Rust on zinc/aluminium or aluminium/zinc alloy-coated steel roofing

Rust on zinc/aluminium or aluminium/zinc alloy-coated galvanised steel profiled metal roofing

Causes

Erosion of or scratching damage to the zinc/aluminium or aluminium/zinc alloy coating, exposing the steel

Lack of rain washing. Rain washing tends to be less effective on roofs with less than 15 degrees slope

Roofing metal is in contact with or has run-off from cedar or redwood, or timber treated with a copper-based preservative treatment 

Roofing has run-off from translucent (PVC) sheeting 

Corrosion of the steel in a corrosive environment such as near a swimming pool, or in coastal, industrial or geothermal regions

Inadequately primed end and/or side laps

Use of lead flashings

The roofing is near the end of its serviceable life 

Listed under: finishes, metal, aluminium, copper, steel, zinc, plastic, polyvinyl chloride (PVC), timber and wood-based, paint, timber, Roof, cladding, flashings, durability, corrosion, rust, lead, roof cladding, profiled metal, translucent sheets

Cause

Erosion of or scratching damage to the zinc/aluminium or aluminium/zinc alloy coating, exposing the steel

Repair

  • lightly sand the corroded areas to remove rust. Do not use a wire brush. Prime with zinc-rich primer and a metal primer, then apply at least two coats of acrylic paint
  • replace badly corroded sheets or sheets with holes; ensure the new roofing is compatible with the existing
  • reroof when it is not economic to repair existing. If the old profiled metal roof has met minimum Building Code durability provisions (it has lasted at least 15 years) a building consent is not required; if not, a consent must be obtained before reroofing.

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Cause

Lack of rain washing. Rain washing tends to be less effective on roofs with less than 15 degrees slope

Repair

  • lightly sand the corroded areas to remove rust. Do not use a wire brush. Prime with zinc-rich primer and a metal primer, then apply at least two coats of acrylic paint
  • replace badly corroded sheets or sheets with holes; ensure the new roofing is compatible with the existing
  • reroof when it is not economic to repair existing. If the old profiled metal roof has met minimum Building Code durability provisions (it has lasted at least 15 years) a building consent is not required; if not, a consent must be obtained before reroofing.

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Cause

Roofing metal is in contact with or has run-off from cedar or redwood, or timber treated with a copper-based preservative treatment 

Repair

  • lightly sand the corroded areas to remove rust. Do not use a wire brush. Prime with zinc-rich primer and a metal primer, then apply at least two coats of acrylic paint
  • replace badly corroded sheets or sheets with holes; ensure the new roofing is compatible with the existing
  • reroof when it is not economic to repair existing. If the old profiled metal roof has met minimum Building Code durability provisions (it has lasted at least 15 years) a building consent is not required; if not, a consent must be obtained before reroofing.

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Cause

Roofing has run-off from translucent (PVC) sheeting 

Repair

  • lightly sand the corroded areas to remove rust. Do not use a wire brush. Prime with zinc-rich primer and a metal primer, then apply at least two coats of acrylic paint
  • replace badly corroded sheets or sheets with holes; ensure the new roofing is compatible with the existing
  • reroof when it is not economic to repair existing. If the old profiled metal roof has met minimum Building Code durability provisions (it has lasted at least 15 years) a building consent is not required; if not, a consent must be obtained before reroofing.

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Cause

Corrosion of the steel in a corrosive environment such as near a swimming pool, or in coastal, industrial or geothermal regions

Repair

  • lightly sand the corroded areas to remove rust. Do not use a wire brush. Prime with zinc-rich primer and a metal primer, then apply at least two coats of acrylic paint
  • replace badly corroded sheets or sheets with holes; ensure the new roofing is compatible with the existing
  • reroof when it is not economic to repair existing. If the old profiled metal roof has met minimum Building Code durability provisions (it has lasted at least 15 years) a building consent is not required; if not, a consent must be obtained before reroofing.

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Cause

Inadequately primed end and/or side laps

Repair

  • lightly sand the corroded areas to remove rust. Do not use a wire brush. Prime with zinc-rich primer and a metal primer, then apply at least two coats of acrylic paint
  • replace badly corroded sheets or sheets with holes; ensure the new roofing is compatible with the existing
  • reroof when it is not economic to repair existing. If the old profiled metal roof has met minimum Building Code durability provisions (it has lasted at least 15 years) a building consent is not required; if not, a consent must be obtained before reroofing.

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Cause

Use of lead flashings

Repair

  • replace lead flashing with lead-free soft edge flashing; alternatively, isolate the lead flashing from the roofing by painting
  • lightly sand the corroded areas to remove rust. Do not use a wire brush. Prime with zinc-rich primer and a metal primer, then apply at least two coats of acrylic paint
  • replace badly corroded sheets or sheets with holes; ensure the new roofing is compatible with the existing
  • reroof when it is not economic to repair existing. If the old profiled metal roof has met minimum Building Code durability provisions (it has lasted at least 15 years) a building consent is not required; if not, a consent must be obtained before reroofing.

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Cause

The roofing is near the end of its serviceable life 

Repair

  • reroof when it is not economic to repair existing. If the old profiled metal roof has met minimum Building Code durability provisions (it has lasted at least 15 years) a building consent is not required; if not, a consent must be obtained before reroofing.

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