Soil classification for a new home: what classes A to P mean for slabs and site costs

A soil classification is the class an engineer assigns to a house block under AS 2870-2011, the Australian standard for residential slabs and footings. The classification shows how much the ground is expected to move as moisture levels change and helps determine the footing and slab design for the home. The Australian Building Codes Board (ABCB) Housing Provisions use these classifications for residential footings and slabs.

The main classes are A, S, M, H, E and P¹. Class A covers most sand and rock sites with little or no ground movement from moisture changes. Class S is slightly reactive clay, Class M is moderately reactive clay or silt, Class H is highly reactive clay and Class E is extremely reactive clay. The higher-reactivity classes generally indicate greater expected ground movement. Class P is a problem site rather than a reactivity grade, and a filled site can be classified from A to P under AS 2870.

AS 2870-2011 splits Class H into H1, for high ground movement, and H2, for very high ground movement². It can also add a D suffix where deeper moisture-related ground movement needs to be considered. Where the depth of moisture change is 3 m or more, which the Housing Provisions associate with dry climates, a Class M site becomes M-D. The footing design then follows AS 2870 rather than the standard Housing Provisions tables¹.

The Queensland Building and Construction Commission (QBCC) sets out the expected movement for each reactive soil class. A Class S site is expected to move up to 20 mm, a Class M site 20 to 40 mm, a Class H1 site 40 to 60 mm, a Class H2 site 60 to 75 mm and a Class E site more than 75 mm under normal seasonal conditions³. The QBCC adds that some Class E sites in south-east Queensland have moved 100 to 150 mm or more.

This movement happens as clay absorbs and loses moisture. CSIRO explains that clay swells as it absorbs water, which can push up on the footings. When the soil dries and shrinks, it can reduce the support beneath them².

Under the Australian Building Codes Board (ABCB) Housing Provisions 2022, the standard footing and slab designs apply to Class A, S and M sites. Class H, E and P sites require a design under AS 2870 rather than the standard Housing Provisions tables.

The Housing Provisions also set limits on when their standard designs can be used. The footing must be on a Class A, S or M site with generally even ground support, the slab must be no more than 18 m long or wide, and the supporting structure must have no more than two trafficable floors and no wall higher than 8 m¹². Other limits apply to conditions such as aggressive soils, alpine areas and sites that need specific earthquake design.

The required footing depth generally increases as the soil becomes more reactive. For a clad-frame house, the minimum strip footing depth is 300 mm on a Class A site and 400 mm on Class S and Class M sites. For a full-masonry house, it increases from 300 mm on Class A to 500 mm on Class S and 900 mm on Class M, with heavier reinforcement also required as the class increases¹.

A house on a reactive clay site can continue to move as the soil gains and loses moisture. The Queensland Building and Construction Commission (QBCC) states that footings designed to AS 2870-2011 are intended to allow for normal seasonal ground movement. Conditions such as nearby trees, poor drainage or leaking plumbing can change moisture levels around the footings and may cause additional movement.

The QBCC sets out recommended distances between trees and houses on reactive soil. For a single tree, the recommended distance is at least the tree's mature height on Class M and H sites, 1.5 times its mature height on Class E sites, and twice the mature height for rows or groups of trees³. The QBCC also notes that tree roots searching for water can spread sideways by a distance equal to the tree's height.

The QBCC also recommends avoiding sprinkler systems next to the house on Class H and E sites because they can change soil moisture levels around the footings³.

CSIRO recommends paving around the building that extends at least 900 mm from the walls, with a greater distance on highly reactive soil. The paving should slope away from the building at a minimum fall of 1:60 to help direct water away from the footings².

Some cracking can occur as the ground moves. Under the AS 2870-2011 damage classification, cracks less than 1 mm wide are classed as very slight, cracks under 5 mm as slight, and cracks between 5 mm and 15 mm as moderate².

The soil class for a specific block is in its site classification report. Until that report is completed, the soil class is not known². The report is commissioned as part of the building process rather than published.

In New South Wales the soil assessment may form part of the builder's preliminary agreement. In Victoria, the relevant information forms part of the builder's foundation information. In Queensland, a site classification report is lodged with the plumbing permit application where required.

Once the soil class has been determined, it may appear in several building documents:

• The site classification report itself, which records the soil conditions and classification for the block.
• The engineer's footing design, which the NSW Planning Portal describes as part of the engineering information used to determine the site, soil and wind classifications for the house design.
• In Victoria, the foundation information the builder must provide to the owner after the amount owing for it has been paid⁵.

BuildStreet does not hold a soil class for any lot, because no public dataset reviewed for this article records that information for each block. Its catalogue price cards state that a builder's advertised base price does not include site costs and upgrades, and its lot report lists slope and site conditions among the factors that cannot be determined from the two-dimensional parcel shape alone¹⁶.