There is an old Welsh saying that translates as follows: "Only the Welsh can quarry slate because the rocks don't speak English." This is quite the anthropological linguistic riddle — but it does put me in mind of how few people understand the language of slate.

Not the language of geology — that's another kettle of fish — but the language of slate, from the quarrier to the roofer. The following is a brief explanation of the most common slate terms. Whether you are an architect specifying a historic restoration, a contractor ordering material, or a homeowner trying to understand a repair estimate, this vocabulary will help you communicate clearly with the professionals you work with.

Quarrying Terms

These terms describe the geological and operational aspects of extracting slate from the earth. Vermont's slate belt has been quarried for over 150 years, and the vocabulary reflects both the geology of the region and the craft traditions of the quarriers who worked it.

Overburden

Unusable material burying the slate seam.

The Rock (also, The Stone)

Slate.

The Bed

The original horizontal deposits of the material that metamorphoses into slate.

Joints

Breaks in the slate formation, often consisting of a seam of different material.

The Seam

The continuous run of the slate mass.

The Strike

The direction of the seam, described by points of the compass.

The Dip

The angle of the seam to the horizontal.

Blocks

The rectangular blocks cut from the quarried stone.

Splitting & Processing Terms

Once blocks are extracted from the quarry, they are split and trimmed into roofing slates. This process requires intimate knowledge of the stone's structure — particularly the cleave and grain — and is one of the most skilled operations in the slate industry.

The Cleave

The plane on which the slate is split (perpendicular to the main plane of compression).

The Grain

A plane perpendicular to the cleave in which the slate may also be split, though with much greater difficulty — a process called sculping.

The Face

The exposed surface of the cleave.

A Book

The slate split from the same block.

Chips

Books of slate that have not been trimmed.

Natural Cleft

The natural face (without honing).

The natural cleft surface is what gives slate its distinctive texture and appearance. Unlike honed or sawn stone, natural cleft slate retains the subtle ridges and variations created by the splitting process — each piece is unique. This quality is especially prized in historic preservation work, where matching the character of original roofing material is essential.

Explore our Slate Encyclopedia for a comprehensive reference on slate geology, grading, and installation. Browse the Knowledge Center for more technical resources.