Laying floor joists

Take the joist cross-section, centre spacing and span direction from the drawing and structural calculation. Check that the delivery matches the calculation: size, strength class (e.g. C24), CE marking and moisture content. Load-bearing dimensions are never guessed.

Cross-section, spacing and support are structural matters — use the calculation, not rules of thumb.

Set out the joists' centre lines cumulatively from one fixed starting point so that the last spacing is not off. The number of joists follows the fence-post principle (spaces + 1 with a member at both ends). Mark the centre lines clearly on both supports so the joists end up parallel.

Place the joists with the support specified by the project — joist hangers, trimming or bearing on a wall plate. Joist hangers and trimmer brackets must have all the specified nails/screws fitted, and the bracket's load-bearing capacity follows the supplier's data sheet. Check that the bearing length on the wall plate is sufficient.

Sight along each joist and find any bow. Always lay the joist with the crown (the convex side) upwards, so that the load straightens it slightly over time instead of making the floor sag. Mark and sort out badly bowed pieces for shorter spans.

Crown upwards — the load works with you, not against you.

Check that the joists are level and that the top edges are flush in one plane — it is the top edge the subfloor rests on. Correct height differences with the means the project allows (wedges/packing at the bearings), so that you avoid an uneven floor.

Slender joists can topple about their weak axis during installation and under load. Fit bridging/cross-bracing (herringbone or solid strutting) between the joists as instructed, so that they are held upright and distribute load to neighbouring joists. Brace temporarily until the subfloor ties the layer together.

Lay a vapour barrier, impact sound or sound insulation where the project requires it, before the floorboards are closed. With a vapour barrier, overlaps and penetrations must be tight (overlap min. 100 mm) — see the guide «Installing a vapour barrier».

Lay the subfloor (tongue-and-groove chipboard flooring or plywood) with staggered board joints and glue in the tongue and groove where the system specifies it. The board thickness is chosen according to the joist/bearer spacing, and the screw spacing follows the supplier's instructions. Staggered joints give a stiff, squeak-free floor.

Photograph joists, supports and brackets before the subfloor and ceiling close up the structure. Document deviations and clarify them with the designer before you close up — a hidden deviation is expensive to find again. Photo documentation of concealed structures is part of quality assurance.