10 Common MGP10 Span Table Mistakes (and How to Avoid Them)

Updated 2026-03-05 · MGP10 Span Tables

Span tables are reliable — right up until a small wrong assumption sends you to the wrong cell. Almost every span-table problem is one of a handful of avoidable mistakes. Here they are, with the fix for each, so you can build in confidence instead of a callback.

The usual suspects

1. Reading the wrong grade. An MGP12 or F-grade span is not an MGP10 span. Fix: confirm the stamp and read the matching grade table.
2. Wrong spacing column. Mixing up 450 and 600 mm centres. Fix: read the column for your actual set-out.
3. Single vs continuous mix-up. Using continuous values for a member that isn't truly continuous. Fix: see single vs continuous span; when unsure, use single.
4. Guessing the wind class. Estimating N-class from a map. Fix: get a proper wind assessment.
5. Mis-measuring load width. Underestimating FLW or RLW. Fix: calculate it from the real layout.
6. Ignoring roof mass. Reading the sheet column for a tiled roof. Fix: match sheet vs tile.
7. Forgetting deflection. Picking a size that "looks strong." Fix: trust the table, which includes deflection limits.
8. Missing point loads. A beam or post landing on a lintel. Fix: tables assume distributed loads — point loads need engineering.
9. Wrong treatment. Using untreated timber outdoors. Fix: match the hazard level (e.g. H3 for decks).
10. Using an old edition or random web numbers. Fix: work from the current official tables.

Why we don't just hand you a number

You will notice this site teaches you to read the tables rather than printing spans for you to copy. That is deliberate. The right span for your build depends on inputs only you (and your engineer) can confirm for your site, and on the current edition of a regulated standard. A number lifted from a web page — including one we might publish — is exactly how the mistakes above happen. The Span Spec Builder instead helps you assemble a complete, correct lookup to take to the authoritative source.

A quick pre-cut checklist

• Grade confirmed (MGP10) and reading the MGP10 table?
• Correct member table, spacing and span type?
• Wind class assessed, not guessed?
• Load width and roof mass correct?
• Deflection handled by the table, no point loads?
• Current edition, and signed off by a professional?

Keep going

Go back to basics with how to read span tables, or gear up with our framing tools & reference picks.

Frequently asked questions

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