Zinc White Rust
This section on White Rust is intentionally written in as non scientific wording as possible.
Zinc is a very durable non-ferrous metal that will keep out the elements for many years, when installed correctly.
Zinc when exposed to the elements will form its own natural self patina layer, and even when scratched will self heal; this process will vary from location to location and can take many years.
For zinc to form its own patina it needs to be able to get regularly wet and then dry, if however, it gets wet and the water is allowed to rest on the zinc, White Rust will form. The first sign of White Rust occurring on the surface is a white stain which may increase to a white powder. If this situation persists the White Rust will eventually corrode through the zinc and holes will appear.
Zinc should never be installed flat, because it is a fully supported system and not bonded to the substrate the zinc will have some degree of ‘oil canning’. To allow the water to freely run off the roof it needs to be at such a pitch as to not leave small puddles of water behind. The minimum pitch for roofing is 3 degrees and we would recommend that you strive to get as much above this as possible.
There is no treatment for White Rust, once it has formed it cannot be removed, however, if moisture is prevented from resting for long periods on the surface the White Rust will not get any worse.
One of the most common situations for White Rust to occur on zinc is when the coil of zinc is allowed to get wet by the elements or is stored in such away as condensation can form between the layers on the coil. Although in this case when the coil is profiled into a panel or formed into a flashing and the conditions allow the zinc to get wet and dry, the White Rust will not get any worse, however, it will still leave indelible marks and be unsightly.
Care needs to be taken when specifying the substructure build up of a roof so as to avoid condensation occurring on the rear side of the zinc, if condensation is allowed to persist it can corrode the zinc from the inside out. We recommend that you seek advice when drawing up a specification.
We do not recommend the installation of zinc which already shows signs of White Rust being present.
