Building on a sloping block in Adelaide: the NCC rules

How can a house meet a sloping block?

The Housing Provisions contain separate clauses for these, and do not rank them. Clause 3.2.1 covers site cut and fill using un-retained embankments. Clause 4.2.6, headed “Slab edge support on sloping sites”, covers footings and slabs on the low side, and one of its options is edge beams “stepped in accordance with AS 2870”; clause 4.2.7 covers stepped strip footings. Clause 3.3.3 and Part 6.2 deal with the ground beneath suspended floors.

Which arrangement, or which combination of them, suits a particular block is chosen by the designer and the engineer for that site, working from its classification, its levels and its drainage. The NCC provisions quoted here do not compare them.

For a planning-consent application within Schedule 8 clause 2 of the Planning, Development and Infrastructure (General) Regulations 2017, the elevation drawings must show “existing and proposed ground levels” and “proposed internal floor levels (relative to adjacent ground levels)”.

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Cut and fill on a sloping block: the Table 3.2.1 slope ratios

Table 3.2.1 of the Housing Provisions, “Un-retained embankment slope ratios”, gives a maximum slope ratio (H:L) for each soil class, separately for a site cut and for compacted fill. Stable rock (Class A): site cut 8:1, compacted fill 3:3. Sand (Class A): 1:2 for both. Firm clay (Class M-E): site cut 1:1, compacted fill 1:2. Soft clay (Class M-E): site cut 2:3, and for compacted fill “Not suitable”.

In Figures 3.2.1a and 3.2.1b, H is drawn as the height of the embankment and L as its horizontal length. The table refers to clause 4.2.2 for the material description of each soil class.

Where a slope would be steeper than the table allows, or the soil is not described in it, the table’s note requires a retaining wall; that, and clause 3.2.1’s depth, height and compaction limits, are set out in the retaining walls note. The site-classification conditions on Part 3.2 are set out in the site classification note.

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Cut and fill near the boundary or a neighbour’s house

Clause 3.2.1(1) keeps an un-retained cut out of the zone of influence of existing structures and the allotment boundary. Figure 3.2.1a, titled “Site cut commencing at the allotment boundary or affecting an adjoining property”, illustrates it, and its note explains that the angle of line A-A is the maximum slope ratio from Table 3.2.1, “taken from the bottom of the footing and identifies the area suitable for excavation.” Figure 3.2.1b does the same for fill.

Both figures carry the note: “Consideration must be given for drainage of surface water, particularly where fill affects an adjoining property.” Figure 3.2.1b draws a swale drain between the toe of the fill and an adjoining house, and clause 3.2.1(2) requires fill to “have surface water diverted away from any existing structure on the property or adjoining allotment in accordance with 3.3.3.”

Trenches for drains are covered too. Clause 3.3.2 requires “excavation for drains adjacent to existing footings” to be within the area Figure 3.3.2 shows as safe, and the figure’s note states that the slope ratio “is determined using Table 3.2.1.”

The slab on the low side: NCC clause 4.2.6

Clause 4.2.6 of the Housing Provisions sets out how “footings and slabs installed on the low side of sloping sites” are supported. Edge beams are to be “supported by controlled fill in accordance with 4.2.5(b)”; or “supported by deepened edge beams or bulk piers designed in accordance with AS 3600”; or “deepened (as per AS 2870) to extend into the natural soil level” with the required bearing capacity; or “stepped in accordance with AS 2870.” Where a cut of the natural ground is used, it must be in accordance with Part 3.2.

Clause 4.2.5(b) allows edge beams connected to the slab to sit on natural soil with an allowable bearing pressure of not less than 50 kPa, or on controlled fill “extending past the perimeter of the building 1 m with a slope ratio not steeper than 2 horizontal to 1 vertical”, compacted in accordance with 4.2.4(a)(iii). Clause 4.2.4 sets maximum depths for controlled fill (sand up to 800 mm, clay up to 400 mm) and rolled fill (sand up to 600 mm, clay up to 300 mm) under a slab, with layer limits. The explanatory information on Option 1 of Figure 4.2.5 states that fill beyond these depths “will need to be installed in accordance with H1D4(1).”

The same explanatory information gives the reason for the clause: “Where soil is disturbed by building work and the like, the bearing capacity can be dramatically altered. This is typically the case for sloping sites where cut and fill procedures are used.” South Australia’s variations to Part 4.2 replace clauses 4.2.8 and 4.2.10 and do not vary 4.2.3 to 4.2.7.

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Stepped footings on a sloping site

Clause 4.2.3(2) sets the base of a footing excavation: for flat sites “generally level but may slope not more than 1:40 to allow excavations to drain”; “for sloping sites at an angle of not more than 1:10”; and “for stepped footings in accordance with 4.2.7.”

Clause 4.2.7 requires stepped strip footings to “have a base that is horizontal or be sloped at not more than 1:10; or be stepped in accordance with one of the methods shown in Figure 4.2.7.” The figure shows four methods: Methods A, B and C for steps of not more than 200 mm, and Method D for a step of more than 200 mm but not more than 2D, with D marked as the depth of the footing.

These footing provisions sit within limits. Under NCC 2022 Volume Two clause H1D4(2), Section 4 of the Housing Provisions may only be used where, among other conditions, “the footing is on a Class A, S or M site (classified in accordance with AS 2870) with a uniform bearing capacity”. H1D4(1) also lists AS 2870 and AS 3600 as routes. The explanatory information to H1D4 adds: “For the purposes of H1D4(2)(e), split level slabs are considered as one slab.” The footing design for a particular house is set by the engineering for that site.

A suspended floor over the slope: the ground underneath

Clause 3.3.3(c) of the Housing Provisions requires that “the ground beneath suspended floors must be graded so that the area beneath the building is above the adjacent external finished ground level and surface water is prevented from ponding under the building”. Clause 6.2.1(2) repeats that the ground beneath a suspended floor is to be graded in accordance with 3.3.3, and requires the subfloor to be cleared of debris and vegetation and to contain no dead air spaces.

Table 6.2.1b sets the minimum clearance between the ground and the lowest horizontal member of the subfloor: 150 mm, or 400 mm where termite inspection is required. Its note adds: “On sloping sites the 400 mm clearance required by (1) may be reduced to 150 mm within 2 m of external walls in accordance with Figure 6.2.1b.”

Surface water and subsoil water on a sloping block

Clause 3.3.3 requires surface water to be diverted away from a Class 1 building. Around a slab-on-ground, the finished surface must fall “25 mm over the first 1 m from the building” for reasonably impermeable surfaces in low rainfall intensity areas, among other cases, and “50 mm over the first 1 m from the building in any other case”. The slab must sit not less than 100 mm, 50 mm or 150 mm above the external finished surface, depending on the conditions the clause lists.

The explanatory information to 3.3.3 lists factors that can change the appropriate slab height and fall, including the local topography and the effect of excavation on a cut and fill site.

For water below the surface, the explanatory information to clause 3.3.4 states: “Typical locations of subsoil drainage systems are on the uphill side of cut and fill sites, adjacent to deep footings, behind retaining walls and adjacent to basement walls.” It also warns that in clay soil “subsoil drains can alter the long-term moisture content in the soil”, and that in such conditions they “should only be used where there are no other options for dealing with subsoil water.” Clause 3.3.4 applies “where a subsoil drainage system is installed to divert subsurface water away from the area beneath a building”, and then requires a uniform fall of not less than 1:300 to an external silt pit or sump.

Where the collected water finally goes is a stormwater question: clause 3.3.5 requires the position and manner of discharge to be “to the satisfaction of the appropriate authority.”

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Site levels on the plans, and who assesses them

For a planning-consent application where the building work is not within Schedule 8 clauses 1 or 3, Schedule 8 clause 2(1)(a)(iv) of the PDI (General) Regulations 2017 requires the site plan to show “existing ground and floor levels (if relevant), and proposed finished floor levels and proposed site (or “bench”) levels, including in relation to the top of any kerb level, showing the height and location of any earthworks or retaining walls (if relevant)”. For building consent, clause 4(1)(a) asks for “the levels of the site and of the floors of the proposed building in relation to any street drainage channel or council drain” and “the method of drainage and services proposed to be used”.

Planning rules that bear on site levels, earthworks and overlays are checked for the address. The relevant authority for an application is set by Part 6 Division 5 of the Planning, Development and Infrastructure Act 2016, which provides for assessment panels, the State Planning Commission, the Minister, assessment managers, accredited professionals and councils to act as relevant authorities in the cases the Act and Regulations set out, and, for assessment against the Building Rules, a building certifier (section 99).

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