Effective: 17 July 2026
The terrain category describes the surface roughness of the surrounding area within approximately 500 m of your building site, and is one of four factors used to determine your wind classification (defined in AS 4055). A lower number means less obstruction and higher effective wind speeds. Click a category below for its full definition and examples.
TC1 — Very Exposed / Open (treeless plains, open water)
Very exposed open terrain with very few or no obstructions, and all water surfaces.
Examples: flat, treeless, poorly grassed plains; open ocean; rivers, canals, bays, and lakes.
TC1 sites experience the highest effective wind speeds for a given region. Buildings on exposed coastal headlands, flat farming land with no tree cover, or adjacent to large open water bodies typically fall into this category.
TC2 — Open / Grassland (farmland, scattered obstructions)
Open terrain including grassland with well-scattered obstructions having heights generally from 1.5 m to 5 m, with no more than two obstructions per hectare.
Examples: farmland and cleared subdivisions with isolated trees or clumps of trees at least equivalent to house size; uncut grass.
TC2.5 — Transitional (outer urban fringe, acreage)
Terrain with a few trees or isolated obstructions. TC2.5 is an intermediate category between TC2 and TC3, representing developing outer urban areas with scattered houses or clumps of trees equivalent to house size, or large acreage developments with fewer than 10 house-size obstructions per hectare.
Examples: outer suburban fringe with scattered housing; acreage blocks with a few large trees.
TC3 — Suburban / Semi-Built (suburban housing, light industrial)
Terrain with numerous closely spaced obstructions having heights generally from 3 m to 10 m. The minimum density shall be at least the equivalent of 10 house-size obstructions per hectare.
Examples: suburban housing estates; light industrial estates; clumps of trees greater than house size.
TC4 — Dense Urban (city centres, multi-storey surroundings)
Terrain with numerous large, closely spaced obstructions — typically well-developed urban areas with multi-storey buildings providing significant wind shielding at ground level.
Examples: dense city centres and inner-urban areas with buildings of three or more storeys.
Which category goes on the order form?
Terrain category is required on the AMIA Custom Products order form when your wind classification has not been provided by an engineer. If the exact category is uncertain, contact your local council, your builder, or a structural engineer. Selecting a terrain category lower than actual conditions (i.e. less sheltered) is always the conservative choice — it results in a higher wind rating and a stronger product specification.
Full details on the Terrain Category methodology are available on request from AMIA’s technical team — sales@amia.com.au or 02 6652 5255.
