Roof Restoration · Six guides · Sheet 1
Coastal corrosion · NCC Housing Provisions, Part 7.2Coastal corrosion and metal roofs
The National Construction Code grades a roof’s surroundings from low to very high corrosion by how far it sits from breaking surf, sheltered bays and aggressive industrial areas, and sets the minimum metal coating a roof sheet needs in each. Within about 100 metres of breaking surf, none of the metallic coatings in its table is listed as suitable.
General information, not building advice. The NCC’s sheet roofing provisions are the place to check; this guide summarises them in plain words.
Distance from the surf, in five bands
The code measures from breaking surf, which it defines as salt water where waves break on an average of at least 4 days a week, not counting white caps or choppy water. It adds that breaking surf does not normally occur in sheltered water such as Port Phillip Bay, Sydney Harbour, the Swan River and the Derwent River.
- Within 100 mVery highNo metallic coating in the table is listed as suitable.
- 100 to 200 mVery highOnly sheet with a metallic and an organic coating is listed.
- More than 200 mHighIn surf areas, reaching from several hundred metres to about 1 km inland.
- More than 1 kmMediumOn surf coasts, out to about 10 to 50 km inland.
- Remote inlandLowDry rural areas away from the coast and from pollution.
A summary of the “typical” locations and notes in Table 7.2.2a of the NCC 2025 Housing Provisions. The code’s own words say each category is typical only, and that the extent of each depends on wind, wave action and topography.
What each category accepts
The table names minimum metal coatings in accordance with Australian Standard AS 1397, in two columns: steel with a metallic coating only, and steel with a metallic and an organic coating. Each coating is named with its metal: Z with galvanised, AZ with aluminium/zinc and AM with aluminium/zinc/magnesium.
| Category | Typical place, surf coasts | Metallic coating only | Metallic and organic coating |
|---|---|---|---|
| Low | Remote inland | Z450, AZ150 or AM125 | Z275, AZ150 or AM100 |
| Medium | More than 1 km from breaking surf | Z450, AZ150 or AM125 | Z275, AZ150 or AM100 |
| High | More than 200 m from breaking surf | AZ150 or AM125 | AZ150 or AM100 |
| Very high | 100 to 200 m inland from breaking surf | Not suitable | AZ200 or AM150 |
| Very high | Within 100 m of breaking surf | Not suitable | Not suitable |
Where a metallic coating is not a suitable form of protection, the code says the roof sheeting must be of a type designed and manufactured for that environment. The table also gives each category’s corrosion rate for mild steel, from 1.3 to 25 micrometres a year for low up to 80 to 200 for very high.
What the table’s notes add
The notes under the table add detail the table itself leaves out. Read on the NCC’s sheet roofing page, they say:
- Much of five coastal cities is medium. The code puts much of metropolitan Wollongong, Sydney, Newcastle, Perth and the Gold Coast in that category, and says it can reach 30 to 70 km inland in South Australia.
- Low has many exceptions. Many areas at least 50 km from the sea are low, including cities and towns such as Canberra, Ballarat, Toowoomba and Alice Springs, and some suburbs of Melbourne, Hobart, Brisbane and Adelaide more than 1 km from the sea. The code adds that each of these has many exceptions in more corrosive categories.
- Sheltered bays have their own scale. Around them, high extends up to 50 m inland from the shoreline, and medium from about 50 m to about 1 km, stretching 3 to 6 km inland in some seasons or in semi-sheltered bays.
- Industry counts as well as salt. Very high includes aggressive industrial areas with a pH of less than 5, and medium includes the land for several kilometres around large industries such as steelworks and smelters.
- Pockets are worse than the average. Every location in the table has variations of greater corrosion severity, and if they are significant, the code says the design must allow for the most severe environment.
Screws, flashings and gutters must get along
Where different metals are used in a roof, including flashings, fasteners, guttering and downpipes, the code says they must be compatible with each other, and its tables give a yes or no for each pairing in two kinds of air: low, and medium to very high. Three rules apply on top of the tables: no lead materials upstream from aluminium/zinc coated materials, no lead on a roof that is part of a drinking water catchment, and no copper upstream from galvanised materials. A selection of the pairings:
Stainless steel screw, aluminium/zinc sheet
A stainless steel (300 series) accessory or fastener on aluminium/zinc or aluminium/zinc/magnesium coated steel is not acceptable in medium to very high air, and acceptable in low.
Lead flashing, aluminium/zinc sheet
A lead accessory against aluminium/zinc or aluminium/zinc/magnesium coated steel is not acceptable in either kind of air. Lead raises other questions too: lead and older roofs.
Zinc-coated fastener, copper
A zinc-coated steel or zinc fastener in contact with copper or copper alloys is not acceptable in either kind of air.
Aluminium/zinc fastener, aluminium/zinc sheet
An aluminium/zinc or aluminium/zinc/magnesium coated accessory on the same kind of sheet, or on zinc-coated steel, is acceptable in both.
The full set of pairings is in Tables 7.2.2b to 7.2.2e of the NCC’s sheet roofing provisions. The code’s explanation is short: it is there to prevent corrosion caused by the chemical reaction between materials that touch.
Leaks, flashings and rust
Queensland’s building regulator, the QBCC, says it often receives complaints about roof leaks, and that while some come from defective work, leaks are often caused by a lack of maintenance and cleaning. Its list for roofs is short: do not let leaves and debris build up in gutters and downpipes, and repair cracked or broken tiles, guttering or flashing.
Flashings matter more than their size suggests. The QBCC calls inadequate roof flashing a major cause of leaking roofs and one of the top ten defects its inspectors find, and says flashings, apart from malleable ones fitted on site, should be purpose-made, machine-folded and of a material compatible with everything else in the roof’s stormwater drainage.
In its advice on adding solar panels to an older home, Your Home, the Australian Government’s guide to sustainable homes, says to replace rusty or degraded roofing materials first, taking the chance to weatherproof the roof properly. Replacing a roof brings its own questions: approvals and licences.
