Slate differs itself from other stones by its natural cleavage, which allows it to split easily. Quarried slate for roofing application is dense and extremely durable. Slaty cleavage refers to extraordinarily closely spaced, parallel planes of weakness that give slate the ability to split into very thin layers.
There are variations in the chemical composition of some slate making it unacceptable for roofing applications, and this slate is referred to as schist. See image below for slate quarried in another country that would be considered unacceptable for roofing application.
This image is of a slate roof that was installed >10 years ago and is already suffering catastrophic failure.
The slate that is labeled as non-weathering or unfading will stay true to its original color, there might be a slight change in the appearance, but overall, there will be minimal change in appearance.
Weathering / Semi Weathering
Slate labeled with this designation will change in appearance over time. The extent of that change is hard to pin down, but due to experience with that specific vein, the quarry knows there will be changes in appearance over time.
With either the non-weathering or the weathering designation, the quarry is only able to make an approximation of what the slate will look like based on their previous experience with that vein of slate.
Each quarry has there own designations for the color/variants of their slate, listed below is the colors currently at Camara slate with there variations. 90% of the slate roofs we have installed in the last 15 years has been Camara Slate, and we feel they are the best source of slate in the country.
Thickness / Weight
Slate Roof Components