Footing Sizes & Site Classification (AS 2870)

The requirement

Your block gets a letter from A to P under AS 2870 before you build (NCC 2025, Housing Provisions clause 4.2.2). The letter says how much the ground will move. Footing and slab concrete must reach at least 20 MPa (N20) (clause 4.2.10). The exact grade and cover for your job come off your engineered plan.

Why does the ground move? Clay swells when it gets wet and shrinks when it dries. That movement is what cracks slabs and footings. AS 2870 is the Australian rule book for house footings and slabs.

Part 4.2 has scope limits set by H1D4(2). Check those before using its details. AS 2870 and a Performance Solution are other possible paths. The designer must choose a path that covers the soil, loads and building.

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. That’s the layer with grass roots in it.
Keep the trench clean. No loose earth, tree roots, mud or debris in the bottom when you pour.
Keep the base roughly level. It may fall up to 1:40 so water can drain away.
On loose sand, or ground that washes away easily, the footing bottom must sit at least 300 mm below the finished ground level.

The grade and the cover. Housing Provisions clause 4.2.10 sets a floor. Footing and slab concrete must reach at least 20 MPa (grade N20) at 28 days. Never pour below that. The design and exposure conditions may call for a higher grade. Do not choose it from soil reactivity alone. Part 4.2.11 also gives cover rules for work within its scope. Other design paths can use different details. So the exact numbers 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 out the concrete volume

Use the concrete sizes on the approved plan. Embedment is the depth from ground level to the base of the footing. It is not the thickness of concrete to pour. A strip, wall and trench fill each have their own shape. Work out each part on its own. Where parts meet, count that concrete once.

This example uses a straight concrete strip with the same size all along:

Total concrete length: 24 m.
Concrete width: 0.45 m.
Concrete thickness, measured from its top to its underside: 0.5 m.

These are example sizes, not code limits or a footing design. Check the shape and size on the plans. Then check how deep the base must sit. It must meet the soil and frost rules for the site.

Volume = length × concrete width × concrete thickness.

24 × 0.45 × 0.5 = 5.4 m³.

That is the volume before any extra for waste or uneven ground. Measure the trench or forms. Agree the extra amount and final order with the supplier. The tool does the sum. It does not choose safe footing sizes.

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 design requirements. It does not make site class alone a concrete-grade selector. Use the specified grade and cover on the approved design.
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 as it gets wet or dries out. NCC 2025 requires classification to AS 2870 before you build (Housing Provisions clause 4.2.2). The classes run A, S, M, H, E and P. A soil tester or engineer sets the class, not you.

What do the site classes A to P mean?

A is the calmest: mostly sand or rock, little movement. S is slightly reactive clay. M is moderately reactive. H is highly reactive. E is extremely reactive and moves the most. P is a 'problem' site: soft or loose soil, landslip, mine subsidence, or dodgy fill. It always needs an engineer.

What concrete grade do I use for my footings?

The NCC sets a floor of 20 MPa, grade N20 (Housing Provisions clause 4.2.10). Never pour below that. The design and exposure conditions can require more. Site class alone does not choose a concrete grade. Check the cover rules in the adopted Housing Provisions and the design. Read the real numbers off your engineered plan.

How do I prep the trench before I pour?

Housing Provisions clause 4.2.3 sets the rules. Strip the topsoil. Keep the trench clean: no loose dirt, tree roots, mud or rubbish. The base can be roughly level, with a fall up to 1:40 so water drains. On loose sand or ground that washes away, the footing must sit at least 300 mm below 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.