Slate is probably the most durable roofing material. When properly installed, according with climatic and environmental conditions, a good quality slate cover can last for hundred years. A house with a slate roof will increase its value with the years…blah blah blah.
I´m sure you have heard this before in the mouth of a slate distributor. Of course every salesman always says his product is the best. However, this time I´m going to give you accurate and comparative data about this. In 2017, some researchers from the Architectural School of the Burgos University, Spain, checked and compared the performance of zinc sheets and roofing slates installed in the Church tower of Saint Mary of Aranda de Duero (Burgos). Due to an inappropriate restoration work, the original slate cover had been substituted by zinc sheets. In humble my opinion, a crime for which someone should be whipped. Anyway, these researchers noticed that the zinc sheets were severely weathered and had to be replaced. As a part of the restoration study, they performed a comparative test between the original material, roofing slate, and the new one, zinc sheets. They used a Kesternich Chamber, in which one can reproduce exposure to a sulphur dioxide (SO2) atmosphere, in other words, urban environment. This chamber accurately determines the effect of aggressive pollutant agents and meteorological weathering.
Results showed how roofing slate resisted to this aggressive environment much better that zinc sheets. To check the alterations, the authors used Scanning Electronic Microscopy, which allowed to see the shape, amount and degree of these alterations. Even when the alterations were similar in both materials, slate did not lose structural integrity and continued doing its work.
The conclusion of this study is straightforward: if it works, do not fix it. The original cover was made of roofing slate. During the restoration of the tower, a new material, zinc sheets, was introduced, altering the aspect and performance of the cover. After some years, it had to be replaced again due to “bad behavior”. So before choosing the material for your roof ask some professionals and do not think that “modern” materials are better than “old” ones.
Want more information? Take a look at the original paper:
Garabito, J., Rodriguez, A., Garabito, J. C., & Calderon, V. (2017). Durability of slate and zinc sheets in the rehabilitation of historical heritage. A case study. Construction and Building Materials, 135, 212-224. doi:10.1016/j.conbuildmat.2016.12.224