A flooring schedule that names the species, the board width, the grade and the finish, then says nothing about how the boards are fixed down, has handed the most consequential decision on the page to whoever turns up on site with a compressor.

That decision sets the finished floor level, which sets the door heights, which were probably ordered weeks ago. It sets how the floor sounds under a heel, which in an apartment is somebody else’s problem as much as yours. And it decides whether a damaged board in year twelve gets lifted out in an afternoon or chiselled off the slab.

A solid floor cannot float

Real wood flooring in engineered form can be laid loose, clicked together and left sitting on the subfloor as one raft. The cross-bonded core is what makes that safe.

A solid board has no core. It takes up and gives off moisture across its full width, and a solid floor laid loose moves as a single mass and ends up wherever that mass takes it. So solid wood flooring is either bonded to the substrate or mechanically fixed to something. There are three ways to do that here, and they are not interchangeable.

The three ways solid wood flooring goes down

Fully bonded. Boards glued directly to the screed. Lowest build-up of the three, the most solid underfoot, and no hollow sound anywhere on the floor. It also asks the most of the slab, because nothing sits in between to absorb a deviation. Lifting a board later means cutting it out.

Secret-nailed to plywood. A plywood deck, usually 12 to 18mm, goes down first, bonded or mechanically fixed to the slab. Boards are then nailed or stapled at an angle through the tongue so no fixing shows on the face. The ply gives you a forgiving layer over a slab that will not come flat, and a single board of solid timber flooring can be lifted later without disturbing its neighbours.

Over battens. Timber battens at around 400mm centres with a void beneath, boards fixed across them. Highest build-up by a distance, and the only one of the three that lets you run services under the floor or correct a badly out-of-level slab without screeding. The void also changes how the floor sounds, which cuts both ways depending on who is listening.

Build-up is a drawing decision

Between a bonded floor and a battened one sits 50mm or more of difference in finished floor level. That is a door leaf height, a threshold detail against stone or tile, a level change at the balcony door, and the proportion of the skirting.

None of it can be resolved on site once joinery has been ordered. Fix the total build-up as a dimension on the drawing, and fix it before the door schedule goes out.

The same applies at every junction with another material. Where solid timber flooring meets tile, stone or a lift lobby threshold, draw the detail rather than describing it. A written instruction to match levels gets interpreted at the last minute by somebody who does not have both products in front of them.

Holding the board while it moves

Fixing solid wood flooring down does not stop it moving. It decides where the movement goes.

Adhesive is the clearest case. A hard-setting adhesive restrains the board against the slab, and the movement it would otherwise have made turns up somewhere else, usually at the joints or across the face of the board. An elastic system, MS polymer or a flexible PU, holds the board while allowing it a small amount of independent movement. Above roughly 150mm board width, elastic is the sensible default.

Width is the other half of it. A 190mm plank moves close to twice as far across its width as a 120mm plank in the same conditions. Wide solid boards look magnificent and they ask more of everything around them: a flatter slab, a more forgiving adhesive, a bigger gap at the wall.

Leave 10 to 15mm at the perimeter and carry it under the skirting. On runs over about 8 metres, or passing through a doorway into another room, an intermediate expansion allowance earns its place. A floor with nowhere to go pushes at the wall and lifts in the middle, and by the time that shows itself, the remedy is a lifted floor rather than an adjustment.

Slab flatness and acclimatisation sit ahead of all of this, and we have covered both in our piece on herringbone and chevron. The tolerances there apply to a plank floor in the same way.

Impact sound, if it is an apartment

Solid wood flooring bonded straight to the slab passes impact energy into it. In a landed house nobody notices. In an apartment the household below does, and management corporations here increasingly ask for a resilient layer under any hard floor.

That layer goes under the plywood or under the battens rather than directly beneath the boards, which makes it part of the fixing decision rather than something you add once the method is settled. Raise it at drawing stage or it becomes a build-up problem later.

When the floor goes in

The sequence for solid wood flooring is short, and it gets broken anyway.

After the wet trades. After the ceiling. After the first coat of paint. And after the air conditioning has been commissioned and is running.

That last one is the item that gets compressed when a programme slips, and it is the one that should not be. A floor delivered into a space that will be conditioned, then laid while that space is still open to the weather, has been asked to settle at one moisture level and then live at another.

Cover the floor once it is down, and let the finish cure before anything is loaded onto it.

What to fix before the package goes out to tender

•     The fixing method, named.
•     Total build-up from slab to finished floor, as a dimension.
•     Adhesive or fixing system, and whether it is elastic.
•     Resilient layer, where the project is an apartment.
•     Perimeter and threshold details, drawn.
•     Who tests the slab for moisture and flatness, and at what point in the programme.
•     The moisture content the boards will be delivered at.

Send us the floor plan with the slab levels and the door schedule marked. We will come back with the fixing method we would use on it, the build-up dimension you can draw to, and what we need from the main contractor before delivery.