Topographic multiplier
The topographic multiplier is the AS 4055 factor for wind speeding up over hills, ridges and escarpments; it can push a site's wind classification and bracing demand up.
Ask Chalkline about this →The topographic multiplier is one of the four inputs AS 4055 uses to set a site’s wind classification, alongside the wind region, the terrain category, and shielding. It accounts for the way wind speeds up as it is forced over the top of a hill, ridge, or escarpment: a house on an exposed crest sees a higher wind speed than the same house on flat ground in the same region, so the multiplier increases the design wind speed for that site.
Because it feeds the wind classification, a high topographic multiplier can push a site up a class or more, and that flows straight into the structural design: more wall bracing, heavier tie-down and roof fixings, and higher-rated windows and doors. A site at the foot of a slope is barely affected; a site right on the brow of a steep ridge can be affected a lot. It is the reason two neighbouring blocks on a hillside can carry different wind classifications.
Do not eyeball the topography. The wind classification, and the multiplier within it, is assessed under AS 4055, and on a complex or very exposed site it may need a wind engineer working to AS 1170.2 rather than the simplified AS 4055 housing method. Get the classification confirmed before pricing the frame and roof, because a hilltop block can carry materially more bracing and tie-down than a base quote assumes. See AS 1684 timber framing.
Also known as: Topographic class, hill-shape multiplier, Mt.
Category: Structural / Wind.
Related
See also
References
- AS 4055:2021 Wind loads for housing, Standards Australia (verified 2026-05-10)
- AS 1170.2 Structural design actions, Wind actions, Standards Australia (verified 2026-05-10)
Last updated: 2026-05-30. Verified: 2026-05-10. Quarterly review for currency.