This is the question we get asked in the showroom more than any other, and it is usually decided by one measurement and two things you can see from standing up.
First, work out what you actually have
Solid hardwood is one piece of wood, normally 3/4 inch thick, with roughly a quarter inch of usable wood above the tongue. That is enough to be sanded flat four to six times across the life of the floor. A solid floor that has only been done once or twice has plenty left.
Engineered hardwood is a real wood veneer over a plywood core. Everything depends on how thick that veneer is:
| Veneer thickness | What you can do |
|---|---|
| Under 2mm | Do not sand. A screen-and-recoat is the only option. |
| 2mm | One light sanding, once, by someone careful |
| 3mm | One or two sandings |
| 4 – 6mm | Two or three sandings, close to solid |
You can measure it at a floor vent or a doorway threshold. Lift the register and look at the cut edge of a board. If the top layer of grain is thinner than a credit card, it cannot be sanded.
The three signs that decide it for you
Cupping. If the boards are higher at their edges than in the middle, water has come up from underneath. Sanding a cupped floor before the moisture problem is fixed and the wood has re-flattened just makes it worse. You flatten it wet, it dries, and it crowns in the opposite direction.
Black stains that follow the grain. Pet urine that has gone through the finish into the wood turns black as it reacts with the tannins. Sanding does not reach it. Those boards get cut out and replaced, or the floor does.
Nail heads or a grey, fibrous surface. If you can see fasteners, or the wood has gone soft and splintery, the wear layer is gone. There is nothing left to take off.
What each option costs
- Screen and recoat — buffing the old finish and putting a fresh coat over it. Cheapest by a wide margin. Only works when the finish is dull or lightly scratched and nothing has gone through to bare wood.
- Full sand and refinish — taking the floor back to raw wood and building the finish up again. Typically a third to a half of what a new floor costs.
- Replace — $8 to $15 per square foot installed for new hardwood.
Sanding is almost always the cheaper answer when it is available at all. The reason to replace a sound floor is that you want a different species, a different width, or a different color than the wood you have can give you.
Two things people get wrong about refinishing
Stain cannot lighten wood. It only ever goes darker. If you have red oak and you want a pale Scandinavian floor, stain will not get you there. That needs a different species or a new floor.
Not every board has to match. Where boards are cut out and replaced, new wood in the same species will take stain slightly differently for the first year and then settle in. On a floor that is being fully sanded and refinished afterwards, patched areas usually disappear. On a prefinished floor that is not being sanded, they will not.
Where solid hardwood should not go
Below grade, and directly on a concrete slab without a proper subfloor between. Solid wood moves with moisture, and a slab passes moisture up into it year-round. Basements and slab-on-grade rooms want engineered wood, or a different material entirely.
Prefinished or finished in place
Prefinished boards arrive with a factory-cured finish, usually aluminium oxide, which is harder than anything that can be applied in a house, and it comes with a small bevel on every edge. Finishing in place gives a flat surface with no bevels between boards, and lets you match an existing floor exactly, but the house has to be empty while it cures.
Neither is better. Prefinished is faster and tougher; site-finished looks more like one continuous floor.