Load width (framing)

Load width in residential framing is the horizontal distance of roof or floor that a structural member (lintel, beam, rafter, joist) is responsible for carrying to a wall plate, beam, or other support. It is one of the four inputs to every AS 1684 span table:

Stress grade of the timber (e.g. MGP10, MGP12, F11).
Member spacing (e.g. 600 mm centres).
Load width (this).
Wind classification (N1, N2, N3, N4, C1, C2 per AS 4055).

Get load width wrong and the member is sized for the wrong load: undersized, deflects in service, fails frame inspection.

How to calculate load width:

For a lintel carrying roof load: half the roof span from one side, plus half the next clear span on the other side. A lintel mid-house typically carries half the front roof span; an external corner lintel may carry one-quarter or one-half of one rafter run, depending on the corner geometry.

For a floor joist: half the joist span on each side; usually the same as the joist’s tributary area.

For a wall plate: the half-distance between the wall plate and the next support line (an internal load-bearing wall, a beam, etc.).

Common errors that drive undersized framing:

Counting full span instead of load width. A lintel under a 6 m roof span has 3 m load width, not 6 m. Sizing the lintel from “6 m” produces a member at roughly twice the required capacity, but the more common error is the opposite: builders glance at the load width as half the span and undersize when the span is misread.
Forgetting roof slope effects. A pitched roof’s load width on a lintel is the horizontal projection, not the slope length. Long-rafter steep pitches do not carry more load than short-rafter shallow pitches at the same span.
Missing internal load-bearing walls. A lintel that “looks like” it carries the front roof span actually only carries to the next internal load-bearing wall behind it. The internal wall takes the rest.
Mistaking a beam for a wall plate. A beam at one end of the load width transfers the load to a point support; the load is concentrated, not distributed. Span tables don’t apply.

Reading the AS 1684 span table:

A typical residential lintel table column header reads:

Maximum span (mm) for MGP12 stress grade, supporting roof, member at 600 mm centres, wind class N3, load width 2400 mm

Match the load width on the drawings against the column. Switching to the 1800 mm column or the 3000 mm column changes the allowable span by 15-25%.

Also known as: roof load width (RLW); floor load width (FLW); tributary load width; supported load distance.

Category: Structure.

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