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Solid boards are milled from one piece of wood, usually 18–22 mm thick, with a tongue on one side and a groove on the other. They are fixed to the subfloor: blind-nailed through the tongue into a timber subfloor, or glued down on screed.
What makes solid wood different
Solid wood moves more than any other floor. A wide board can change width noticeably between a dry winter and a humid summer. Most installation problems, such as gaps, cupping and crowning, come from moisture, not from workmanship.
Key numbers
| Rule | Typical value |
|---|---|
| Acclimatise | Until the boards reach the room's moisture content, often 5–7 days |
| Moisture difference | Boards and timber subfloor within 2% (wide boards) to 4% (narrow strip) |
| Expansion gap | About 15 mm, or the board thickness |
| Fixings | One cleat or staple every 20–25 cm, and 5–7 cm from each end |
| End joint stagger | At least 15 cm, more for wide boards |
Installation in short
- Measure the moisture of the subfloor and of several boards with a pin moisture meter.
- Lay underlay paper and mark the joist positions on the walls.
- Run boards across the joists where possible.
- Snap a straight chalk line for the first row at the gap distance plus board width from the wall. Walls are not straight; the line is.
- Face-nail the first row on the wall side, then blind-nail through the tongue.
- Rack out several rows of boards on the floor before nailing to plan joint spacing and colour.
- Use the flooring nailer from the third row on. The last rows are face-nailed or glued where the nailer cannot fit.
- Sand and finish if the boards are unfinished, then fit skirting.
Never install solid wood straight onto fresh screed, over a damp crawl space, or in a room that has not been heated yet. Wait until the building is dry and lived-in.