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Internal pressure in wind design

Internal pressure is the upward push wind exerts on a roof from inside. A dominant opening raises it, adding to external suction to increase total uplift.

By Chalkline. Last updated 11 June 2026. Verified 11 June 2026.

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Internal pressure is the wind-driven pressure that builds up inside a building when air can enter through an opening. When wind hits the windward face of a building and finds an opening, it forces air inside and raises the pressure in the enclosed space. That pressurised air pushes outward in all directions, including upward against the underside of the roof. At the same time, wind flowing over the roof is creating suction (negative pressure) on the outside of the roof surface. The two effects act in the same direction: one pulls the roof up from outside, the other pushes it up from inside. Together, total uplift is substantially higher than external suction alone.

Also known as: internal wind pressure, Cp,i (the internal pressure coefficient used in AS/NZS 1170.2 calculations).

Category: Structural wind loads, AS/NZS 1170.2, AS 4055.

The dominant opening concept

The key variable is whether the building has a dominant opening: one opening whose area is large relative to the combined leakage area of all other wall and roof surfaces. When one opening dominates, most of the wind entering the building comes through it, and the internal pressure approaches the external pressure at that face (verified 2026-06-11: Frontiers in Built Environment, “Minimising damage to houses by designing for high internal pressures”, 2022, https://www.frontiersin.org/journals/built-environment/articles/10.3389/fbuil.2022.970673/full).

A fully sealed, tight building with no dominant opening carries a low internal pressure coefficient. As soon as a single large windward opening exists, the internal pressure coefficient rises sharply. AS/NZS 1170.2:2021 treats buildings in cyclonic wind Regions C and D differently from non-cyclonic regions precisely because of this: below 25 m, those buildings must be designed assuming a dominant opening will occur, because debris and high wind loads make it practically certain that a window or door will fail during a severe event (verified 2026-06-11: AS/NZS 1170.2:2021 Clause 5.3.2.3, consistent with Frontiers in Built Environment 2022).

Under AS 4055:2021, any site in a C-class wind classification (C1 to C4) inherits this dominant-opening design assumption for the structural design of the roof and wall connections. The uplift forces tabulated in AS 1684 for C-class sites already include the internal pressure contribution (verified 2026-06-11: ABCB NCC 2022 Volume Two Part H1).

Why cyclonic regions are designed to assume a breach

In a cyclone, debris travels at extreme speeds. A single piece of debris, a snapped branch, a tile from a neighbour’s roof, can breach a window or door in seconds. Once that breach occurs, the building effectively has a dominant opening on the windward face. Internal pressure jumps, and any roof connections that were specified only for the sealed-building case are now undersized.

The code response is to design for the breached condition from the start, rather than betting on the envelope staying intact. Cyclone shutters, impact-rated glazing, and rated garage doors are the construction-side answer: they reduce the probability that a dominant opening forms under debris impact, keeping the building in the sealed-design case rather than tipping it into the dominant-opening case.

Garage doors: the classic failure

The garage door is usually the largest single opening in a residential building. A standard residential roller door that has not been wind-rated can fail inward or outward under cyclonic gusts. When it fails inward, the garage becomes a pressure vessel: internal pressure in the garage spikes, and the force transfers directly to the roof above it. Roof-to-wall connections that were correctly sized for the design wind class but not for a dominant-opening internal pressure scenario can then pull out, and the roof section over the garage separates from the building (verified 2026-06-11: JCU Cyclone Testing Station Information Bulletin 4, Wind Resistance of External Overhead Roller and Sectional Doors, https://www.jcu.edu.au/cyclone-testing-station/documents/Information-Bulletin-4.pdf; Frontiers in Built Environment 2022).

The fix is structural as well as product: specifying a wind-rated door to AS/NZS 4505:2012 that matches the site’s wind class is the minimum, but the structural frame around the door opening, the lintel, jambs, and fixings, also needs to be sized to transfer the door’s wind load rating into the structure (verified 2026-06-11: NCC 2022 Housing Provisions, AS/NZS 4505:2012; https://directory.bnd.com.au/home-improvement-building/garage-doors-australia/australian-garage-door-safety-standards-compliance-ncc-asnzs-4505-and-installation-regulations/).

What this means on site

  • In cyclonic wind classes (C1 to C4 under AS 4055), the dominant-opening internal pressure is a design assumption, not a worst-case scenario. The tie-down schedule already prices it in. Confirm your garage door, windows, and external doors are all rated to the site’s wind class; substituting a lower-rated product undermines that design assumption.
  • In non-cyclonic classes (N1 to N6), the design typically assumes a sealed envelope with low internal pressure. That assumption is sound as long as windows and doors stay intact. It is why storm shutters can make sense even on N3 to N4 sites in severe storm country: they keep the building in the sealed-design case.
  • A dominant opening is not limited to a failed opening. Deliberately leaving a large door open, a garage door left open as a cyclone approaches, puts the building immediately into the dominant-opening pressure state and can exceed the design limits of the roof connections.

References

  1. Frontiers in Built Environment, “Minimising damage to houses by designing for high internal pressures”, 23 November 2022. https://www.frontiersin.org/journals/built-environment/articles/10.3389/fbuil.2022.970673/full (verified 2026-06-11).
  2. JCU Cyclone Testing Station, Information Bulletin 4: Wind Resistance of External Overhead Roller and Sectional Doors. https://www.jcu.edu.au/cyclone-testing-station/documents/Information-Bulletin-4.pdf (verified 2026-06-11).
  3. Australian Building Codes Board, NCC 2022 Volume Two Part H1 Structure. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h1-structure (verified 2026-06-11).
  4. Standards Australia, AS/NZS 1170.2:2021 Amd 1:2023 Structural design actions, Part 2: Wind actions, Clause 5.3.2. https://store.standards.org.au/product/as-nzs-1170-2-2021-amd-1-2023 (verified 2026-06-11).
  5. Standards Australia, AS/NZS 4505:2012 Garage doors and other large access doors. https://store.standards.org.au/product/as-nzs-4505-2012 (verified 2026-06-11).

See also

Last updated: 2026-06-11. Verified: 2026-06-11. Quarterly review for currency.

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