Wind Classification for Houses: AS 4055 N1, N2, N3 and Cyclonic C1 to C4, How Terrain, Shielding and Topography Set the Class, and a Free Calculator (2025 Guide)
A house site's wind classification, N1 to N6 or C1 to C4 in cyclonic regions, is read from AS 4055:2021 Table 2.2 using four site conditions: the wind region, the terrain category within 500 m, the topographic class and the shielding class. This guide explains how each is found, with its clause, where the NCC uses the class, and a calculator that applies Table 2.2 to your site.
Every set of house drawings carries a wind class, and the N2 or N3 on the general notes decides a lot: the bracing, the tie-downs, the window rating, the roof fixings, and whether the simple recipes in the Housing Provisions can be used at all. The class belongs to the site, not the suburb. It is worked out from four site conditions in AS 4055:2021, the Standard the National Construction Code (NCC) 2025 calls up for houses.
This guide takes each condition in the Standard's order, then shows how Table 2.2 turns them into a class and where the NCC uses it. The calculator in section 9 applies the table to your site.
In one minute
A site wind class is N1 to N6 in non-cyclonic Regions A and B1, or C1 to C4 in cyclonic Regions B2, C and D (AS 4055:2021 Clause 2.1). N2 is a design gust wind speed of 40 m/s at the ultimate limit state (Table 2.1(A)).
Four site conditions decide it (Clause 2.2): wind region, terrain category, topographic class and shielding class, read in Table 2.2. Regions C and D add the distance from the coast.
AS 4055 covers Class 1 and 10a buildings within its geometric limits (Clauses 1.1, 1.2): eaves 6.0 m and roof 8.5 m above averaged ground, width 16.0 m, length five times the width, roof pitch 35°. Otherwise, use AS/NZS 1170.2 (Clause 2.101).
The hill limits the shielding. Full shielding counts only on T0 to T2, partial only on T0 to T3 (Clause 2.5). In Regions B2, C and D trees never count as shielding.
NCC 2025 calls up AS 4055:2021 incorporating Amendment 1 (Schedule 2) and limits most of the Housing Provisions' structural recipes to N3 (Volume Two H1D5 to H1D8). Above N3 or C1 the site is a high wind area (Schedule 1).
1. What a wind classification is
AS 4055:2021 Wind loads for housing gives site wind speed classes for Class 1 and 10a buildings: houses, and private garages, carports and sheds (Clause 1.1). Our building classes guide explains the classes. There are ten site wind classes (Clause 2.1). N1 to N6 apply in the non-cyclonic Regions A and B1, and C1 to C4 in the cyclonic Regions B2, C and D.
Each class stands for a design gust wind speed at two limit states (Tables 2.1(A) and 2.1(B)). N2 is 40 m/s for strength, the ultimate limit state, and 26 m/s for serviceability. N3 is 50 m/s and 32 m/s. Each class up is roughly a 50% increase in wind pressure (Clause A.3.1).
C1 to C4 carry the same speeds as N3 to N6, but they are not interchangeable. The note to Table 2.1(B) warns that Section 3 may give different pressures for the same speed, and the commentary explains why: the cyclonic pressures include the effect of large openings (Clause A.4.1(d)).
The procedure has three steps (Clause 2.101). Find the site class from Table 2.2. Turn it into a roof class and a wall class: an N2 site gives N2r roofing and N2w walls, windows and doors (Clause 2.6, Tables 2.6.1(A) and 2.6.1(B)). Then evaluate the pressures from Sections 3 to 5, or use the NCC referenced documents written for each class. Bracing uses the site class itself (Clause 2.6.2).
Which edition. NCC 2025 lists AS 4055:2021 incorporating Amendment 1 (June 2024) in Schedule 2, against Volume Two H1D6 and H1D8 and Housing Provisions 2.2.3. That clause names AS/NZS 1170.2 or AS 4055 for the wind actions on a house (2.2.3(1)(c)). This guide and the calculator use the amended text.
2. Where AS 4055 stops
The Standard covers Class 1 and 10a buildings only (Clause 1.1). The house must also fit four geometric limits (Clause 1.2):
Not more than 6.0 m from averaged ground level to the underside of the eaves.
Not more than 8.5 m from averaged ground level to the highest point of the roof, not counting chimneys.
A width of not more than 16.0 m, including roofed verandas and excluding eaves up to 0.9 m, and a length of not more than five times the width.
A roof pitch of not more than 35°.
Averaged ground level is natural ground where earthworks are minor, or a level averaged through the modified land where they are major (definition 1.4.1). The commentary applies the 16.0 m to the tallest section, and notes that three-storey houses may not fit (Clause A.2.2).
Outside those limits, or where Table 2.2 reads N/A, wind loads come from AS/NZS 1170.2 (Clause 2.101). The two Standards are not mixed: a design takes its parameters from one or the other (Clause 1.2).
The NCC leans on the same limits. Window installation under Housing Provisions Part 8.2 and the Part 6.3 steel member tables are tied to buildings within Clause 1.2 of AS 4055 (Volume Two H1D8(1), H1D6(6)). The ABCB protocol for structural software applies within the same five dimensions (Housing Provisions 2.2.5(2)).
3. The wind region
The region comes from Figure 2.2 of AS 4055, which shows Regions A, B1, B2, C and D as AS/NZS 1170.2 gives them (Clause 2.2(a)). AS 4055 does not use the sub-regions of Region A (Clause A.3.2). The NCC prints the map as Figure 2.2.3 of the Housing Provisions, free on the NCC website. In words:
Region A, non-cyclonic, covers most of the continent and Tasmania, including Perth, Adelaide, Melbourne, Hobart, Canberra and Sydney.
Region B1, non-cyclonic, is south-east Queensland and north-east New South Wales between about 25° and 30° south, up to 200 km inland, including Brisbane.
Region C, cyclonic, is the coastal strip across the north, from south of Carnarvon in Western Australia around to the Queensland coast near Bundaberg.
Region B2, cyclonic, is the band behind Region C, the Western Australian coast from about 27° to 30° south, including Geraldton, and several offshore islands.
Region D, cyclonic, is the north-west coast of Western Australia from about the Tropic of Capricorn, south of Exmouth, to beyond Port Hedland.
The map's notes set the coastal boundaries in from a smoothed coastline by 50, 100, 150 and 200 km, and give islands within 50 km of the coast the region of the adjacent coast (Housing Provisions Figure 2.2.3).
The region also sets the design risk. For a house the NCC uses an annual probability of exceedance of 1 in 500, or 1 in 1000 in Region D (Housing Provisions Table 2.2.3b, Importance Level 2), and AS 4055 derives its classes on that basis (Clause A.3.2(a)).
In Regions C and D the distance from the coast matters too. Table 2.2 gives their classes in bands from 0 to 50 km, measured from the smoothed coastline, or in Region C inland of Region D in Western Australia, from the C and D boundary (Clause 2.2). Read the table as printed: the note to Clause 2.2 says no interpolation should be used in it.
4. The terrain category
The terrain category measures the roughness of the ground the wind crosses. AS 4055 takes the lowest effective roughness from any direction within 500 m of the site, so one smooth sector governs (Clause 2.3). It may be judged on the terrain expected five years after design (Clause 2.3 and Note 1). Substantial, well-established trees may count as obstructions in every region.
| Category | What it looks like | Obstructions | Clause |
|---|---|---|---|
| TC1 | Open ocean, bays, rivers, canals and lakes; flat, treeless, poorly grassed plains | Very few or none | 2.3(a) |
| TC2 | Grassland, farmland, cleared subdivisions with isolated trees, uncut grass | 1.5 m to 5 m high, no more than two per hectare | 2.3(b) |
| TC2.5 | Developing outer urban areas with scattered houses or house-sized clumps of trees; large acreage | Fewer than 10 house-size obstructions per hectare | 2.3(c) |
| TC3 | Suburban housing, light industrial estates, clumps of trees larger than houses | 3 m to 10 m high, at least the equivalent of 10 house-size obstructions per hectare | 2.3(d) |
The four terrain categories of AS 4055:2021 Clause 2.3, paraphrased.
Three rules decide most real sites (Clause 2.3):
Small gaps stay suburban. In urban areas, roads, rivers, small lakes and canals less than 200 m wide count as TC3. So do parks under 250 000 m², provided they are not within 500 m of each other or of open country.
The edge of a development. A site less than 500 m from the edge of a development takes the terrain category that adjoins it.
No TC4. Terrain Category 4 of AS/NZS 1170.2, for city centres, does not apply (Note 5).
Smooth features that touch are added together. A 150 m freeway reserve beside a 100 m creek reserve is wider than 200 m in total, so the area is treated as TC2; separated by two rows of houses, they stay separate (Clause A.3.3). At the coast, a site needs to be more than 500 m from the ocean shore before it can be TC3 (Clause C.2). For wooded land, the commentary reads fewer than 2 large trees per hectare as TC2, 2 to 10 as TC2.5 and more than 10 as TC3 (Clause A.3.3).
5. The topographic class
The topographic class measures the effect of rising ground on the wind at the house, from T0 for flat ground to T5 (Clause 2.4). It comes from the slope of the hill, ridge or escarpment through the site and from where the site sits on it (Table 2.4).
Take one cross-section through the site, roughly at right angles to the contours, where they are closest together (Clause A.3.4). Along it:
The bottom is where the average slope falls below 1 in 20, such as a creek or flat, and the ground does not keep falling at more than 1 in 20 for at least 200 m (Clause 2.4, Figure 2.4(A)).
The top is where the average slope at the crest falls below 1 in 20, such as a saddle, peak or plateau, and the ground does not keep rising for at least 200 m.
The height H is the difference between them. The lower, middle and top thirds are thirds of that height (Figure 2.4(B)).
The slope is the steepest measured slope in the top half of the cross-section through the site (Clause 2.4). The figure and worked examples measure it as the rise of the top half, H/2, over its horizontal length, Lu (Figure 2.4(A), Appendix B).
Table 2.4 then gives the class. Its slope bands break at 1:20, 1:10, 1:7.5, 1:5 and 1:3. A site in the lower third is T0 whatever the slope, and one in the middle third reaches T2 at most. In the top third the class also depends on H: 10 m or less, more than 10 m up to 30 m, or more than 30 m. The small-hill band is for hills about as high as the houses on them (Clause A.3.2(g)).
Escarpments. A rise is an escarpment where the maximum slope is steeper than 1 in 10 on one side and less than 1 in 20 on the other (Clause 2.4). Its top-third zone extends the same distance, d, past the crest as it runs below it (Figure 2.4(B)), so a site set back on the plateau can still be in the top third. Near any crest, check the far side too: a slope whose top is within d of the site should also be considered, and where directions give similar slopes, the one giving the highest class is used (Clause A.3.4).
In Appendix B, a site 15 m below the crest of a 110 m escarpment, whose top half rises 55 m over 260 m (1:4.7), is in the top third and is T4.
6. The shielding class
Shielding is the effect of the buildings close around the house, judged on what is expected five years after design (Clause 2.5 and Note 7). There are three classes:
Full shielding (FS): at least two rows of houses, or similar-sized permanent obstructions, within 100 m and surrounding the house (Clause 2.5(a)). It suits typical suburbs of 10 or more houses per hectare. Road reserves and open areas under 20 m wide beside the site may be ignored (Note 2).
Partial shielding (PS): at least 2.5 houses or sheds per hectare, such as acreage (Clause 2.5(b)). The second row of houses facing open parkland, open water or an airfield may be classed as partial (Note 3).
No shielding (NS): no permanent obstructions, or fewer than 2.5 per hectare, such as the row of houses or a single house facing open parkland, open water or an airfield (Clause 2.5(c)).
The weakest side decides. Full shielding needs two full rows of housing on all sides; one full row on a side makes the site partial, and an open side makes it unshielded (Clause A.3.5). A typical street under 20 m wide does not break full shielding if there are two rows of houses beyond it.
The hill limits the shielding. Full shielding applies only on T0, T1 and T2, partial shielding only up to T3 (Clause 2.5(a), (b)). Table 2.2 follows suit: T3 has partial and no-shielding columns, and T4 and T5 have only no shielding. A house on an exposed crest gets no credit for its neighbours, which is why the topographic class is settled first.
Trees. In Regions A and B1, permanent, closely spaced trees taller than the house give full shielding when they are as deep as three rows of houses, and partial when as deep as two (Clause 2.5(a) and Note 4). In Regions B2, C and D, trees and vegetation shall not be used as shielding (Clause 2.5), because tropical cyclones can strip them (Clause A.3.5).
7. Reading the class from Table 2.2
Table 2.2 has a row for each region and terrain category and a column for each allowed pairing of topographic and shielding class. The cell is the site wind classification, a whole class (Clause 2.2). A few patterns help to check a result:
In Region A, a suburban TC3 site on flat ground is N1 with any shielding. On a T1 rise it is N2 unless fully shielded.
A smoother terrain category or a higher topographic class tends to raise the class, and Region B1 reads one or two classes above Region A for the same site.
Region B2 gives C classes from C1 up. In Regions C and D, a split cell steps the class down as the site moves inland.
Some cells read N/A, at the exposed end of the table: one cell in Region B2, open elevated sites near the coast in Region C, and more in Region D. N/A means the site is beyond the Standard and is designed to AS/NZS 1170.2 (Table 2.2 key, Clause 2.101).
8. What the class changes in the NCC
The NCC's deemed-to-satisfy recipes for houses are written for a range of classes, and most stop at N3:
| Housing Provisions part | Wind class it covers | Where the limit is |
|---|---|---|
| Masonry veneer, cavity and single-leaf masonry, isolated piers and masonry accessories (Parts 5.2 to 5.6) | Not more than N3 | Volume Two H1D5 |
| Structural steel members (Part 6.3) | Not more than N3, within AS 4055 Clause 1.2 | Volume Two H1D6(6) |
| Metal sheet roofing (Part 7.2) | Not more than N3 | Volume Two H1D7(2)(c) |
| Roof tiles (Part 7.3) | Not more than N3, pitch 15° to 35°, tiles to AS 2049 | Volume Two H1D7(3)(b) |
| Window installation (Part 8.2) | Not more than N3, within AS 4055 Clause 1.2 | Volume Two H1D8(1) |
| Glass in fully framed windows (Part 8.3) | Tables for N1, N2 and N3 | Housing Provisions 8.3.3 |
| Masonry veneer wall ties | Light duty to N2, medium duty above | Housing Provisions 5.6.5(1) |
| Roof tile fixing | One pattern for N1 and N2, a denser one for N3 | Housing Provisions 7.3.2 |
NCC 2025 provisions that change with the site wind class, summarised from the clauses cited.
Above N3, these elements follow the Standards the same clauses list, such as AS 3700, AS 1562.1, AS 2050, AS 2047 and AS 1288. The NCC names the threshold too: a high wind area is a region subject to a design wind speed of more than N3 or C1 (Schedule 1). Its definition of design wind speed sets N4 to N6 and C2 to C4 apart as the classes "covered in the ABCB Housing Provisions Part 2.2" (Schedule 1, Table 4).
Cyclonic regions add rules of their own. In Regions B2, C and D, metal roof assemblies, their connections and immediate supporting members must stay in place under the low-high-low pressure sequence of Table H1D7 (Volume Two H1D7(2)(a)). Timber framing in cyclonic areas follows AS 1684.3 (H1D6(4)). Garage doors in openings up to 3 m high in Regions B2, C and D follow AS/NZS 4505 (Housing Provisions 2.2.4(1)); the Class 10a sheds and garages guide covers the rest of what a garage needs.
9. Work out your site's class
Choose the wind region (and for Regions C and D the distance from the smoothed coastline), the terrain category and the shielding. For the topography, pick the class, or work it out from the hill's height H, the horizontal length of its top half Lu and the third the site sits in. The drawing shows the terrain, the hill, the house and its neighbours, and the class. The cards give the design gust wind speed, the roof and wall classes and any rule that limited an input, each with its clause.
10. Worked examples
| Site | Region | Terrain | Topography | Shielding | Class |
|---|---|---|---|---|---|
| Suburban house on a flat street | A | TC3 | T0 | FS | N1 |
| The same suburb, top third of a 40 m ridge | A | TC3 | T3 | FS not counted, PS read | N3 |
| The same, on a 25 m ridge | A | TC3 | T2 | FS | N2 |
| First row of houses facing a bay | A | TC1 | T0 | NS | N3 |
| Second row; third row, within 500 m of the shore | A | TC1 | T0 | PS; FS | N2; N2 |
| Farmhouse on open grassland | A | TC2 | T0 | NS | N2 |
| The Standard's racking example (Appendix D) | B1 | TC2.5 | T2 | PS | N4 |
| Acreage 20 km and 40 km from the coast | C | TC2.5 | T0 | PS | C2; C1 |
| Suburb 12 km from the coast | C | TC3 | T0 | FS | C1 |
Classes read from AS 4055:2021 Table 2.2. Check each in the calculator above.
The suburb on a ridge. The ridge is 40 m high and its top half rises 20 m over 120 m, a slope of 1:6, between 1:7.5 and 1:5. The house is in the top third and H is more than 30 m, so Table 2.4 gives T3. Its neighbours would give full shielding, but that does not count on T3 (Clause 2.5(a)); partial shielding does (Clause 2.5(b)). Region A, TC3, T3, PS reads N3.
Lower the ridge to 25 m with the same 1:6 slope (12.5 m over 75 m). H is now between 10 m and 30 m, the class is T2, full shielding counts again, and the site reads N2. The same suburb reads N1 on the flat, N2 on the 25 m ridge and N3 on the 40 m one.
The bay. Water within 500 m makes every site TC1 (Clause 2.3). The first row has no shielding and reads N3. The second row may be partially shielded (Clause 2.5 Note 3) and reads N2, as do the fully shielded rows behind it, until they are more than 500 m from the shore and become TC3, N1. Appendix C sorts a waterfront suburb the same way (Table C.3).
Region C. For acreage with partial shielding on flat ground, the cell reads C2 from 0 to 35 km and C1 from 35 to 50 km. A site 20 km from the smoothed coastline is C2, and one 40 km in is C1. A fully shielded suburb on flat ground reads C1 across the whole 50 km.
11. Common mistakes
Reading the terrain from the street. The category is the smoothest sector within 500 m in any direction (Clause 2.3). A large park, open water or the edge of the estate inside that circle pulls it down.
Full shielding with one open side. The weakest side decides (Clause A.3.5). A house facing a park or reserve wider than 20 m, or the first row facing water, has no shielding on that side (Clause 2.5(c); Appendix C).
Shielding on a hilltop. Full shielding stops at T2 and partial at T3 (Clause 2.5). Settle the topographic class first.
Trees as shielding in the north. In Regions B2, C and D, trees and vegetation shall not be used as shielding (Clause 2.5).
The wrong slope. Table 2.4 uses the slope of the top half of the cross-section through the site, not a bank beside the house or the whole hill (Clause 2.4). Near a crest, check the far side too (Clause A.3.4).
12. Where to read the source
AS 4055:2021 Wind loads for housing, incorporating Amendment No. 1 (2024), is purchased from Standards Australia. The parts used here are Clauses 1.1, 1.2, 1.4.1, 2.101 and 2.1 to 2.6, Tables 2.1(A), 2.1(B), 2.2 and 2.4, Figures 2.4(A) and 2.4(B), and Appendices A to D. AS/NZS 1170.2:2021 defines the wind regions and covers buildings outside AS 4055.
NCC 2025 Volume Two H1D5 to H1D8 and Schedules 1 and 2, and Housing Provisions 2.2.3 to 2.2.5, 5.6.5, 7.3.2 and 8.3.3, are free on the NCC website. The wind region map is Figure 2.2.3 in Housing Provisions Part 2.2.
Inside AEC Assistant the Code text is searchable by clause, and on a paid plan the Standards are too. Ask what wind class a house in the top third of a 40 m ridge needs and the answer comes back with the clause it was read from. This article is general guidance. The building surveyor or certifier for your project determines compliance.
Questions this guide answers
What is wind classification N1, N2, N3?
They are site wind classifications from AS 4055:2021 for houses in the non-cyclonic wind Regions A and B1, which run from N1 to N6. Each stands for a design gust wind speed: N2 is 40 m/s and N3 is 50 m/s at the ultimate limit state (Table 2.1(A)). In the cyclonic Regions B2, C and D the classes are C1 to C4.
How do I work out the wind classification for my site?
Find four things: the wind region, the terrain category within 500 m of the site, the topographic class from the hill the site is on, and the shielding class from the houses around it (AS 4055:2021 Clause 2.2). Then read the class from Table 2.2. In Regions C and D the distance from the smoothed coastline also matters.
What does an N2 wind rating mean?
N2 is a site wind classification with a design gust wind speed of 40 m/s at the ultimate limit state and 26 m/s at serviceability (AS 4055:2021 Table 2.1(A)). In Region A a suburban TC3 site on flat ground is N1, and on a T1 rise it is N2 unless fully shielded. Roofs on an N2 site are specified as N2r, and walls, windows and doors as N2w.
What is the difference between N and C wind classes?
N classes apply in the non-cyclonic Regions A and B1, and C classes in the cyclonic Regions B2, C and D. C1 to C4 carry the same design gust wind speeds as N3 to N6 (Tables 2.1(A) and 2.1(B)), but the Standard's pressures for cyclonic classes include the effect of large openings, so they are not interchangeable. The NCC also adds cyclonic rules, such as the low-high-low test for metal roofing (Volume Two H1D7(2)).
What is terrain category in wind classification?
It measures the smoothest ground the wind crosses in any direction within 500 m of the site (AS 4055:2021 Clause 2.3). TC1 is open water and bare plains, TC2 open grassland and farms, TC2.5 acreage and outer urban land with fewer than 10 house-size obstructions per hectare, and TC3 suburbs with at least 10. It may be judged on the terrain expected five years after design.
When can't AS 4055 be used?
When the building is not Class 1 or 10a, when it is outside the geometric limits of Clause 1.2 (eaves more than 6.0 m or roof more than 8.5 m above averaged ground, width more than 16.0 m, length more than five times the width, or roof pitch over 35 degrees), or when Table 2.2 reads N/A for the site (Clause 2.101). Wind loads are then found with AS/NZS 1170.2.
