The Nature and
Characteristics of Slate

Natural slate is a microcrystalline, fine grained metamorphic rock formed 400 million to 550 million years ago, with its beginnings as sedimentary silt washed into ancient seas. These sediments accumulated on the sea floor for 150 million years, forming a bed of clay and shale several thousand feet thick. During later geological upheaval, these sedimentary shale beds were lifted from the ocean floor, folded, and buried on the North American continent. Sediments accumulated on the sea floor were exposed to extreme pressures which lead to the chemical and mineralogical transformations you see today. As a result, the sedimentary bedding plains of the original clay and shale are totally independent of the metamorphic grain of the resultant slate bed. Natural slate has many uses which include roofing material, floor tile, countertops, wall cladding, blackboards, and even tub and shower enclosures.

Slate Colors and Slate Weathering Designations

The color of a natural slate is derived from its chemical and mineral composition. Chlorite produces green slate, hematite the purples, carbon the grays and blacks, and hematite and iron oxide the reds.

Descriptions of color can vary widely from supplier to supplier, but generally, roofing slate produced in North America falls under the general color descriptions of black, gray/black, gray, green, gray/green, purple, variegated purple, mottled purple/green, and red.

One of the unique and aesthetic advantages of natural roofing slate is the subtle variation in color, shade, veining, and grain of each individual slate shingle on a roof. These color variations should be expected.

Metamorphic rock
Natural Roofing Slate
 Roofing Slate Color 1
Roofing Slate Color 2
  Roofing Slate Color 3
Roofing Slate Color 4

In order to obtain the best blending of these colors, contractors should draw from several pallets at the same time when taking slate to the roof for installation.

There are two color adjectives that have been used in the roofing slate industry that explain the change of color known as weathering and fading. Slate can be known as “weathering” and “non-weathering”, or “fading” and “non-fading”. It is important to note that in many cases, and in current usage, the terms “unfading” and “non-weathering” and “fading” and “weathering” are often used interchangeably. Unfading slates more or less maintain their original color on the roof. Weathering is used to describe slates that would show various percentages of color change slowly as the slates were exposed to the elements.

Slate Fading

Historically, the term “fading” was used to describe slates that, upon exposure to the elements, had a discernible, marked, and generally uniform lightening in color due to their mineral makeup. In some cases, the color change and mineral makeup had no impact on the longevity of the slate on the roof, and in other cases, the color change was due to accelerated deterioration of deleterious minerals that would lead to a shorter life expectancy of these slates.

Where slates were known not to have these “fading” characteristics, the slates were described as “unfading” or “non-fading.” In current usage, “unfading” is the most common reference for this color characteristic. These slates more or less maintain their original color on the roof. Even for these unfading slates, color change on a roof can take place as the result of environmental pollutants, most commonly in the form of high sulphur content acid rain.

Slate Weathering

Historically, the terms “weathering” or “semi-weathering” were used to describe slates that would show various percentages of color change – to tones of buff, tan, and brown – slowly, over varying periods of time, as the slates were exposed to the elements.

Color weathering is due to the presence of calcite and iron compounds, primarily fine-grained iron pyrite in the form of limonite. In the quantities present, these minerals are not deleterious and do not affect the life expectancy of the material. The percentage of color change in these weathering slates can vary from quarry to quarry, or even within the stone of a given quarry. The process can become apparent in as little as a few weeks or take six months or longer. As this is a natural process, dependent on the mineralogy of the stone, color change percentages cannot be assured and expectations should be discussed with the supplier prior to ordering material. In current usage, “semi-weathering” is the most common reference for this color characteristic, as few quarries, if any, produce roofing slates that will exhibit color weathering in 100 percent of their material.

Although it is always important to blend slates from multiple pallets when installing a slate roof to ensure a proper blending of tones, it is even more important that this be done when working with weathering or semi-weathering slates to avoid unsightly blotches of earth tones on the roof.
In usage today, “unfading” is commercially used to describe slates that are both (or either) non-fading or non-weathering. Non-weathering slates are also referred to as unfading slates. Few if any producers present their product today as fading.

Slate Color Designations and Descriptions

This color weathering (or absence of color weathering) and color fading (or absence of color fading) produces either the “weathering” or “unfading” designation of slates. This has led to the development of commercial color descriptions for roofing slate with the slate color preceded by the words unfading (least likely to change color in any way over time), semi-weathering (most likely to have a moderate percentage of slates change color to buff, tan, and brown) or weathering (most likely to have a significant percentage of slates change color to buff, tan, and brown) to signify the color stability of slate from a given source. It is these color designations that have been used in describing colors in this manual. Quarries should be consulted directly for an explanation of the color change characteristics of any of their products during the design stage.

The wide variety of natural slate colors available offers limitless opportunities to create unique roofs. Slate roofs can be laid from a single color or a custom combination of various colors to suit any architectural desire. It is a common practice to blend various colors of slates, with both unfading and semi-weathering characteristics, to create a truly unique custom roof.

It should be noted that the color changes associated with semi-weathering and weathering slates are not a result of the presence of deleterious minerals in the slate that will affect expected service life. On the other hand, the excess presence of deleterious minerals, such as large inclusions or high concentrations of oxidizable iron pyrite or high concentrations of calcium carbonate, will lead to unsightly staining or accelerated decomposition of a roofing slate when exposed to the atmosphere.

North American Roofing Slate Producing Regions

At the time of this writing, natural roofing slate is being produced in the United States in Vermont, New York, Pennsylvania, and Virginia. In Canada, natural roofing slate is being produced in Quebec and Newfoundland/Labrador. North American producers have proven deposits of stone and a wealth of experience and expertise in the quarrying, fabrication, classification, packaging, and application of natural slate roofing. With their long established industry involvement and by their membership in the National Slate Association they have exhibited a commitment to produce and market a world-class product.

Vermont and New York

This region, straddling the Vermont/New York state border and including Bennington and Rutland counties in Vermont and Washington County in New York, produces a full range of slate colors and all of the colored slates manufactured in the United States. Roofing slate has been quarried here since the 1840s.

The common and commercially important colors produced in this region are:

Semi-weathering Gray/Green, also known as Sea Green

Green or gray/green when first extracted from the quarry, a percentage of these slates will weather to tones of buff and brown upon exposure to the elements. The degree of this weathering will vary from quarry to quarry and range from less than 10 percent to more than 90 percent. Because many quarries produce this color of slate, a range of color, texture and weathering characteristics should be expected.

Semi-weathering Gray, also known as Vermont Gray

Clear gray or gray with small black markings when first extracted from the quarry, a percentage of these slates may weather to tones of buff and brown upon exposure to the elements. Texture can range from smooth to medium.

Semi-weathering Strata Gray, also known as Mottled Gray/Black

This is a medium gray tone slate with black or darker gray stripes. A percentage of these slates will weather to tones of buff and brown upon exposure to the elements. It is a medium- to heavy-textured slate.

Semi-weathering Variegated Purple

A blend of clear purple and purple slates with green markings when first extracted from the quarry, a small percentage of these slates may weather to tones of buff and brown upon exposure to the elements. Because stone characteristics vary from quarry to quarry, a range of color, texture, and weathering characteristics should be expected.

Semi-weathering Gray/Black, also known as Vermont Black

Depending on the source, these slates can range from a blend of medium and dark gray shades with black and darker gray linear markings to dark gray and black slates without the darker markings. A percentage of these slates can weather to tones of buff and brown upon exposure to the elements, and they are of medium to heavy texture. Because many quarries produce this color, a range of color, texture, and weathering characteristics should be expected.

Unfading Mottled Green and Purple

These are purple slates with large and frequent inclusions of green in almost every piece. Predominantly purple with inclusions of green (dark mottled), some pieces are primarily green with inclusions of purple (light mottled). These slates will hold their original color when exposed to the elements. It is generally a medium-textured slate.

Unfading Gray/Green

These are light green slates of a consistently uniform shade. They will hold their original color when exposed to the elements and are generally of medium texture.

Unfading Green

A blend of green slates ranging from bright green to gray/green tones. These slates will hold their original color when exposed to the elements and are generally of medium texture.

Unfading Gray

These are battleship gray slates with small black or darker gray markings. These slates will hold their original color when exposed to the elements and are generally of medium texture.

Unfading Purple

This is a deep purple slate with occasional inclusions of green. It is generally of medium texture and will hold its original color when exposed to the elements.

Unfading Red

This is a bright red slate that will hold its original color when exposed to the elements. It is generally A medium textured slate and available in limited production.

Pennsylvania

The region of Lehigh and Northampton counties in eastern Pennsylvania produces a range of black slates. Roofing slate has been quarried in this “Soft-Vein” District since 1805 or 1812, according to original source material.

Semi-weathering Black, also known as Pennsylvania Black or Pennsylvania Gray

This slate can range from blue-gray to blue-black and be smooth-, medium-, or rough-textured. The material has been marketed in years past as Varitone, Colortone, Blue/Gray, Blue/Black, Cathedral Gray, Gothic Blue/Gray, Slate Gray, and Storm Blue-Gray, to name a few. A percentage of these slates will weather to tones of buff, brown and light gray when exposed to the elements.

Virginia

The region of Buckingham County in central Virginia produces black slates. Roofing slate was first produced there in 1787.

Unfading Black, also known as Buckingham-Virginia Black, Buckingham Black, and Buckingham

These are blue/black slates with a mica sheen. These slates will hold their original color when exposed to the elements and are generally of medium texture.

Quebec

The region of Temiscouata County in southeastern Quebec produces black slates. Roofing slate was first produced there in 1910.

Unfading Black, also known as North Country Black

These are blue/black slates of a consistent uniform shade with subtle darker vertical markings. These slates will hold their original color when exposed to the elements and are smooth-textured.

Newfoundland/Labrador

Eastern Newfoundland produces purple and green slates. Roofing slate was first produced there in 1850.

Unfading Purple

These are clear purple slates in two distinct shades – deep purple and plum – that are typically not separated at the quarry. These slates will hold their original color when exposed to the elements and are smooth- to medium-textured.

Unfading Green

These slates range from blue/green to gray/green in color, will hold their original color when exposed to the elements and are of smooth to medium texture. The slates are typically provided as a mix of the full range of colors.

Sizes & Thicknesses

Standard dimensions of roofing slate begin at 12 inches in length and increase by 2 inch increments up to 24 inches. The standard shape for natural roofing slate is a rectangle with the long dimension laid running parallel to the slope. Standard thickness is 1/4″- 3/8″ and can be as thick as 1″. For the purposes of slate, a roofing square is defined as a sufficient number of pieces of slate shingles of a given size required to cover 100 square feet of open roof when laid with a 3 ” headlap. There are many different styled cuts that can be produced as seen below.

Nail Holes, Packaging & Storage of Slate

Each natural roofing slate should be installed with a minimum of two nails. Field slate holes are normally punched or drilled by the producer with two nail holes located one-quarter to one-third down the slate length, measured from the head of the slate, and 1 1/4″ to 1 1/2″ in from each side. It is standard to drill, rather than punch, slates that are thicker than 3/8″.

Slate should always be packaged and stored on its edges (never flat) on a solid base. Rows of slate in crates, pallets, or storage should be tightly packed on their sides or ends with adequate headers on the pallet sides to support them and hold them in place. Depending on size, two to three rows of slate can be stacked vertically as long as each row is separated by two pieces of wood lath to provide support for each row.

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