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$vuetify.icons.faPhone1300 650 620

Corrosion protection for sheet metal roofing

When designing and specifying metal roofing it is important to take into account a building’s location and whether that building’s roof will be exposed to either airborne salt in coastal areas or the effects of airborne pollution in heavy industrial areas. Unless a material that is ‘fit for purpose’ is used, the result may be corrosion of the metal roofing that may lead to costly repairs and disputes with the client.

The National Construction Code (NCC) provides information on appropriate corrosion protection of metal roofing. You should abide by the provisions outlined in the NCC to ensure compliance and proper protection of the roof.

The table below has been taken from NCC Volume Two and provides information on the correct level of protection required for steel roofing based on the type of steel used, the environment it is used in and exposure to salt water.

Acceptable Corrosion Protection for Sheet Roofing 

ENVIRONMENT LOCATION  MINIMUM METAL COATING IN ACCORDANCE WITH AS 1397
 Metallic coated steel  Metallic and organic coated steel

Low

(Mild steel corrosion rate

1.3 to 25 µm/y)

Typically remote inland areas.  

Z450 galvanized or

AZ150 aluminium/zinc or

AM 125

aluminium /zinc/magnesium

 

Z275 galvanized or

AZ150 aluminium/zinc or

AM 100

aluminium/zinc/magnesium

 Typically more than 1km from sheltered bays.
 

Medium

 

(Mild steel corrosion rate 25 to 50 µm/y)

 Typically more than 1km from breaking surf or aggressive industrial areas.  

Z450 galvanized or

AZ150 aluminium/zinc or

AM 125

aluminium /zinc/magnesium

 

Z275 galvanized or

AZ150 aluminium/zinc or

AM 100

aluminium/zinc/magnesium

 Typically more than 50m from sheltered bays.
 High (Mild steel corrosion rate 50 to 80 µm/y)  Typically more than 200m from breaking surf or aggressive industrial areas.  

AZ150 aluminium/zinc or

AM 125

aluminium /zinc/magnesium

 

AZ150 aluminium/zinc Or

AM 100

aluminium/zinc/magnesium

 Typically within 50m from sheltered bays.  

AZ200 aluminium/zinc or

AM 125

aluminium /zinc/magnesium

 

AZ200 aluminium/zinc or

AM 100

aluminium/zinc/magnesium

 

Very High

(Mild steel corrosion rate 80 to 200 µm/y)

 Typically extends from 100m inland from breaking surf to 200m inland from breaking surf, or within 200m of aggressive industrial areas.  Not suitable  

AZ200 aluminium/zinc or

AM 150

aluminium/zinc/magnesium

 

Typically within 100 m of

breaking surf

 Not suitable  Not suitable
 

 

The NCC provides clear definitions of environments as Low, Medium, High and Very High risk, and defines breaking surf as “any area of salt water in which waves break on an average of at least four days per week, but does not include white caps or choppy water.

The table shows that, for metallic and organic coated steel, the level of galvanized or zinc/aluminium coating varies and one size does not fit all locations.

The Australian Standard for hot dip zinc coated and aluminium/zinc coated steel, AS 1397, provides guidelines on the nature of various coating classes and typical applications for both galvanized and zinc/aluminium roofing or steel.

The Standard notes that a Z450 galvanized and an AZ150 zinc/aluminium coating is the recommended coating for typical exterior applications (e.g. roofing, cladding and accessories) and that Z275 is acceptable for general purpose coatings, whereas AZ200 coating is a heavy duty coating designed for culverts and box gutters.

According to the NCC, the AZ200 (zinc/aluminium) coating is the only one suitable for use in Very High risk coastal or industrial areas. It would be prudent to check with your supplier that the correct material is being provided for use in these areas. 

To find out more, contact HIA Building and Planning Services.

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