Floor imposed actions for houses in Australia and New Zealand (AS/NZS 1170.1)

AS/NZS 1170.1:2002 (with Amendments 1 and 2), Structural design actions Part 1: Permanent, imposed and other actions, is one joint standard for Australia and New Zealand. Section 3 sets the imposed actions due to use and occupancy. Table 3.1 gives reference values for floors, each as a uniformly distributed action in kPa (1 kPa = 1 kN/m²) and a concentrated action in kN, and Table 3.2 covers roofs. AS/NZS 1170.0 sets out how the actions are combined.

In Australia the National Construction Code calls up AS/NZS 1170.1 for permanent and imposed actions (NCC Housing Provisions 2.2.3). In New Zealand, Verification Method B1/VM1 cites it, with changes set out in B1/VM1 Section 2.3.

For most houses the usual route is not a full engineering design. In Australia, timber-framed houses are normally framed from the AS 1684 span tables under the NCC Housing Provisions, and in New Zealand light timber-framed houses follow NZS 3604. Those tables already allow for the loads. The values on this page apply when a member is designed by calculation, for example a beam, lintel or wall line outside the span tables.

The Table 3.1 values are reference values: where the intended use of the structure gives a higher action, such as heavy equipment, use that instead. A concentrated action goes where it has the worst effect, or at its known position.

The Table 3.1 values on this page include no allowance for partitions, and neither does the app's table. Partitions are part of the weight of the building, so include them in the permanent action G: work out their weight from the drawings and the unit weight of the construction (NCC Housing Provisions 2.2.3).

Where partitions could be added or moved later, read the provision for partitions in your copy of AS/NZS 1170.1 and add the allowance it gives to G. Do not count partitions as part of the 1.5 kPa imposed action.

Roofs not used for floor-type activities take the Table 3.2 imposed action, not Table 3.1. In Australia, structural elements of such roofs (R2(i)) take 0.12 + 1.8/A kPa, where A is the plan area supported in m², but at least 0.25 kPa, so 0.25 kPa once A reaches about 14 m². The concentrated action is between 0.5 and 1.4 kN depending on access (1.4 kN is typical), applied on its own, not with the distributed action. In New Zealand, B1/VM1 2.3.1.1 replaces this with a flat 0.25 kPa and 1.1 kN. The R2(i) roof action is not combined with wind (AS/NZS 1170.0). Roofs used as terraces or for other floor-type activities take at least 4.0 kPa (Table 3.1, C3).

A two-storey timber-framed house. Upper-floor joists span 4.0 m from each external wall onto an interior bearing wall on the ground floor, so the wall's tributary width is 4.0 / 2 + 4.0 / 2 = 4.0 m. The roof trusses span between the external walls, so the interior wall carries no roof load. Ground-floor storey height 2.7 m.

Assumed values (check them for the actual build-up): upper floor permanent action 0.6 kPa (joists, flooring, finishes and the ceiling below); interior stud wall 0.4 kPa of wall area; partitions shown on the drawings and carried elsewhere. Imposed action 1.5 kPa (Table 3.1, A1 general areas).

Strength combinations, AS/NZS 1170.0 clause 4.2.2. Permanent action only: 1.35G = 1.35 × 3.48 = 4.7 kN/m. Permanent and imposed action: 1.2G + 1.5Q = 1.2 × 3.48 + 1.5 × 6.00 = 4.2 + 9.0 = 13.2 kN/m. The second combination governs. Use 13.2 kN/m to check the studs, the plates, and any lintel or beam under the wall.

Unfactored total: G + Q = 3.48 + 6.00 = 9.5 kN/m. Serviceability checks such as deflection use G with the short-term or long-term factor on Q (ψs, ψl) from AS/NZS 1170.0 Table 4.1 for the use of the floor; take them from the standard.

1.35G governs only where G is more than ten times Q (0.15G > 1.5Q), which happens on members that carry mostly roof or self-weight. Where the wall continues to a footing, add the ground floor's tributary load and the footing itself.

• Using the A2 value of 2.0 kPa for a house. A2 is for hotels, wards and other residential uses; general areas in a self-contained dwelling take 1.5 kPa.
• Using 1.5 kPa for every balcony. A house balcony 1 m or more above the ground takes 2.0 kPa and 1.8 kN, and a balcony shared by several dwellings takes at least 4.0 kPa.
• Forgetting the concentrated actions. Flooring, stair treads and short members are often governed by the 1.8 kN, 2.7 kN or the 2.2 kN/m tread load, not the kPa value.
• Designing a garage slab or suspended garage floor for 1.5 kPa. Garages are type F: 2.5 kPa and 13 kN, or 9 kN for a domestic garage with a timber floor.
• Using the Australian roof formula in New Zealand. Under B1/VM1 the roof structural elements take a flat 0.25 kPa and 1.1 kN.
• Adding partitions to the imposed action, or leaving them out altogether. Partitions belong in the permanent action G.
• Checking deflection with factored loads, or strength with unfactored loads. Keep the 1.2G + 1.5Q totals apart from the service totals.