Joist Spacing Calculator

Divide the span by the spacing to get gaps and add one for the joist at the far end. A 4 m floor at 400 mm centres needs eleven joists.

Commonly 400 or 600 mm, and the correct figure comes from a span table for the timber grade, joist section and the load carried. It is set by building code rather than preference.

Deeper, by a wide margin. Bending stiffness goes with the cube of depth and only linearly with width, so a 47 × 220 joist is dramatically stiffer than a 94 × 110 of the same cross-sectional area.

Span tables are usually set by strength and a deflection limit, not by perceived comfort. A floor can be entirely safe and still feel lively — closer spacing or deeper joists fixes it.

Cross-members that stop joists twisting and spread point loads between them. Most codes require it at intervals on longer spans, and it makes a noticeable difference to how a floor feels.

Only within limits set by code — typically shallow notches in defined zones near supports, and holes on the neutral axis. Notching in the wrong place removes exactly the material carrying the load.

It depends on section, grade, spacing and load, which is exactly what a span table encodes. Those tables are set by local building code and they are not interchangeable between countries.

The maximum sag allowed, usually expressed as a fraction of the span such as span over 360. It is a serviceability limit rather than a strength one — a floor can be strong and still fail the deflection check.

I-joists and open-web joists span further for the same depth and weight and are more dimensionally stable. They also behave very differently in a fire and have specific rules about notching and holes.

Doubling the width roughly doubles the strength but only doubles the stiffness, whereas increasing depth helps by the cube. Where headroom allows, deeper is far more effective than wider.

By hangers, by bearing on top, or by a ledger. Hangers must be the right type for the joist and fully nailed — partly nailed hangers are a common and serious defect.

Movement between the subfloor and the joist, usually a fixing that has loosened. Screwing and gluing the subfloor down is the durable fix; more nails is not.