What is Pyrite in Roofing Slate?

What is pyrite in roofing slate?

Pyrite is iron sulphide (also known as “fool’s gold”) which is commonly found in roofing slates. Pyrite has an unfortunate reputation of rusting on the roof, but what many people don’t know is that you can have 2 types of pyrite inclusions; stable and unstable.

Stable Pyrite Inclusions

A stable inclusion is found in a T1 slate. These inclusions tend to be visible flecks of “gold” with refined edges, typically a cube shape which, ironically, when found in its raw form, is really quite a beautiful thing. When on the roof, and exposed to the elements for a number of years, these inclusions, if anything, will rust as they are, without bleeding down the roof. These are completely harmless, they don’t discolour the slate and, most importantly, don’t affect the performance of the slate.

stable pyrite in roofing slate

(a small, clearly defined fleck of pyrite)

Unstable Pyrite Inclusions

Unstable pyrite is sometimes very difficult to spot as the inclusions tend to be a lot smaller but greater in numbers, covering large areas of the roofing slates (see image below). These inclusions are commonly found in T2 slates. While these inclusions very rarely compromise the performance of the slate, they do rust and bleed down the roof, creating a bit of an eye sore. We recommend checking the A1-S1-T1 DOP (declaration of performance) prior to making an order, to ensure you’re receiving a T1 slate.

unstable pyrite inclusion in roofing slate

(clusters of unstable pyrite inclusions which can rust and bleed down the slate)


As stated above, not all pyrite is going to discolour the slate. A certified T1 slate is almost guaranteed not to rust, whereas a T2 slate can be prone to it. Here at Buy Roof Slate, we openly divulge all details of our slates, including their origin, to our customers. Your peace of mind and satisfaction is our number 1 priority. If you’d like to discuss this further with one of our knowledgeable staff, please don’t hesitate to get in touch.

2 thoughts on “What is Pyrite in Roofing Slate?

  1. I believe I have T2 on my roof, only after 2 years I’m already seeing rust bleeding down my roof.

    Is this normal performance and how do I stop it

    • Hi David,

      We’re sorry to hear that. In all honesty, there isn’t a great deal that can be done to prevent it. We’ve known some people to periodically clean their rusting slates, but ironically, this only speeds up the oxidising process. Unfortunately, there isn’t a great deal that can be done. Most of the time, rust problems are simply aesthetic, but if you have slates which are starting to break due to iron ore, then I recommend getting in touch with the roofers or the supplier. Most slates carry a guarantee which might cover this, so it’s always worth looking in to. Best wishes, Buy Roof Slate

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