wear layer's thickness and how flat the floor is. For engineered floors it gives a floor: where the wear layer is less than 3/32 in. (2.38 mm), sanding is not recommended. It also notes that factory aluminum oxide finishes are hard to cut, that bevelled board edges take a lot of wood to sand flat, and that poor sanding removes more wood than it needs to. NWFA runs a program that identifies engineered floors made to be sanded (Hardwood Floors, How Many Times Can a Wood Floor Be Sanded?). Convention

So the useful question for a refinisher is not "can this floor be sanded?" but "how much wood is above the tongue, or how thick is the wear layer, and how much will one sand take?" One way to answer it is to look at a board's edge where one is exposed, such as at a floor vent register; ask the refinisher how they will measure it. Write the answer on the installed register; it is the number every later refinishing decision needs. Ask the authority

Who does the work, and what to ask

This manual does not teach sanding. A full sand is a contractor's job: it takes heavy machines, removes wood that does not come back, and makes fine dust. Before you hire, ask:

• How thick is the wear layer, how did you measure it, and how much will this sand remove?
• Will you try a recoat first, and what would tell you it is not enough?
• Which finish will you use, how long before furniture and rugs go back, and which cleaner does that finish maker name?
• For a home built before 1978: are you EPA or state certified for lead-safe work, and how will you protect painted trim, baseboards and the rest of the house from dust?

See it: Rosa's gaps and Harold's wear layer See it

• NWFA, Problem Prevention: cupping and gapping explained, and when to call a professional.
• NWFA, Refinishing Your Floors: maintenance coat against full sand and refinish.
• Hardwood Floors, How Many Times Can a Wood Floor Be Sanded?: wear layers, the 3/32 in. floor for engineered, and what each sand removes.
• USDA Forest Products Laboratory, Wood Handbook (PDF): hardness tables, moisture content targets and why wood changes color indoors.

Practice: identify, measure, protect Practice

Minimum: the floor's type and finish on the installed register (or "unknown" with who can settle it), one humidity reading, pads under the heaviest furniture, and the cleaning items on the care schedule. Time: about 30 minutes, plus a day for the hygrometer. Materials: any paperwork for the floor, an indoor hygrometer, felt protector pads, a breathable door rug, the finish maker's care sheet if you can find it, and the installed register and care schedule (or paper).

1. Gather the paper. Box ends, receipts, invoices, a refinisher's note, the home's sale listing. Write the floor's maker, product, species and finish on the installed register, or "unknown" and who might know.
2. Name the construction. Solid, engineered or unknown. If a board edge is visible at a threshold or stair nosing, look at it; leave registers and trim in place unless the person who services the floor says otherwise.
3. Place the hygrometer. Put it in the room, away from a heat vent or a window, and read it the next day. Write the reading and the date on the care schedule, and add a reminder for the opposite season.
4. Protect the floor. Pads under chair and table legs and under anything that moves; a breathable rug at each exterior door and at a busy pivot point.
5. Set the cleaning items. On the care schedule: sweep or dust-mop as needed, vacuum weekly on the bare-floor setting, clean monthly with the finish maker's cleaner, and "no wet mop, no steam" in the maker-says column.

Your home Your home

Check your work Check your work

Criteria

• Does the installed register say solid, engineered or unknown, and name the finish (or say who can)?
• Is there a dated humidity reading, and a reminder for the other season?
• Are pads under the furniture that moves, and is there a rug at the door?
• Does the care schedule list weekly and yearly items, with "no wet mop, no steam" and the finish maker's cleaner?
• If a refinish is on your mind, is the wear layer written down as a measured number or as "open", with who settles it?

Worked example

Rosa's register reads: red oak strip, solid; finish unknown (ask the refinisher); wood above the tongue measured by the refinisher at the vent register. Her care schedule has a January and a July humidity check, the January reading of 22 percent, and a note to discuss a humidifier with an HVAC contractor. It passes. It does not say the floor can take another sand until the measurement is in, and it does not choose a humidifier. Harold's register reads "engineered; wear layer unknown, check at vent register". That also passes, because the open fact is written as open with its route to an answer.

Common mistakes

• Reading Janka as a scratch rating. It measures dents in solid wood; the finish takes the scratches.
• Treating every gap as damage. Gaps that come and go with the seasons are the floor following the air; gaps that stay through the humid months are the ones to report.
• Jumping to a full sand. A recoat removes almost no wood and can be tried first; the wood a sand removes is gone for good.
• Using a steam mop "just this once". NWFA, Shaw and Bona all say no steam on wood.
• Assuming an engineered floor can or can't be refinished without checking. The wear layer decides it.

What is still unresolved

How much wood your floor has left is settled by a refinisher measuring it. Why a floor cups is settled by an inspector or restoration professional who finds the moisture source. Whether old paint in the house contains lead is settled by testing, in Stains, damage and older homes. Specific stains and damage on wood, and when to repair, refinish or replace, are in the same chapter. The products to keep off wood are in Cleaning by surface, and how a vinyl or laminate floor that looks like wood differs is in Vinyl, laminate, linoleum and cork.

Knowledge check and scenario

Checklist

• The installed register names the floor type and finish (or says "unknown" and who can settle it), and the care schedule has its weekly and yearly items.
• I read the room's humidity and wrote it down with the date, and set a reminder for the other season.
• Felt pads are under the furniture that moves, and a breathable rug is at the door.
• The cleaner I use is one the finish maker names, and nobody uses a wet mop or steam on this floor.
• I wrote my refinisher questions on the comparison sheet and added a recoat item to the care schedule. (Go deeper; optional)