The building surveyor flagged the timber stair off our Geelong deck for slip resistance on the treads and nosings. What classification do external stair treads need, and does a nosing strip get us out of treating the whole tread?

The surveyor is right to raise it, and yes, a nosing strip can get you out of treating the whole tread. An external deck stair is exposed to the weather, so the wet column applies: the tread surface needs P4 or R11, or the tread can instead have a nosing strip rated P4, each tested to AS 4586. [1] The two routes are stated as alternatives, so you pick one. [1]

Why wet, not dry. Table 11.2.4 has separate dry and wet columns. The Standard's explanatory information says a wet surface is one that is normally wet or likely to be made wet, including areas exposed to the weather. [2] A stair off an open deck is the textbook case. The dry figures (P3 or R10) are for internal stairs that only get wet from an accidental spill. [1] [2]

The two compliance routes for treads, 11.2.4(2). Treads must have either: [1]

• (a) a tread surface with a classification not less than Table 11.2.4, tested to AS 4586; or
• (b) a nosing strip with a classification not less than Table 11.2.4, tested to AS 4586.

The figures, Table 11.2.4: [1]

• Tread surface: P3 or R10 dry; P4 or R11 wet
• Nosing or landing edge strip: P3 dry; P4 wet

What the nosing strip route does and does not do. It removes the need for the rest of the tread to meet the tread surface classification. It does not remove testing: the strip itself must be classified to AS 4586 at P4 or better for a wet stair. [1] Note the strip row lists only a P classification, with no R alternative, so an R11 rating on a strip product does not answer the table. [1] Ask the supplier for a P4 wet result on the actual strip.

Do not forget the landings. Where a landing's edge leads to the flight below, it needs either the tread-level classification for not less than 190 mm from the stair nosing, or a compliant nosing strip. [1] If your stair has a landing partway down, or a landing at the top rather than the deck floor itself, check that edge too.

Timber finishes change the result. The ABCB's advisory note (guidance, not a mandatory provision) says timber stairs, particularly polished treads built on site, may need an in-situ test once the clear finish is applied to confirm the AS 4586 classification. [3] Pre-coated timber can be pre-tested by the manufacturer, and nosing strips, adhesive tapes or applied finishes can be assessed by a laboratory test from the supplier or an in-situ wet pendulum test. [3] So a bare timber test result may not hold once the treads are oiled or coated.

Practical next step. Decide the route. If you are treating the treads, get a P4 or R11 wet result for the tread as finished, not the raw board. If you are using nosing strips, get the product's AS 4586 P4 wet result and note it on the stair detail, then check any landing edges against the 190 mm rule. Send that to the surveyor as the compliance evidence.

Victorian edition note. NCC 2025 has been the sole operative edition in Victoria since 1 May 2026, unless the building surveyor certified substantial design progress before that date under s10(2) of the Building Act 1993. The answer holds either way: NCC 2022 has the same two routes and an identical Table 11.2.4. [4]