Quick answer
Solid hardwood flooring is flooring milled from a single piece of wood from top to bottom, usually 3/4 inch thick with tongue-and-groove edges, nailed or stapled to a wood subfloor and finished either at the factory or on site.
Because the whole board is wood, it can be sanded and refinished many times. It also swells and shrinks with indoor humidity, which limits where it can go.
Quick facts
The key facts about Solid Hardwood Flooring, with sources:
- NWFA definition
- A solid piece of wood from top to bottom 1
- Common thickness
- 3/4 in strip and plank (one maker’s guide) 2
- Grade levels (one maker)
- On or above grade only; not in full bathrooms 2
- Wood subfloor moisture (one maker)
- 12% or less, within 3% of strip or 2% of plank flooring 2
- Wood moisture at 70°F
- About 6% at 30% RH and about 9% at 50% RH 4
- Expansion space (one maker)
- 3/4 in at walls 2
Key takeaways
- Solid hardwood is one piece of wood through its full thickness, so it can be sanded and refinished many times.
- Wood gains and loses moisture with the air; a floor that is installed too wet or too dry will gap, cup or buckle.
- Most makers allow solid wood only on or above grade, over a wood subfloor.
- Moisture testing of both the flooring and the subfloor is part of a proper install.
- Refinishing older floors can disturb lead paint on trim in pre-1978 homes.
What makes hardwood “solid”
The National Wood Flooring Association (NWFA) sorts real wood floors into three groups. Solid wood flooring is “a solid piece of wood from top to bottom.” Engineered flooring is built from glued layers, and composite engineered flooring has real wood only on top 1.
Most solid floors are oak, maple, hickory or another hardwood species, milled into strips (narrow boards) or planks (wider boards) with a tongue on one edge and a groove on the other.
Bruce, a large U.S. maker, sells 3/4 in solid strip and plank for staple, nail or cleat installation, and also glue-down in some cases 2.
The defining trait is movement. Wood is hygroscopic: it takes on and gives off moisture with the surrounding air.
The U.S. Forest Products Laboratory explains that wood swells as it gains bound water and shrinks as it loses it, and that this can lead to gaps in flooring and other problems 4. Everything about installing solid wood is about controlling that movement.
How a solid wood floor stays flat
- The wood reaches balance with the house. Boards move toward the equilibrium moisture content (EMC) set by indoor temperature and humidity 4.
- The installer measures before nailing. Bruce asks for a wood subfloor at 12% or less and within 3% (strip) or 2% (plank) of the flooring 2.
- Fasteners tie it to the subfloor. Boards are blind-nailed or stapled through the tongue into a wood subfloor, usually across the joists 2.
- A gap at the walls takes the swelling. Bruce’s layout allows a 3/4 in expansion space, hidden by base trim 2.
- A vapor retarder slows moisture from below. Bruce calls for one over wood subfloors before nail-down, with seams lapped 4 in or more 2.
- Indoor airTemperature and humidity
- Equilibrium moisture contentWood settles to match the air
- Board swells or shrinksMostly across its width
- Gaps, cupping or bucklingIf conditions swing too far
- Back to step 1
The cycle repeats every season; steady humidity keeps it small. Source: [4]
Types of solid hardwood flooring
| Type | What it is | How it is installed | Notes |
|---|---|---|---|
| Strip flooring | Narrow 3/4 in boards 2 | Nailed, stapled or cleated 2 | Classic look in older homes |
| Plank flooring | Wider 3/4 in boards 2 | Nailed; some products glued 2 | Tighter moisture match: within 2% of subfloor 2 |
| Thin solid (5/16 in) | Thinner solid strip | Staple-down or glue-down 3 | Fewer refinishes than 3/4 in |
| Unfinished | Sanded and finished on site | Nailed, then sanded and coated | Custom stain and smooth seams |
| Prefinished | Finished at the factory | Nailed; ready to walk on sooner | Small edge bevels are common |
Example: will this oak floor gap in winter?
A remodeler is installing solid oak plank in a living room in October. The Wood Handbook’s equilibrium equation shows how much moisture the wood will settle at as the home’s humidity changes at 70°F 4. Bruce’s guide sets the install limits 2.
| Condition | Figure | What it means |
|---|---|---|
| Summer, 50% RH at 70°F | Wood settles near 9% moisture 4 | Boards at their widest |
| Winter with heat, 30% RH at 70°F | Wood settles near 6% moisture 4 | Boards shrink; small seasonal gaps appear |
| Very dry winter, 20% RH at 70°F | Wood settles near 4.5% moisture 4 | Wider gaps and possible checking |
| Subfloor reading at install | 11% (example) | Below the 12% maximum 2 |
| Flooring reading at install | 9.5% (example) | Within 2% of the subfloor for plank: OK 2 |
The example readings are illustrations; the limits are from the guide. Bruce suggests checking about 40 boards per 1,000 sq ft to get a fair average 2. Keeping winter humidity near the middle of the range is what keeps seasonal gaps small.
Where solid hardwood shows up in a remodel
On the quote. Species, grade, width, thickness, prefinished or site-finished, fastening method, moisture testing, vapor retarder, subfloor repairs, transitions and stair nosings. Removal of old floors is separate.
On the plans. Board direction matters. Bruce says to run boards perpendicular to the floor joists unless the subfloor has been reinforced 2.
Over the subfloor. Loose or squeaky subfloor panels must be screwed or nailed down first, and water-damaged panels replaced 2.
In older homes. Existing solid floors are often refinished rather than replaced, which is cheaper on material but can raise lead and dust questions.
Solid wood is limited to on or above grade
Bruce’s 3/4 in guide says solid hardwood may be installed on or above grade, not below, and not in full bathrooms. Over concrete, it needs a suitable wood subfloor 2. Below grade, engineered wood is the usual wood choice.
Solid hardwood pros and cons
Strengths
- Real wood throughout its thickness 1.
- Can be sanded and refinished many times over its life.
- Nailed floors feel solid underfoot.
- Damaged boards can be cut out and replaced.
Limits: moisture, grade and radiant heat
Moisture is the core limit. The Wood Handbook notes wood shrinks and swells most across the grain, which in a floor means across the width of each board 4.
Over a damp crawl space, a floor can cup; Bruce calls for crawl spaces at least 18 in high with a 6- to 20-mil polyethylene ground cover and perimeter vents equal to at least 1.5% of the crawl space area 2.
Grade is the second limit. Basements and slabs on the ground bring moisture from below, which is why makers keep solid wood on or above grade 2.
Heat is the third. Radiant floor heating dries wood quickly. Many makers restrict solid wood over radiant heat or set strict limits; check the product guide before planning it.
What drives the cost of solid hardwood, and how long it lasts
We do not publish flooring prices. Cost depends on species and grade, board width (wider boards often cost more and need tighter moisture control), prefinished versus site-finished, subfloor repairs, the vapor retarder, stairs and nosings, and removal of old floors.
Site-finished floors add sanding, staining and several coats of finish, plus drying time when rooms are off limits.
A well-kept solid floor can last for generations, because sanding exposes fresh wood. Each full sanding removes some thickness, so the number of refinishes depends on how much wood sits above the tongue. The flooring calculator estimates material, and the cost guide covers labor drivers.
How a solid wood floor is installed
A flooring contractor usually does this work. A permit is rarely needed for flooring alone, but structural subfloor or joist repairs can need one.
- Finish wet work (drywall mud, paint, concrete) and run the HVAC so the house is at living conditions.
- Deliver flooring to the conditioned site and acclimate it until moisture targets are met 2.
- Measure and record subfloor and flooring moisture with a wood moisture meter 2.
- Fix squeaks and loose panels; check flatness to 3/16 in in 10 ft or 1/8 in in 6 ft 2.
- Lay a qualifying vapor retarder over the wood subfloor, lapped at least 4 in 2.
- Snap a starting line that leaves the 3/4 in expansion space, and run boards across the joists 2.
- Blind-nail or staple each board; face-nail the first and last rows.
- Sand and finish on site, or install trim for prefinished floors.
Caring for a solid hardwood floor
- Keep indoor humidity steady; EPA suggests below 60%, ideally 30% to 50%, for the home overall 7.
- Use the cleaner recommended for your finish, and avoid wet mopping.
- Put felt pads under furniture and mats at entries.
- Recoat before the finish wears through to bare wood.
- Clean up spills and pet accidents quickly.
Warning signs and when to call a pro
- Cupping (edges raised): moisture from below; check the crawl space or slab.
- Crowning (centers raised): often a cupped floor sanded flat too soon.
- Buckling (boards lifting off the subfloor): major water exposure or no expansion space.
- Wide winter gaps: very dry indoor air 4.
- Soft or spongy areas: subfloor or joist damage; may need a structural look.
Refinishing can stir up lead dust
In homes built before 1978, sanding floors often means sanding near painted baseboards and trim. EPA says most such homes still contain some lead-based paint and that paid work disturbing painted surfaces requires an RRP-certified firm using lead-safe practices 6.
Standards and rules that apply
NWFA definitions and guidelines. NWFA defines solid, engineered and composite engineered wood flooring and publishes installation guidelines that some makers defer to when their own are silent 1.
Maker installation instructions. These set moisture limits, grade levels, fastening and expansion space. Bruce states that failing to follow its instructions may void the warranty 2.
Building codes. Model residential codes govern the floor structure under the wood (joists and subfloor), not the wood finish itself.
Lead rules. EPA’s RRP rule (40 CFR 745) applies to paid work that disturbs paint in pre-1978 homes, which many floor refinishing jobs do at the edges 6.
Solid vs engineered hardwood vs laminate
| Floor | Construction | Below grade? | Refinishing |
|---|---|---|---|
| Solid hardwood | One piece of wood 1 | No, on or above grade 2 | Many times |
| Engineered hardwood | Cross-layered plies with a wood wear layer 1 | Yes, per maker 5 | Depends on wear layer |
| Laminate | Printed image on fiberboard | Per maker | No |
| Vinyl plank | Vinyl or polymer core | Usually yes | No |
Solid hardwood myths
- Myth Gaps mean the floor was installed badly.
- Reality Small seasonal gaps are normal as wood dries in heated winters 4.
- Myth Solid wood can go anywhere engineered wood can.
- Reality Makers limit solid wood to on or above grade 2.
- Myth Acclimation means waiting a set number of days.
- Reality The goal is a moisture match, measured with a meter 2.
- Myth Builder’s paper always works as a vapor retarder.
- Reality Bruce says common kraft paper and red rosin generally do not qualify 2.
- Myth Thicker finish coats protect it from humidity.
- Reality Finish slows but does not stop moisture exchange; humidity control matters 4.
Climate decides how much a solid floor moves
Indoor humidity follows the outdoor climate and the home’s HVAC. In cold regions with long heating seasons, indoor air can drop toward 20% to 30% RH, pulling wood down toward 4.5% to 6% moisture 4 and opening gaps.
In humid regions without good dehumidification, floors take on moisture and cup. Homes over crawl spaces need a ground cover and venting to keep moisture from below in check 2.
When solid hardwood is the right call
Choose solid hardwood for main living areas on or above grade, over a sound wood subfloor, in a home where you can keep humidity reasonably steady and plan to stay long enough to refinish.
Choose engineered wood for basements, slabs and many radiant-heat floors, and a non-wood floor for full bathrooms. When bids differ, compare moisture testing and prep line by line with the quote comparison tool.
Questions about Solid Hardwood Flooring
What is solid hardwood flooring?
Solid hardwood flooring is milled from a single piece of wood from top to bottom, usually 3/4 inch thick with tongue-and-groove edges. The National Wood Flooring Association defines it that way to separate it from engineered flooring, which is made of glued layers.
Is solid hardwood better than engineered?
Neither is better everywhere. Solid hardwood can be refinished more times and suits main floors over wood subfloors. Engineered hardwood is more stable with humidity changes and can go below grade and over slabs. Pick by location, humidity control and how long you will keep the floor.
Can solid hardwood be installed in a basement?
Usually not. Makers such as Bruce allow solid hardwood only on or above grade. Basements bring moisture from the slab and surrounding soil. For a wood look below grade, engineered hardwood or another floor rated for basements is the normal choice.
Why does my hardwood floor have gaps in winter?
Because the wood is drying out. Heated winter air is dry, and the Wood Handbook shows wood at 70°F settling near 6% moisture at 30% humidity versus about 9% at 50%. The boards shrink across their width. Gaps that close again in summer are normal; humidification reduces them.
How long should hardwood acclimate before installation?
Until its moisture content is within the maker’s target, not a fixed number of days. Bruce asks for a wood subfloor at 12% or less and the flooring within 3% of it for strip or 2% for plank, checked with a moisture meter on many boards.
How many times can solid hardwood be refinished?
Several times, depending on how much wood sits above the tongue and how much each sanding removes. A skilled refinisher takes off as little as possible. Recoating the finish before it wears through to bare wood reduces how often a full sanding is needed.
What causes hardwood floors to cup?
Cupping means the board edges sit higher than the centers. It usually means more moisture below the board than above, such as a damp crawl space, a slab or a leak. Find and fix the moisture source first; sanding a cupped floor too soon can leave it crowned later.
Which direction should hardwood run?
Across the floor joists in most cases. Bruce says boards should be installed perpendicular to the floor joists unless the subfloor has been reinforced to prevent sagging. For looks, many installers also run boards along the longest wall where the structure allows.
Do I need a vapor retarder under hardwood?
Many makers require one over wood subfloors. Bruce calls for a vapor retarder before nail-down, with seams overlapped at least 4 inches, and notes that common brown kraft paper and red rosin paper generally do not qualify. Asphalt-saturated felt usually does.
Sources
- National Wood Flooring Association, Engineered Wood Flooring Refinishable Program (includes NWFA Definition of Real Wood Flooring), retrieved .
- Bruce (AHF Products), Installation Instructions: Solid Wood Flooring 3/4 in Strip and Plank, retrieved .
- Bruce (AHF Products), Solid Wood Flooring 5/16 in Strip Installation Instructions, retrieved .
- USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282), Chapter 4: Moisture Relations and Physical Properties of Wood, retrieved .
- Hardwood Floors Magazine (NWFA), Engineered Wood Floors Have a Comfort Zone Too (2022), retrieved .
- U.S. EPA, Lead Renovation, Repair and Painting Program, retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The definition is NWFA’s. Installation limits (grade levels, moisture match, vapor retarder, flatness, crawl space, expansion space, board direction) are from Bruce’s 3/4 in solid guide and labelled as one maker’s rules.
Moisture-content values were computed from the Wood Handbook’s EMC equation (4–5) at 70°F and agree with its Table 4–2. The worked example’s meter readings are illustrative inputs, marked as such. Lead guidance from EPA. No prices are given.
Suggest a correction. We fix errors and say what changed.