The rust usually comes from iron inclusions.
Some types of slate naturally contain tiny amounts of iron pyrite — also called fool’s gold.
- When water and oxygen reach those iron minerals, they oxidize (rust).
- The oxidation creates reddish-brown stains that can spread across the surface of the slate or streak down the roof.
This is most common with lower-grade or cheaper slates, which have more impurities.
Prevention or remedy:
- Use high-quality slate (low iron content).
- Always use copper or stainless steel nails and fixings.
- Avoid cleaning with acidic solutions, which can make rusting worse.
- Minor rust stains can sometimes be cleaned with a gentle oxalic acid solution, but results vary.