Analysis & loading · Wind, snow & cyclone
Cyclonic wind: roof uplift and tie-down chain calculator
Roof uplift on a house or shed in any wind region, including the cyclonic Regions C and D. Regional speed VR = F(a − bR^−0.1), site speed with terrain and shielding multipliers, design pressures with external Cpe, internal Cpi (+0.7 for a dominant opening unless the envelope is debris-rated), the local pressure factor at edges, and the net uplift after 0.9G carried through the batten-to-rafter fixing and the rafter/truss tie-down. Flags the cyclic (fatigue) testing required for cladding in Regions C and D.
Standard: AS/NZS 1170.2:2021 Sec 2–5 (VR, Mz,cat, Cpe, Cpi, Kl) · AS 4055 / AS 1684.3 tie-down · AS 4040.3 low-high-low cyclic test
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Wind region | C (cyclonic) | |
| Annual return period R (IL2 = 500) | 500 | yr |
| Terrain category | TC2 | |
| Average roof height | 6 | m |
| Shielding Ms | 1 | |
| Topographic Mt | 1 | |
| Direction Md | 1 | |
| Roof external Cpe (uplift −) | -0.9 | |
| Internal Cpi | +0.7 dominant opening (not debris-rated) | |
| Local pressure factor at edges / corners | 2 | |
| Combination factor Kc | 0.9 | |
| Roof dead load | 0.4 | kPa |
| Batten spacing | 0.9 | m |
| Truss / rafter spacing | 0.9 | m |
| Truss span | 9 | m |
| Batten-to-truss fixing capacity φNj | 5 | kN |
| Truss tie-down capacity φN | 15 | kN |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Regional speed VR | 69.4 | m/s |
| Mz,cat (TC2, z = 6 m) | 0.928 | |
| Site speed Vsit | 64.4 | m/s |
| Design pressure qz = 0.6V² | 2.492 | kPa |
| Net pressure, general roof | -3.59 | kPa |
| Net pressure, edges (Kl) | -5.61 | kPa |
| Net uplift after 0.9G, general / edges | -3.23 / -5.25 | kPa |
| Batten fixing force at edges | 4.25 | kN |
| Truss tie-down force (general) | 13.07 | kN |
| Equivalent wind classification | N5 / C3 |
- ✓ Batten fixing ≥ demand (edge zone) — 4.25 vs 5 kN
- ✓ Truss tie-down ≥ demand — 13.07 vs 15 kN
- ✗ Cyclonic region: cladding and fixings proven by low-high-low cyclic testing (AS 4040.3 / TR440) — required
Method, formulas and assumptions
- VR = F(a − bR^−0.1): A 67/41, B 106/92, C 1.05(122 − 104R^−0.1), D 1.1(156 − 142R^−0.1) (AS/NZS 1170.2 Table 3.1).
- Vsit = VR·Md·Mz,cat·Ms·Mt; qz = 0.6Vsit² Pa; p = qz(Cpe·Ka·Kc·Kl − Cpi·Kc) with Ka = 1 and Cdyn = 1.
- In Regions C and D, assume Cpi = +0.7 (dominant opening) unless doors, windows and the envelope are rated for debris impact (Cl 5.3.2).
- Tie-down chain: batten → truss → top plate → stud → bottom plate → slab, each connection checked for the same uplift; fixings in C and D are subject to fatigue under cyclic loading.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.