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 W: www.deltapanels.com     Sydney Depot:  11 The Old Northern Road, Narellan, NSW 2567   +61 2 4611 7712
ABN: 11 147 861 292
Version Date: 
2025-11-25
13
STEP 1         DETERMINING CORRECT WIND SPEED
Step 1.4        Topographic Effect
This step measures the effect of wind on a proposed structure based on its site location on a slope, ridge, mountain or 
escarpment, then the actual height/slope of that particular location. Topographic classifications are divided into zones T0, 
T1, T2, T3, T4 and T5. Use the following table to determine the applicable topographic classification of a proposed structure. 
If the slope, ridge, mountain or escarpment exceeds 30 metres please refer to AS 4055:2021.
TOPOGRAPHIC
CLASSIFICATION
EFFECT
MAXIMUM SLOPE OF 
MOUNTAIN / RIDGE / ESCARPMENT
LOWER 
THIRD
MID 
THIRD
TOP THIRD 
OVER 
TOP
Less than 1:20
(2.9o)
T0
T0
T0
T0
T0
T0
Greater than & equal to 1:20 to Less than 1:10
(≥ 2.9o to < 5.7o)
T0
T0
T1
T1
T1
T0
Greater than & equal to 1:10 to Less than 
1:7.5
(≥ 5.7o to < 7.6o)
T0
T1
T1
T2
T2
T0
Greater than & equal to 1:7.5 to Less than 1:5
(≥ 7.6o to < 11.3o)
T0
T1
T2
T2
T3
T1
Greater than & equal to 1:5 to Less than 1:3
(≥ 11.3o to < 18.4o)
T0
T2
T2
T3
T4
T2
Greater than & equal to 1:3 
(18.4o)
T0
T2
T3
T4
T5
T3
SITE LOCATION ON SLOPE / MOUNTAIN / RIDGE / ESCARPMENT

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