Terrain category
Terrain category (TC1 to TC3) describes ground surface roughness within 500 m of a building site, used in AS 4055 and AS/NZS 1170.2 to determine the site wind class.
Ask Chalkline about this →A terrain category (TC) is a measure of the surface roughness of the land surrounding a building site, assessed within approximately 500 m of the site. Under AS 4055:2021 and AS/NZS 1170.2:2021, the terrain category is one of four factors used to determine the site wind classification. A more sheltered terrain category (TC3, typical residential suburbs) reduces the wind class compared to an exposed terrain category (TC1, open coastal or flat plains).
The four terrain categories used in AS 4055:2021 are:
- TC1: totally unobstructed open flat terrain with minimal surface irregularities, such as open coastal headlands, inland flat plains, and airfields.
- TC2: open areas with scattered low obstructions, such as rural farming land, open parkland, and airport surrounds.
- TC2.5: moderately scattered obstructions, such as agricultural land with cane fields or low-rise scattered structures.
- TC3: typical residential estates with buildings and vegetation providing consistent surface roughness; the most common category for suburban residential work.
Also known as: TC, terrain class.
Category: Compliance & approvals.
Why it matters for builders: the terrain category affects the final wind classification assigned to a site. A site on the edge of a residential estate (TC2 or TC2.5) will often attract a higher wind class than the same site in an established suburb (TC3). The building designer or engineer assesses terrain category; the builder should confirm it is documented in the structural details.
Related
- AS 4055 wind loads for housing, the standard that uses terrain category as one of four site factors in the wind classification process
See also
Last updated: 2026-05-08. Verified: 2026-05-08. Quarterly review for currency.