Galvanic corrosion occurs when two different metals oxidise and corrode by an electrolyte such as water. Galvanic corrosion is the reason why contact between two different types of metals should always be avoided.
However, there are three circumstances that must be met in order for this type of corrosion to occur;
- There must be two different types of materials
- The metals must be in contact with each other
- They must be subject to an electrolyte 
If these three conditions are met then there is a very high likelihood that corrosion will occur. The severity of the corrosion is dependant on the types of metals that are coupled together. The galvanic chart below shows the nobility of metals. The farther apart the metals are from each other on this chart, the greater the corrosion that occurs. 
The typical old ‘Queenslander’ houses tend to be fitted with galvanised roofing materials when originally constructed. However, many more modern homes now use Colourbond & Zincalume roofing. The old galvanised roofing is compatible with lead flashings; however, these newer roof types are not due to their metallic coatings. BlueScope steel states that if building design is such that water will flow from lead flashing onto this roof type then it should be painted. 
Pictured below is an example of galvanic corrosion occurring due to two different metals found by Brisbane Roofing Solutions upon a roof inspection. The corrosion is due to one metal being Cathodic (Zincalume) and one being Anodic (Galvanised) being in contact with each other. The Zincalume tends to be protected but the Galvanised suffers great corrosion very quickly.
Galvanic corrosion is also something to consider prior to installing solar panels. This occurs when the aluminium frame or the brackets used to join the panels has corroded slightly, flowed below and dissolved onto the galvanised roof. There it has re-deposited on the galvanised iron roof which leads to corrosion and inevitable problems in the future.
If your roof is currently showing signs of corrosion, a roof restoration may be able to salvage the current roof and save you from a costly roof replacement. This is done by pressure cleaning, applying a rust inhibitor (e.g Penatrol) to treat existing surface rust, followed by 1 coat of metal primer, and 2 coats of Globalcote Elastacote Acrylic Membrane Roof Coating to prevent and stop any further corrosion. An inspection is advisable on these old roofs to determine whether the roof is worth restoring or a new roof is required.
Another option that is available that may be of some interest is the use of polycarbonate sheeting in some situations e.g. patio roofs etc. The polycarbonate sheeting used by Brisbane Roofing Solutions comes in Clear, Opal, Bronze & Grey. These sheets let light in whilst also reflecting away some heat (see chart below for details). These are a good option as well as they are not subject to galvanic corrosion.
If you suspect your roof is suffering from galvanic corrosion or you would like your roof inspected contact Brisbane Roofing Solutions today!