From Quarry to Roof

Manufacturing Greenstone Slate

The Greenstone Slate Company has 58 quarries in what is known the world over as the Slate Valley of Vermont. The Slate Valley is famous for its very high quality slate. The slate is removed from the ground, cut with diamond tipped saws and then hand split by Greenstone trained craftsmen. Then it is trimmed and drilled (or punched) to create roofing slates. It is manufactured, crated, stored and shipped from our facilities in Poultney, Vermont. We offer tours of our manufacturing process, so if you are coming to Vermont let us know and we will set up an appointment for a tour. You can also learn all about the history of slate and the Slate Valley of Vermont at the Slate Valley Museum, located in nearby Granville NY. (The slate valley does not stop at the state line. It extends into the state of New York.)

Step 1 – The Quarry

Mining slate involves finding a vein of stone, and then strategically extracting it. Once exposed and broken into rough pieces, it is carted out of the quarry and stacked at near the cutting area. Each rough piece of raw stone weighs between 3 and 4 tons. After examining each stone, a jackhammer is used to create smaller more manageable pieces for the first phase of cutting.

Step 2 – Cutting the Slate

After the stone is broken into smaller pieces, they are brought into a warehouse where radial saws cut the slate into thick blocks approximately the size of the final slates. The blades used are diamond-tipped and cost about $1000 each. Greenstone uses between sixty and one hundred of these blades every year. At each station there are two saws, one to cut the length and another for the width. A third craftsman cuts off the excess layers from the top and bottom of the blocks so that all sides of the slate are smooth before moving on to the splitting phase of the slate process.

Step 3 – Splitting the Slate

Splitting is by far the most specialized skill involved in the entire process of mining and shaping slate. Each piece is hand split from the blocks created in the “cutting” stage. The first step is to split the block into pieces that are twice the thickness of the final slates. These pieces are then split in half. Due to the delicate nature of this stage a huge amount of slate becomes damaged due to an unexpected split or crack. In all, 85% of slate becomes unusable or damaged during the manufacturing process.

Step 4 – Trimming the Slate

Trimming the slate is the final step in shaping the stone. Every slate is individually trimmed to your specifications. If desired, this step also removes the cut edge and gives each piece the common look of slate tiling that we are accustomed to seeing on most roofs today.

Step 5 – Punching the Slate

If you are using slate for tiling a roof, the slate must be punched for nail holes. Nails cannot be driven directly through the slate as the stone would break and split. Slate is punched in small groups or one by one, to ensure that nail holes are placed at the ideal point for strength and durability.

Step 6 – Shipping

The final step of the slate process is packing and shipping. Each and every pallet must be packed by hand to insure the least amount of breakage during shipping. Even the greatest care cannot keep some slates from being damaged, and when estimating your contractor or architect should account for this. Once each pallet is packed tight enough that the slates won’t move and shatter during transport, a flatbed is loaded and the order is delivered.