MGP10SpanTables
MGP10 Guide

Wind Classification and Load Width in MGP10 Span Tables

Updated 2026-02-27 · MGP10 Span Tables

A stylised map of Australian wind classification regionsN1–N2N3N4 / CWind classification (N1–N4, C1–C4) changes which span column you read
Wind Classification and Load Width in MGP10 Span Tables

Two inputs quietly decide a surprising amount of your span, and both are easy to get wrong: the wind classification of your site, and the load width each member carries. Misjudge either and every number you read afterwards is for the wrong situation.

A stylised map of Australian wind classification regionsN1–N2N3N4 / CWind classification (N1–N4, C1–C4) changes which span column you read
Wind classification (N1–N4, or C1–C4 in cyclonic regions) rates the wind load at your site and changes which span values apply.

Wind classification (N1–N4, C1–C4)

Wind classification rates how much wind load a building must resist at its particular site. Non-cyclonic regions use N1 to N4; cyclonic regions use C1 to C4. A higher class means stronger winds, which affects member spans (especially on the roof) and the tie-down and bracing design.

Load width: FLW and RLW

Load width is how much floor or roof a single member has to carry down to its supports:

  • Floor Load Width (FLW) — the slice of floor a bearer (or similar) supports, broadly half the joist span each side.
  • Roof Load Width (RLW) — the slice of roof a roof member supports.
Diagram explaining load width carried by a structural memberMember being designedLoad width = area each member carriesLoad width is how much floor or roof area each member must carry
Load width is the band of floor or roof area that one member must carry — it sets the load, which sets the span.

Why load width swings the span

Load width directly sets the load on the member. Carry twice the area and you carry roughly twice the load, which sharply shortens the span a given size can manage. This is why the bearer and roof tables are organised around load width in the first place — and why a careless FLW or RLW estimate quietly produces an unsafe result.

Getting both right

  1. Have your wind classification assessed properly for the site.
  2. Work out the load width each member carries from your actual framing layout.
  3. Read the table column for your wind class and load width — not the default.

The Span Spec Builder prompts you for wind class and load width so they never get left out of your lookup.

Keep going

Apply this to bearers and roof members, then run through the common mistakes checklist.

Frequently asked questions

What is wind classification?
Wind classification (N1–N4 for non-cyclonic regions, C1–C4 for cyclonic) rates the wind load a building must resist at its site. It depends on region, terrain, shielding and topography, and it changes which span values and tie-down details apply.
How do I find my wind classification?
It is determined by a site-specific assessment following the wind standard (AS 4055 / AS/NZS 1170.2), usually by a qualified person. Do not guess it — the wrong class flows through the whole design.
What is the difference between FLW and RLW?
Floor Load Width (FLW) is how much floor area a member carries; Roof Load Width (RLW) is how much roof area it carries. Both describe the slice of load a member must support, and both are central inputs to the relevant span tables.
Why does load width change the span so much?
Because it directly sets the load on the member. Double the area a bearer carries and you roughly double its load, which sharply reduces the span it can manage. Measuring load width correctly is essential.

Keep reading

Build your lookup

Use the Span Spec Builder to assemble the exact parameters for this member, ready for the official tables or your engineer.

Open the Spec Builder