Slate roofing is one of the most distinguished building materials on the market, gracing many of the world’s most impressive buildings. Cathedrals, palaces, residential homes, and castles have utilized this long-lasting building material as a weathershield for centuries. In fact, it was the king of roofing materials at the turn of the twentieth century. United States slate consumption was at its highest in 1902.
In reality, high-quality slate is found all over the globe…and so is low quality slate. While Vermont has some of the world’s best slate quarries not all Vermont quarries produce high-quality slate. It is very important to know the actual quarry that a particular slate shipment is coming from.
The United States has some of the world’s best slate-producing quarries, but high quality slate is also mined in Spain, Wales, Canada, Brazil, and China. Different quarries produce different quality slate, irrespective of where they are located.
Although the domestic slate industry in China goes back several hundred years, China is a relative newcomer to the international slate industry. In the early years, quality control in Chinese slate production was low. To some degree this is still a problem, but it has greatly improved. With the introduction of new technology, some very good slate has been shipped from this part of the world.
This is simply not true.
The myth of Spain producing low quality slate does have interesting historical roots. Until very recently, almost all of the high-quality black slate from Spain was imported by France. France and Spain are geographic neighbors, and France was Spain’s dominant trading partner for slate. Since France has one of the strictest slate standards in the world, there wasn’t much high-quality black slate left for export to other countries. What was exported was the lower-grade slate which didn’t match the French standards. And thus arose the poor reputation of the Spanish slate industry.
This all changed when the significant downturn in the French economy freed up high-quality Spanish black slate for export to other nations. We are now fortunate to receive some amazing black Spanish slate here in the United States.
While “unfading” slate maintains its color for most of its lifetime, “weathering” slate weathers by slowly changing color over time. This happens when bits of organic material trapped in the slate are exposed to atmosphere. Thus, green weathering slate might turn browner in color over many years.
Slate quality, which relates to its durability as a roofing material, has to do with its low absorption of water, its fracture strength, and its resistance to corrosion. The presence or absence of organic matter doesn’t affect those factors – so the fact that weathering slate changes colors is not an indication of lower quality. Some people want the slate to change color over time and others do not. It’s up to the homeowner and the architect to make a decision as to whether the changing appearance of the material is a desired feature of the roof slate.
In order to qualify as a “True Black” slate, it must fulfill two criteria:
Slate fulfilling these criteria has only been found in Spain. Other countries produce black slates, but we have not yet found any that satisfactorily meet the second criteria.
In general, we’ve found that black slates from countries other than Spain are undesirable.
The American Society for Testing and Materials (ASTM) is an international standards organization that develops technical standards for a wide range of materials. They developed three tests for roofing slate to determine the water absorption, the fracture strength, and the resistance to corrosion. Their highest rating of S1 would be awarded to slate with low water absorption, high fracture strength, and substantial resistance to corrosion. Slate with an S1 rating has a projected life span in excess of 75 years. S2 is rated for 40 to 75 years. S3 is rated for 20 to 40 years.
The maximum absorption test measures how much water the slate rock will absorb. It’s a simple test: a piece of slate rock is weighed dry and then submerged in water for a certain time period. Then the rock is weighed again. Higher quality rock will have absorbed less water and will have gained less weight after being soaked than lower quality rock.
The “Modulus of Rupture” test is designed to determine a slate’s fracture strength – the load required to break the stone. A piece of slate is subjected to an ever-increasing load while bridging two points of a specified distance. The heavier the load required to break the slate, the higher the quality of the slate.
A piece of slate’s resistance to corrosion is tested by submerging the slate in a weak solution of acid for a particular amount of time and then examining the surface of the slate. The deeper the acid solution was able to penetrate the rock, the lower the quality of slate. Generally, harder rock is more resistant to corrosion than softer rock.
These tests are fairly straightforward for a single piece of slate. But unlike the uniformity found in a manufactured product, slate is a natural product and each individual piece is idiosyncratic with lots of variability—particularly in fracture strength. A random sample of poor quality slate could pass tests required of higher quality slate, or vice versa. To combat the variability, the tests are often repeated on about 20 individual pieces of each slate being tested—which can become expensive.
These tests do reveal something about the quality of slate produced by a particular quarry. If slate samples from a particular quarry consistently rate high, then the quarry itself contains at least some good stone. But provenance isn’t enough. The quality of stone in a particular quarry can change depending on the level being mined, among other factors.
If a particular load of slate is going to be installed in a climate which has extreme freeze and thaw weather cycles, it is important to ensure that it has an extremely low rate of absorption – otherwise it is certain to fail over time.