A timber floor is mostly decided before the first board goes down. The ones that fail usually went onto a damp screed, were laid too wet or too dry for the room, or were fitted tight to the walls with nowhere to move.
So the first decision is solid or engineered, and that follows from what’s underneath, whether there’s underfloor heating and how wide you want the boards. Species and grade come after.
Solid or engineered?
Solid boards are timber all the way through. Engineered boards have a hardwood top layer (the wear layer) glued over a core, usually softwood or plywood. The core holds the top layer in check, so engineered boards are generally more stable, which is why makers steer you towards them for wide boards and heated floors.
- The wear layer is the only part you can sand. Top layers of 4mm and 6mm are common on better boards; some thin boards have almost nothing to sand. Ask for the wear layer thickness, not just the overall thickness.
- Solid boards can be sanded back many times and are the traditional choice nailed to joists or battens. They move more, so on concrete or over heating they need more care.
- Underfloor heating: one UK flooring company says never to lay solid boards over underfloor heating and never to let the screed surface go above 27°C. One engineered board maker excludes beech and maple from heated floors. Check that the maker of your boards accepts heating before you order.
Timber choices for floors
| Timber | What to know |
|---|---|
| Oak | The default UK floor, solid or engineered, in every grade from clean to knotty. Hard-wearing and takes oil or lacquer well. Oak shrinks a lot across the grain and moves in humid conditions, so wide solid oak is where an engineered board earns its place. See the European oak page. |
| Ash | Pale, very hard and stable in service. Less common as flooring, so ask what’s actually available. See the ash page. |
| Hard maple | Highly resistant to wear, which is why it goes into high-traffic floors. Pale, and darkens with light. Some makers exclude it from heated floors. See the hard maple page. |
| Pine and redwood boards | The softwood floorboard, usually redwood (Scots pine), nailed to joists. Much softer than the hardwoods, so it dents and wears. The usual match for repairing an existing pine floor. See the redwood page. |
Walnut is the dark option. American black walnut is stable once dry but noticeably softer than oak, so it marks sooner in a hallway or kitchen. See the black walnut page.
Grade names vary by maker but run roughly from prime (clean and even) through select to rustic (large knots, cracks, some sapwood). Grade is about appearance, so look at a sample from the actual batch.
Width, moisture and movement
Timber swells and shrinks across its width as the air gets damper and drier; along the length the change is small enough to ignore. Wider boards move more. One UK maker gives a rough figure of 2mm per metre of floor width for every 1% change in moisture content. The movement calculator works it out for your species and board width.
UK homes usually sit between 35% and 65% relative humidity, and hardwood flooring is typically dried to 6 to 9% moisture content to suit.
Acclimatising advice differs: one maker says to leave boards in the room for 7 to 10 days before fitting, another to lay straight from the packaging. Follow the instructions for your boards. Either way, plastering and screeding should be finished and the building dry first.
Check the subfloor first
A concrete slab or screed can hold water for months, so measure it rather than guess. The BS 8201 test is a hygrometer sealed under a hood on the screed, reading relative humidity. 75% RH is the figure usually quoted, but makers often want less for some fixings: one gives 75% for floating engineered boards, 65% for full glue-down on sand and cement screed, and lower again for calcium sulphate (anhydrite) screeds. Use your board maker’s figure.
On a ground floor, a damp-proof membrane goes between concrete and floor. Ordinary flooring adhesive is not a vapour check.
Fixing and expansion gaps
- Secret nailing: nails at 45° through the tongue into each joist or batten, hidden by the next board. One maker specifies a nail into every joist, none closer than 50mm to a board end, and predrilling when nailing by hand.
- Glue-down: boards fully bonded to the subfloor. Best heat transfer over underfloor heating, and the strictest subfloor moisture limits.
- Floating: engineered boards joined to each other over an underlay, not fixed down. Heat transfers less well than with a glued floor.
Whatever the fixing, leave a gap at every wall, door frame and pipe, hidden under the skirting. Makers ask for at least 8mm and often 13 to 15mm; one works it out as 1.5mm per metre of floor width at each side, never less than 15mm in the UK, so wide rooms need more. A floor fitted tight has nowhere to go when it swells, and it cups or lifts.
For quantities, the coverage calculator takes the area and a wastage allowance, using the boards’ face width rather than the width including the tongue.
Reclaimed boards
- Old fixings: pull or punch every nail and screw before you plane, sand or cut. A metal detector finds the ones you can’t see.
- Lead paint: boards from homes built before the 1960s may carry lead paint. Don’t dry sand it or burn it off. Cambridge City Council advises a water-based stripper or a hot-air gun below 450°C, keeping the surface damp, a mask with a P3 filter, and keeping children and pregnant women away.
- Moisture and size: boards stored outdoors need to dry to suit the room, like new boards, and old boards vary in width and thickness, so sort them first.
Finishes
Oil soaks in and leaves a matt, natural look. A worn or scratched patch can be re-oiled, or spot-sanded and re-oiled, so you look after the floor area by area. Hardwax oil blends oils and waxes in one product. An oiled floor is refreshed with a new coat of oil rather than sanded back.
Lacquer forms a film on the surface and is usually a little shinier. Good lacquer is low-maintenance, but spot repairs are hard to blend in and bad damage usually means sanding the whole floor. Oil and water-based lacquer don’t mix, so find out what’s on an existing floor before you recoat it.
Sources
- The Solid Wood Flooring Company, Storage, fitting and maintenance instructions 06A (2025) (75% and 65% RH screed limits, calcium sulphate limits, 7 to 10 day acclimatising, fitting to BS 8201, wet trades finished first)
- The Solid Wood Flooring Company, Wood floors and the effects of moisture (hardwood dried to 6 to 9%, 35 to 65% RH, BS 8201 Annex A hygrometer test, DPM on ground floors, adhesive is not a vapour check, cupping from swelling)
- The Solid Wood Flooring Company, Wood flooring and underfloor heating advice SWF100 (no solid boards over heating, 27°C screed surface limit, 4mm and 6mm top layers, 8mm to 15mm perimeter gaps, 2mm per metre per 1% movement, floating floors transfer heat less well)
- Junckers, Batten floors: specifier’s and installation instructions C 1.2 (polythene DPM on concrete, secret nailing at 45° into each joist, 50mm from ends, predrilling, gap of 1.5mm per metre of floor width at each side, 15mm minimum in the UK, no on-site acclimatising)
- Kährs, Engineered wood guide (2026) (beech and maple excluded from underfloor heating, layered construction, grade names from prime to rustic, oil and lacquer finishes, oiled floors recoated with oil)
- American Hardwood Export Council, Species guide (hardness, stability and flooring use of oak, ash, hard maple and black walnut)
- The Solid Wood Flooring Company, UV oiled vs lacquered finishes (oil easier to repair, lacquer spot repairs hard to blend, oil and water-based lacquer incompatible)
- Cambridge City Council, Lead paint in your home (lead in some paints until the mid-1960s, no dry sanding, water-based stripper or hot-air gun below 450°C, P3 mask, children and pregnant women)
