Understanding Deflection in Span Tables

Updated 2026-02-15 · MGP10 Span Tables

Here is the thing most people miss about span tables: they are not just stopping your floor from collapsing. They are also stopping it from sagging or bouncing enough to feel wrong and crack your finishes. That second job — controlling deflection — quietly governs a lot of MGP10 spans.

Two checks, not one

Every structural member has to pass two kinds of check:

• Strength — it must not break or overstress under load.
• Serviceability (deflection) — it must not sag, bounce or vibrate excessively in normal use.

A member can be plenty strong yet fail the deflection check. That is why you cannot eyeball it: a stick that "feels solid" may still sag enough over a long span to crack a ceiling below.

What L/300 and friends mean

Deflection limits are written as a fraction of the span. L/300 means the allowable sag equals the span divided by 300; a stricter limit like L/360 allows even less. Different members and load cases use different limits, and the standard decides which applies — the table has already done that maths so the listed span keeps deflection within bounds.

Why floors feel bouncy

Bounce is usually about stiffness and vibration rather than dramatic sag. Long spans, members worked close to their limit, and lighter framing can all feel springy. The fixes all increase stiffness:

• Reduce the spacing (e.g. 600 to 450 mm).
• Go deeper in section (a deeper joist is dramatically stiffer).
• Step up the grade — MGP12 or MGP15 is stiffer than MGP10.

Depth is your best lever

Stiffness rises steeply with depth, so adding depth is usually the most efficient way to stiffen a floor or roof. This is why the tables reward deeper sections with much longer spans — and why a deeper MGP10 joist often beats a shallow higher grade.

Keep going

See how deflection plays out in practice for floor joists and lintels, and avoid the common mistakes.

Frequently asked questions

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