Natural slate, esteemed for its beauty and durability, has been used as roofing tile for centuries. From Loire Valley chateaux to Ivy-League Halls, from Highland manors to Himalayan temples — slate roofs grace and protect distinguished structures around the world.

Slate is available in a variety of colors and grades. To choose slate well, it is best to understand these qualities. The following looks at these matters in some depth.

Colors

In the US, slate colors are divisible into two classifications: "unfading" and "weathering."

The term unfading is applied to slates that, from decade to decade, retain their dominant color. Even after a century (accumulated pollutants notwithstanding), the essential, original shade remains — green remains green, red remains red.

The term weathering is applied to slates whose color changes (typically in weeks or months) to shades of buff, brown or rust. When this occurs variably, with some slates in a given installation retaining their original color, the slates are said to be "semi-weathering." Generally, within a few years, weathering stabilizes, much as a gathering patina on copper eventually stabilizes.

Weathering must not be confused with "fading." Weathering describes a harmless change, whereas fading is the mark of degradation. Such degradation is often caused by an overabundance of certain minerals (such as pyrite, which can oxidize, or calcite, which can effloresce).

High-grade (described below) unfading and weathering slates are pictured (and described very briefly) above right. Fading slates are not shown, as they are problematic and are not recommended. A number of highly figured, rustic, even iridescent, slates are available. Often these are decidedly fading and generally best confined to non-roofing applications indoors. Testing (described below) confirms their unsuitability for roofing in all but the most benign climates.

Reputable quarries and suppliers of slate will apply the appropriate adjective (e.g., "unfading black" or "semi-weathering gray-green"). But it should be noted that the designation, unfading or weathering, is not, by itself, a sufficient assurance of slate quality.

A solid record of use — a sound, 100-year-old slate roof, for example — offers more validation. But if one is very careful, he or she will note that slate deposits are sometimes not homogeneous. The condition of a slate decades after it was quarried may tell more about the quarry at that time than it would about the quarry today. A record of use is a powerful piece of evidence to support or impugn a slate, but it is not the whole story.

Providing additional insight, laboratory tests make possible the classification of slates into grades.

Grades

Various slate qualities can be assessed in a laboratory. Just what qualities matter can vary from country to country, but there is some agreement about the importance of the following: water absorption, acid resistance and strength.

Water absorption

Water itself is not typically injurious to slate, but the chemical and physical processes that it enables are. There are several intimate mechanisms at work.

• With water, invisible calcite grains become gypsum grains, which swell and strain the slate's substrate (interestingly, a chemical process begetting a physical process).
• With water (and free oxygen), iron pyrites oxidize, which, again, swell and strain the substrate. The oxidized pyrite then breaks down leaving voids – which invite deeper water penetration and further opportunity for mischief.
• With water, freeze-thaw cycling induces fatigue as tiny fissures and pores fill, freeze, expand, thaw and contract — again and again.
• And finally, with water, acids enter into solution and degrade certain slate constituents.

So it is that the amount of water slates absorb matters and is often measured when comparing slates.

The principles underlying acid resistance and strength are simple by comparison to those of water-induced processes.

Acid resistance

Acid resistance is one measure of a slate’s durability, an indirect way of detecting the presence of problematic minerals. The test is conceptually straightforward: Slates are immersed in an acid bath and their depth of softening is measured by means of a "scratch test." The lower the depth of softening, the better.

Strength

Slates are knocked about — at the quarry, in transit, during installation and in the normal course of doing proper service on a roof (notably under wind, foot and snow loads).

Assessing the strength (modulus of rupture) of slate is easy: a slate is supported at its ends, a load is applied to its middle and the force required to break the slate is noted.

Together, water absorption, acid resistance and modulus of rupture provide an indication of slate quality. In the US, these tests are detailed in the American Society of Testing and Materials (ASTM) standard C 406.

The standard defines three classifications by estimated service life:

S1: 75+ years
S2: 40-75 years
S3: 20-40 years.

Quarries and suppliers furnish test results upon request.

Finally, it should be noted that ASTM estimates of service life assume a slate thickness of 3/16" — the thinnest slate commonly available in the US. It should be understood that thicker slates, all other things being equal, last longer.

There is a full palate of slate colors and grades available to owners, specifiers and installers. A proper understanding of these facilitates prudent selection.