Footing Sizes & Site Classification (AS 2870)

The requirement

Before you dig, before you pour, someone has to work out how much your ground will move. Clay swells when it gets wet and shrinks when it dries. That movement is what cracks slabs and footings. So the NCC 2025 makes the ground under your house be classified — given a letter — using the standard AS 2870 (Housing Provisions, clause 4.2.2). AS 2870 is the Australian rule book for house footings and slabs.

You have two legal paths. Either you classify the site and then follow the Housing Provisions Part 4.2 build rules, which mirror AS 2870 — or you design the whole footing straight to AS 2870. Either way, the classification comes first. It sets everything downstream.

The site classes. A soil test on your block sets the letter. Here’s what each one means in plain words:

Class	What the ground does
A	Mostly sand or rock. Little or no movement. The calmest.
S	Slightly reactive clay. A bit of movement.
M	Moderately reactive. More movement.
H	Highly reactive clay. A lot of movement.
E	Extremely reactive. Moves the most.
P	“Problem” site — soft or loose soil, landslip, mine subsidence, or reactive/collapsing fill. Always needs an engineer.

You don’t pick the class yourself. A soil tester or engineer bores the block and tells you. Your job is to build to the class they give you.

NCC 2025 Housing Provisions 4.2 · AS 2870

1 footing
2 reinforcement
3 ground

Prepping the trench. Once the footing is designed, clause 4.2.3 tells you how to get the ground ready:

Strip the topsoil — the layer with grass roots in it comes off.
Keep the trench clean — no loose earth, tree roots, mud or debris sitting in the bottom when you pour.
Base roughly level, but it may fall up to 1:40 so water can drain away.
On loose sand or erosion-prone ground, the bottom of the footing must sit at least 300 mm below the finished ground level.

The grade and the cover. The NCC sets a floor: Housing Provisions clause 4.2.10 requires footing and slab concrete to reach at least 20 MPa (grade N20) at 28 days. That’s the minimum you can pour — never go below it. Above that floor, the grade for your site class comes from AS 2870 (reactive sites need more), and the cover over the steel comes from AS 3600 and changes with how exposed the concrete is. So: N20 is the bottom line, and the exact grade and cover for your job come off your engineered plan or from your engineer. If a mate quotes a grade off the top of his head, check it against the plan before you order the truck.

Work it out by hand

The trench prep is code. The volume of concrete you order is just maths — and it feeds straight into the calculator. Say you’ve got a strip footing to pour. These are example numbers, not code minimums — your real sizes come off the plan:

Total length of trench: 24 m
Trench width: 0.45 m
Footing depth: 0.5 m

Volume = length × width × depth:

24 × 0.45 × 0.5 = 5.4 m³

So you’d order about 5.4 cubic metres, then add a bit for spillage and an uneven trench bottom — most blokes tack on 5 to 10 per cent. Swap in your own lengths and the calculator does the sum for you.

Do it in 5 seconds

Part of the NCC rules for Australian chippies — the pillar guide, with the state-by-state adoption tracker.
Digging posts instead of a strip footing? See concrete for post holes.
Working out the pour? Use the concrete volume calculator for slabs & footings.

Sources

NCC 2025, ABCB Housing Provisions Standard, Part 4.2 “Footings, slabs and associated elements” — site classification to AS 2870 (clause 4.2.2) and excavation rules including the 1:40 fall and the 300 mm depth on loose/erosion-prone sites (clause 4.2.3), and the minimum 20 MPa (N20) concrete strength (clause 4.2.10). Confirm at ncc.abcb.gov.au.
AS 2870 — Residential slabs and footings — sets the site classes (A to P) and the concrete strength grade for each class. The grade must be read from AS 2870 or your engineered plan, not assumed.
AS 3600 — Concrete structures — sets the concrete cover over the steel, which depends on the exposure. Check the cover in AS 3600 or on your plan.

Frequently asked questions

What is site classification and who does it?

It's a letter that says how much the ground under your house will move when it gets wet or dries out. The NCC 2025 makes the ground be classified to the standard AS 2870 before you build (Housing Provisions clause 4.2.2). The classes run A, S, M, H, E and P. A soil test on your block sets the class — that's a job for the engineer or soil tester, not something you eyeball on site.

What do the site classes A to P mean?

A is the calmest — mostly sand or rock that barely moves. S is slightly reactive clay. M is moderately reactive. H is highly reactive. E is extremely reactive, so it moves the most. P is a 'problem' site — soft or loose soil, landslip, mine subsidence, or dodgy fill — and it always needs an engineer to design the footing.

What concrete grade do I use for my footings?

The NCC sets a floor of 20 MPa (grade N20) — you can't pour below that (Housing Provisions clause 4.2.10). Above the floor, the exact grade depends on your site class (from AS 2870) and the cover depends on how exposed the concrete is (AS 3600). Read the real numbers off your engineered plan or ask your engineer — but never go under N20.

How do I prep the trench before I pour?

Housing Provisions clause 4.2.3 sets the ground rules. Strip off the topsoil that has grass roots in it. Keep the trench clean — no loose dirt, tree roots, mud or rubbish in the bottom. The base should be roughly level, but it can fall up to 1:40 so water drains out. On loose sand or ground that washes away easily, the bottom of the footing has to sit at least 300 mm below the finished ground level.

Does clause 4.2 tell me to slope the ground away from the house or add termite protection?

No. Falling the ground away from the building, termite protection and damp-proofing are all real rules — but they live in other parts of the NCC, not in the footing clause. Don't read them into 4.2. This clause is about classifying the ground and getting the trench right before the pour.