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California Slate Company, Inc.

3344 Market Street
San Francisco, CA 94114
+1 (415) 864-6862 Work
+1 (415) 814-2667 Fax
+1 (415) 710-9956 Cell
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General Information

Slate Roofing

Slate roofing is one of the most distinguished building products . It graces many of the most impressive buildings in the world. Cathedrals, palaces, residential homes, and castles have incorporated this long lasting building material as a weathershield. 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.

Slate roofing is really nothing more than mud. Mud that has been squeezed and heated. Fine particles of clay and silt build up in thin layers at the bottom of shallow seas or at the mouths of large river deltas. These sediments then turn into a rock called shale. Mountain building processes can heat and pressurize the shale changing its chemistry. The clay and silt crystallize into quartz, chlorite, and different micas, all of which are very durable materials. This is one of the reasons why slate lasts so long. The chemistry change also imparts a negative quality. The metamorphosis that creates slate makes it brittle. This characteristic is responsible for many of the common slate roof failures.

Slate roofing doesn’t absorb much water at all. However, the water it does absorb can and does destroy it over time. Rain water in industrial areas can turn into mild acid. This acid attacks the minerals in the slate and can dissolve them. Water which turns to ice causes tiny micro fractures which blow the slate apart. Freezing and thawing cycles are like mini jack hammers that take layer after layer of slate off a shingle. Slate that dries rapidly between periods of precipitation minimizes this damage. Spaced sheathing or battens allows air to circulate under the slate. This simple technique can add years to your slate roof if you are thinking of building a new home.

If you are considering a new house or building and choose to use slate roofing, you now have a new option. There is a system by which the slate hangs from tracks. There are no nails in the slate that can pinch them. Furthermore the tracks hold the slate off the roof deck allowing adequate ventilation. It is an excellent way to attach slate to our modern plywood sheathed roofs.

Slate Myths

  1. If slate comes from Vermont, it’s a high quality material.

    In reality, high quality slate is found all over the world…and so is low quality slate. While Vermont has some of the best slate quarries in the world, 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. So while some excellent slate does come from Vermont, a Vermont provenance is not, by itself, a guarantee of quality.

  2. Domestic slate is always superior to imported slate.

    In the United States, we have some of the world’s best slate producing quarries. But high quality slate is also mined in Spain, Wales, Canada, Brazil, and China. When selecting a slate product, it is important to know the actual quarry that the particular stone comes from. Different quarries produce different quality slate, irrespective of where they are in the world.

  3. Chinese slate is intrinsically inferior.

    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’s gotten a lot better recently. With the introduction of new technology, some very good slate has been shipped from this part of the world.

  4. All Spanish black slate contains rust (pyrite) and is of inferior quality to black slate from North America.

    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 get some of the most amazing black Spanish slate here in the United States.

  5. Slate that ‘weathers’ or changes color over time is of lower quality than slate that keeps its original color.

    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 weathering slate changes colors when they are exposed to atmosphere. Thus green weathering slate might turn more brown 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 existence or absence of the organic bits which are responsible for the color change 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 home owner and the architect to make a decision as to whether the changing appearance of the material is a desired feature of the roof slate.

Design Considerations for Slate Roofs

  1. What color(s) will best suit the building’s architecture and its surrounding environment?
  2. What weathering characteristics are most appropriate? (e.g. unfading, semi-weathering or weathering.)
  3. What dimensions and thickness best suit the building?
  4. Is a smooth or textured appearance most appropriate?
  5. Are winds, wind driven rain, hail or severe snow-loading expected?
  6. What are the quality expectations regarding the slate delivery and supply, and when, where and how will they be met?
  7. Which slate best suits the aesthetic features of your roof architecture: Hips, Ridges, Valleys, Saddles?

Please don’t hesitate to talk to California Slate Company about your project and needs. We will gladly send you full size samples of any slate in stock upon request.

Technical Characteristics of Different Slates

Four hundred year old existing slate roofs can be found in Europe with stones quarried from Spain, France, Germany, and Wales. In the East Coast of the United States, roofs dating back 150 years can be found with slate mined from Vermont, Maine, Pennsylvania, New York, and Virginia. Few other roofing materials outlast slate.

But not all slate lasts for centuries. The durability of a slate roof depends both on the quality of the construction and the material characteristics of the stone used. The provenance of the slate itself is important because different slate has different characteristics which can affect its durability. What all the longest lasting slates have in common is high density and very low water absorption.

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.

  • Water Absorption Test

    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 (absorption) than lower quality rock.

  • Fracture Strength Test

    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 higher the load required to break the slate, the higher the quality of the slate.

  • Resistance to Corrosion Test

    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 that the acid solution was able to affect 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—all that testing 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 insure that it has an extremely low rate of absorption or it is certain to fail over time.

Chinese Slate

Are you looking for a company that can import the finest Chinese slate available? Would you like to be able to choose from a range of beautiful natural Chinese slate? If the answer is yes, you have co me to the right place.

California Slate Company are importers and exporters of Chinese slate and stone. We trade all over the world as well as buy and sell locally, so we can bring you a rich variety of slate, including the renowned Kentdale Chinese slate range.

Our range of Chinese slate consist of Kentdale Blue/Black Slate, Kentdale Green Slate and Fellside Blue Slate, all of which are produced to European standards with the required properties to surpass the BS 680 tests. Quality is a key factor to the success of our business therefore we rigorously check all Chinese slates during the chamfering and packing stages.

Kentdale Blue/Black Slate and Kentdale Green Chinese Slate range is mined and manufactured to our own specification. Both are a high quality Chinese slate, which have a good surface texture and excellent edge detail. Fellside Blue Slate is manufactured under the strict quality control of Europe’s largest and oldest slate producers, with over 250 years of sourcing the world’s finest slate. Due to the high demand for this quality Chinese slate, it is advised to contact our office for stock availability.

Please contact us for more information.

Vermont Slate

Vermont is one of the oldest and best slate quarrying regions is in the U.S.

A lot of the slate being produced in Vermont has a distinctive feature called Weathering and Semi-Weathering. Weathering is the tendency to change color once it is exposed to the elements. A Weathering slate is a slate where a large number of pieces, more than 50%, will change color from grey or green to varying shades of buff and brown. A Semi-Weathering slate is a slate that exhibits the same tendency but a lesser number of pieces go through the change. This process produces the distinctive, unmistakable “Vermont” look.

This transformation does not mean the slate is of inferior quality. In fact, it has been our experience that the finest slates are Semi-Weathering. There are also unfading slates available from Vermont so when you are considering a Vermont slate roof it is important to understand these characteristics.

Note: We would like to thank Jeffrey S. Levine and the Technical Preservation Services for providing some of the technical information on slate and slate roofs found on this site.