MGP10 Guide

MGP10 Floor Joist Spans: How to Size Them Right

Updated 2026-01-22 · MGP10 Span Tables

MGP10 Floor Joist Spans

Floor joists are where most people first meet a span table, and they are a perfect example of how the variables interact. This guide explains how MGP10 floor joist spans are determined so you can read the table confidently — it is an orientation to the method, not a number to copy.

Floor joists run across bearers at a regular spacing; the joist span is the distance between those supports.

What spans what

In a typical sub-floor, joists span across bearers, and the joist span is the distance it bridges between those supports. The bearers then span between posts or stumps. Getting the language right matters: when a table asks for "span," it means the clear distance the joist itself bridges, not the size of the room.

Spacing changes everything

Joists are usually set out at 450 or 600 mm centres. Spacing matters because it sets how much floor each joist carries:

Closer spacing (450 mm) — each joist carries less, which generally allows a longer span or a smaller section.
Wider spacing (600 mm) — each joist carries more, generally reducing the span it can manage.

The table has separate columns for each spacing, so always read the one that matches your set-out.

Single vs continuous span

A joist supported only at its two ends is a single span. A joist running continuously over three or more supports is a continuous span and can often reach further. These are different columns — read the right one for your real framing.

Going deeper (e.g. 140 to 190 mm) buys span. The table lets you trade size against the span you need.

Strength is not the whole story

A joist that is strong enough not to break can still deflect or vibrate enough to feel bouncy underfoot. Span tables build in deflection limits precisely so floors feel solid, not just stay up. This is why you cannot simply pick the size that "looks beefy enough" — the table has already balanced strength and stiffness for you.

Decks are a different animal

Putting it together

To size an MGP10 floor joist: confirm the grade, pick your spacing, decide single vs continuous, establish the load case (internal floor vs deck), then read the smallest size whose listed span meets your required span — from the current official table. The Span Spec Builder assembles all of this into a clean spec to take to the table or your engineer.

Handy on site

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Keep going

Joists sit on bearers, so read MGP10 bearer spans next, and make sure you understand load width before you finalise anything.

Frequently asked questions

What spacing are MGP10 floor joists usually set at?

Floor joists are commonly set out at 450 or 600 mm centres in residential construction. Closer spacing lets each joist carry less, which generally allows a longer span or a smaller size — but the exact allowable span must come from the MGP10 table for your loads.

Do floor joists span across bearers?

Yes. Joists typically run across the tops of bearers; the joist span is the distance between the bearers (or other supports) it bridges. The bearers in turn span between posts or stumps — see our bearer guide.

Are deck joists the same as floor joists?

No. Decks are outdoor, often carry a different imposed load, and the timber must be treated (e.g. H3). They use their own span tables, so do not apply an internal floor joist span to a deck.

Why does my floor feel bouncy even though the joists are 'strong enough'?

Because strength and stiffness are different checks. A joist can be strong enough not to break yet still deflect or vibrate enough to feel bouncy. Span tables account for deflection limits — see our guide to deflection.