From loadbearing beam to finished plank: here is the floor structure, subflooring in chipboard and OSB, floating floors and floating timber floors, the three floor types (solid, parquet, laminate) and the moisture and movement rules that determine whether the floor stays flat and quiet for years.
Introduction
A floor consists of multiple layers, each with its own function: the load-bearing beam layer or deck, a subfloor that distributes the load and provides a level surface, and the visible floor covering. If the layers are not built correctly — or if wood movement with moisture is forgotten — the result is creaking, gaps and warped boards. The actual sizing of the beams belongs to structural engineering; the construction and laying is the carpenter's job.
Joist floor — the load-bearing storey separator
In a timber house the floor is carried by a beam layer: parallel beams, typically with centre distance cc 600 mm (bound to the subfloor plate's format and the load — the final cc comes from the structural calculation). Between and under the beams the floor separation is built up with insulation and a strapped or directly mounted ceiling. A heavy floor separation with chips/gravel in older houses was precisely a sound-damping mass — in new buildings the sound is instead solved with decoupled ceilings and floating floors (see sound article).
- 01Beams: solid construction timber (C24), I-beams or glulam; size and cc from structural calculations.
- 02Fixed distance between beams is maintained with spacer blocks/shims so they do not twist.
- 03Installations (electrical, plumbing) are run in the joist layer — but holes and notches in load-bearing beams must follow the rules, not be sawn freely.
- 04The ceiling under the beams is often mounted on acoustic spring hangers/sound bars to break the path of impact sound downwards.
The subfloor
The subfloor is the flat load-distributing board above joists or sleepers. The most common is 22 mm flooring chipboard, moisture-resistant grade (P5/P6), with tongue and groove all round so the boards lock together. The joints are glued in the groove, and the board is screwed to each support. OSB is used similarly. The board is laid in staggered bond (offset joints) with a small expansion gap against the walls, because the board also works with moisture.
Scattered floor and floating floor
On a concrete deck, the floor is laid either on battens or as a floating floor. A battened floor consists of timber joists (battens) on the deck — often shimmed or adjusted in height — with subflooring or boards on top; the cavity provides space for insulation and installations. A floating floor, on the other hand, lies loosely (floats) on top of a sound/insulation layer without a fixed connection to the deck, so impact sound and moisture from the concrete do not pass through.
- 01Joist floor: joists on centre-to-centre (often 600 mm), levelled and fixed with shims; insulation between joists; subfloor/boards on top.
- 02Floating floor: a step-sound damping layer (e.g. step-sound board or impact-resistant insulation) directly on the deck, then a floating board or plank structure that must not have rigid contact with walls or deck.
- 03Moisture barrier against concrete decking: new concrete releases moisture for a long time — a moisture barrier/PE film protects the wooden floor until the deck is dry enough (moisture requirement is looked up from the floor supplier).
- 04No rigid bridges: a single screw through a floating floor down into the deck, or a board that touches the wall, eliminates the entire acoustic damping.
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