Clear span

Clear span is the face-to-face distance between supports for a structural member, NOT the overall length of the member. The AS 1684 span tables that builders use to size joists, bearers, lintels, rafters, and beams are keyed to clear span. Measuring to the wrong reference point is one of the most common causes of an undersized member caught at frame inspection.

Definition (worked example):

Measurement	What it includes
Overall length	The full timber length from end to end (e.g. 3,600 mm member)
Centreline span	Centre-of-support to centre-of-support
Clear span	Inner face of support to inner face of support (the gap the member crosses)

For a 3,600 mm lintel sitting on two 90 mm wide jack studs:

• Overall length: 3,600 mm.
• Centreline span: 3,600 - 90 = 3,510 mm (45 mm half-stud each end).
• Clear span: 3,600 - 180 = 3,420 mm (90 mm full-stud each end).

Reading the AS 1684 lintel table at “3,600 mm” gives an oversized lintel; reading at “3,420 mm” gives the right one. Reading at “3,420 mm” but mistakenly setting it out as “3,600 mm centreline” gives a lintel where the inner-face span is now 3,420 mm.

Where clear span applies:

• Lintels (over door and window openings): clear span is opening width (jack-stud face to jack-stud face).
• Beams and bearers: clear span is post-face to post-face, or wall-to-wall face distance.
• Joists: clear span is bearer-face to bearer-face (or wall-plate face to wall-plate face).
• Rafters: clear span is bearer face to ridge or underpurlin face.

Common builder errors:

• Reading span tables at “opening width” but specifying the timber at “overall length”: undersized member; caught at frame inspection.
• Forgetting bearing length: even if clear span is correct, the bearing surface at each end must be ≥ 35 mm typically (AS 1684 Section 6.5). A short bearing acts like the member has cantilevered support and can fail.
• Measuring centreline instead of clear span: less conservative; can result in undersized member.
• Adding 90 mm to both ends without considering continuous bearing: an inner stud joint adds bearing but not span.

Bearing length rule of thumb:

AS 1684 typically requires ≥ 35 mm bearing for most residential members, with deeper bearing for highly loaded members (lintels supporting roof + ceiling + roof load on a long span may need 50-70 mm). Always check the specific load condition.

For builders:

1. Mark the inner face of supports on the frame drawing before pulling clear span. Centreline reading is a common error.
2. Round clear span UP to the next 100 mm increment when reading AS 1684 tables. Tables typically run in 100 mm increments; rounding up gives one size of safety.
3. Have the AS 1684 supplement and span tables on site for the chippy and certifier. Spot-checking a marginal lintel before the wall is sheeted saves rework.

Also known as: face-to-face span, net span, clear distance.

Category: Structural / framing / measurement.

Related

• AS 1684 (timber framing)
• AS 1684
• Lintel

See also

• Framing inspection

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Last updated: 2026-05-15. Verified: 2026-05-15.