Metal roofs

Types of metal roofs

Long-run metal roofs comes in two basic types: fixed-through or clip-on.

Fixed-through corrugated sheeting is not used on slopes below eight degrees. As the name suggests, fastenings to hold the sheets on to the roof are drilled through the roofing material.

Clip-on roofing can be used on slopes as little as three degrees. In clip-on roofing the clips go on to the roofing first, which is then fixed to the roof purlins.

Always check with the roofing manufacturer if you are doing anything to the roof that could either penetrate the metal (such as installing a TV aerial) or cause water to become trapped (such as installing products that prevent leaves lodging in the gutters). It could void your warranty. The correct fastenings need to be used and all debris from the installation needs to be cleared away as it can cause corrosion.

Check with your manufacturer for the most appropriate products to use to clean and repair your roof.

For example, zinc/aluminium coated steel will react with lead flashings, so soft zinc or aluminium flashings are required. Even marking zinc/aluminium coated steel with black lead pencils can cause a reaction and encourage corrosion.

If a problem arises you think is covered by warranty, contact the manufacturer before making repairs.

Steel Roofs

Flashings lifting: Flashings protect vulnerable areas of your roof. If they are compromised, the weathertightness of the roof may be affected. Re-shape lead or aluminium flashings to the roof profile. Consider replacing the flashing with a heavier grade flashing. Re-fix the roofing with additional nails.

Loose nails or screws: Nails or screws hold the roof and flashings in place. If they fail, the roof becomes vulnerable to damage.
Tighten or re-fix the fastenings. Consider using spiral shank nails or screws as these have greater holding power.

Dented/damaged sheets: Weather, debris from trees or walking on the roof can all cause damage. Dents may be pushed out if you can access the underside of the roof. Replace badly damaged sheets.

Buckling/tearing: This is usually due to thermal movement or an insufficient number of joints. Poor fixing can also buckle or dent the metal. Remove and replace the fastenings carefully. Replace buckled sheets using shorter lengths with more joints.

Corrosion: This may be due to atmospheric conditions, not enough cleaning, chimney/flue deposits, steel debris left on the roof, run-off or contact with incompatible products or acidic timber (such as cedar or timber treated with copper-bearing preservatives).

Address the cause of the rust if you can. It’s best to get technical advice from the manufacturer or supplier before repair. Replace any roofing that has severe corrosion or holes.

Bi-metallic corrosion: If you have two different types of metal on your roof and moisture is present, one metal will be relatively protected while the other will suffer accelerated corrosion. This is called galvanic or bi-metallic corrosion. Copper is not compatible with galvanised steel, zinc/aluminium coated steel or pre-painted steel. This is particularly true when copper is in contact with water or where water can flow from it. Ensure overflow from copper pipes is not able to flow onto metal roofing or guttering.

Lead is not compatible with zinc/aluminium coated products. Flashings on metal roofs can be made from the same material as the roofing or from other compatible products. Contact your roof manufacturer for more information.

Runoff order for metals:

1. zinc, aluminium-zinc, aluminium
   
2. steel
   
3. lead copper
   
4. stainless steel
   

In a metal-to-metal contest with moisture as the catalyst, the metals higher on the list will sacrifice themselves for metals lower on the list.

So, it's ok to have run off from zinc to copper but not from stainless steel to zinc.

Corrosion at roof ridges: This can occur from salt, dust or sand deposits catching under the ridges.

Remove the ridging to clean out deposits and remove the rust. Prime the area before replacing. Replace any sheets with badly corroded top portions or ridging.

Pre-painted steel roofing

Pre-painted steel roofing is factory-coated steel. If well maintained and the correct grade is used for the environment, it should need little repair in the first 15 years after installation.

Paint peeling/chalking: This is due to weathering. If failure is within the warranty period, contact the manufacturer. If the coating is not within warranty, remove all loose paint and repaint according to manufacturer’s instructions. Washing the roof may reduce chalking (white powdery substance).

Minor scratches: This is not a weathertightness issue, it’s cosmetic. Scratched roofing can be left as patches of new paint will be obvious. However, if sheets are badly damaged, they should be replaced.

Surface staining: This could be due to water run-off from lead flashing or poor pre-paint preparation.

Carefully lift the flashing, and prime and paint the top and underside with acrylic topcoat to match the roof.

Unpainted metal roofs

The iconic corrugated ‘iron’ roofs you see throughout New Zealand are actually made from galvanised steel.

White rust: This is due to water droplets sitting on the surface. For example, when the material was stored or from a lack of cleaning. Scrub off the deposits – but don’t use a wire brush. Start regular cleaning and consider painting the steel to reduce corrosion in future.

Red rust: You’ll see this on uncoated galvanised steel or on zinc/aluminium coated steel.

It can be caused by:

• run-off from cedar, redwood, copper-bearing preservative (chromated copper arsenate) treated timber, glass or painted steel
  
• run-off from clear or opaque sheeting
  
• a corrosive environment (for example, a swimming pool nearby), or
  
• use of lead flashings or stainless steel fixings.
  

Address the cause of the corrosion. Then sand the corroded areas to remove the rust, prime the affected areas with a suitable coating system that is recommended by the roofing manufacturer. Replace badly corroded or holed sheets.

Corrosion: Dirt and debris collected under the flashings and roofing overlaps encourages corrosion. Clean the roof more often. Carefully clean under the flashings and overlaps.

Corrosion on the underside of galvanised steel sheets can be caused by unexpected extensive condensation, contact with incompatible materials or contact with corrosive materials. It can even be as simple as the area not being primed or washed. By the time this is noticeable the damage is usually severe. Replace damaged sheets. Prime the portions of sheets that overlap to reduce the potential for corrosion.

When patching the roofing use compatible products.

This page was put together with the help of BRANZ.