H1 Soil Classification Explained | Melbourne Sites

H1 is one of the most common site classifications in Melbourne. If a soil report has come back H1, the ground under the proposed house or extension is expected to move vertically by 40 to 60mm over the life of the building as moisture levels change through wet and dry seasons. That movement is not a reason to avoid the site. It is the design input the structural engineer uses to size the slab or footings so the building moves as a stiff unit rather than cracking.

This guide explains what H1 means, how it compares with the neighbouring classes, how the classification is worked out, and what it changes for footing design, landscaping and extensions. For an overview of every class from A to P, see the parent guide on site classification in Melbourne.

What does H1 soil classification mean?

AS 2870, the Australian Standard for residential slabs and footings, groups sites by how much the ground surface is expected to rise and fall as clay soils take up and lose water. The measure is the characteristic surface movement, written as ys. A Class H1 site is defined as a highly reactive clay site with ys greater than 40mm and up to 60mm.

Before the 2011 edition of AS 2870, highly reactive sites sat in a single Class H. The 2011 revision split that band into H1 (40 to 60mm) and H2 (60 to 75mm) so designers could match footing stiffness more closely to the expected movement. Older reports that simply say "Class H" should be read with that change in mind, and a report prepared under the earlier edition may need review before it is relied on for a new design.

H1 compared with Class M, H2 and E

Most of Melbourne's clay suburbs land somewhere between Class M and Class H2. The table below shows where H1 sits and what the step up from the classes either side of it typically means for design.

The practical difference between M and H1 is footing stiffness. On an H1 site the edge and internal beams of a slab are generally deeper and set closer together than on a Class M site, and wall cladding choices matter more, because brittle finishes such as full masonry tolerate less differential movement than lightweight cladding.

How a site is classified H1

A site classification comes from a geotechnical soil test. The tester drills boreholes, logs the soil profile, takes samples and measures how reactive the clay is. The steps below show how the result is reached.

The depth of design suction change is the depth over which seasonal wetting and drying affects the soil. For Melbourne, AS 2870 gives a range of roughly 1.8 to 2.3 metres, which is why a thick layer of reactive basalt clay in the western and northern suburbs, or the reactive clays found through parts of the south east, can push a site into H1 or H2. The calculation is sensitive to how thick the reactive layer is, so two lots with similar surface soil can still classify differently.

Footing systems for H1 sites

Once a site is classified H1, the structural engineer selects and designs a footing system that limits differential movement to an amount the building finishes can tolerate. The common options on Melbourne H1 sites are:

• Stiffened raft slab: a concrete slab with a grid of deep edge and internal beams. This is the most common choice for new houses on H1 clay.
• Waffle pod slab: a slab cast over polystyrene pods with a grid of ribs. It is used on H1 sites but the rib depth and spacing must suit the higher movement.
• Strip footings: concrete strips under load bearing walls, founded deeper on H1 sites to reduce seasonal movement, often with a suspended timber floor.
• Bored piers or screw piles: used where the reactive layer is deep, where trees are close by, or to support an extension at a depth below most seasonal moisture change.

AS 2870 contains deemed to comply footing tables for H1 sites, but those tables have limits on building size, shape and construction type. Larger homes, split levels, heavy masonry, long spans or irregular plans usually need an engineered design. Principal Built Engineering prepares footing design for Melbourne properties based on the soil report, the building layout and the wall construction, so the footing is matched to the actual site rather than a generic table.

Trees, drainage and keeping an H1 site stable

An H1 footing is designed for normal seasonal movement. Problems arise when moisture changes are larger or more uneven than the design assumed. Most of the cracking seen in houses on reactive clay comes from a few avoidable causes.

CSIRO guidance on foundation maintenance for reactive clay sites gives the same message: keep moisture around the house as even as possible through the year. Homeowners on H1 sites who keep drainage working and trees at a sensible distance rarely see more than minor cosmetic cracking.

Extensions and existing houses on H1 clay

H1 matters as much for extensions as for new builds. The new footing will be stiffer and often deeper than the original footings, and the two parts of the building will move differently as the seasons change. Good extension design on an H1 site usually includes:

• A current soil test for the extension area rather than reliance on the original subdivision report
• An articulation joint between the old and new structures where the footings change
• New footings founded to a depth that limits differential movement against the existing house
• Drainage that carries water away from both the old and new footings
• A check of the existing footings if the extension adds load, such as a second storey

For existing homes on H1 sites, a structural engineer report in Melbourne can confirm whether cracking is normal seasonal movement or a sign of a footing problem, and whether repairs such as drainage works or underpinning are warranted. Principal Built Engineering also provides residential structural engineering for renovations, additions and new homes across Melbourne's reactive clay suburbs.

Frequently asked questions

Is H1 soil bad to build on?

No. H1 is one of the most common classifications in Melbourne and thousands of houses are built on it every year. It needs a stiffer footing than a Class M site and more attention to drainage and trees, but with an engineered footing and normal maintenance an H1 site performs well.

What is the difference between H1 and H2 soil?

Both are highly reactive clay. H1 has a characteristic surface movement of more than 40mm and up to 60mm, while H2 is more than 60mm and up to 75mm. An H2 site needs a heavier footing system, and building costs are usually higher as a result.

What footing is used on an H1 site?

The most common footing for a new house on an H1 site in Melbourne is a stiffened raft slab with deep edge and internal beams. Waffle slabs, deeper strip footings and bored piers are also used depending on the building, wall construction and site conditions.

How far should trees be from a house on H1 clay?

AS 2870 guidance suggests a single tree on an H1 site should generally be planted at a distance of about one times its mature height from the house, with greater distances for groups of trees. Where trees must be closer, the footing design needs to allow for the extra drying.

Can an H1 site be reclassified as P?

Yes. A site with H1 clay can be classified P if there are abnormal moisture conditions, uncontrolled fill, recently removed trees or other factors outside the normal classification. A P site needs a specific footing design by a structural engineer.

Need footing design for an H1 site?

If your soil report shows an H1 classification and you need footings designed for a new home, extension or second storey, Principal Built Engineering can review the report and prepare an engineered footing design. Explore the full range of Melbourne structural engineering services, check typical structural engineer costs in Melbourne, or request a quote with your soil report and plans.