How Roofing Slate Is Graded and Sorted

Let me tell you how I came to learn this, because the honest version is a bit embarrassing. My grandfather worked slate, in Jiujiang, on the beds this region has worked for generations. I grew up knowing exactly one thing about slate quality: if it sounds good and splits clean, it's good. That's true as far as it goes. It goes about as far as knowing how to milk a cow because you've seen a milking once.

The first time a European importer asked me to sort an order into "first choice" and "second choice," I thought: that's just quality control, isn't it? I was about fifteen years of experience wrong.

Here is what fifteen years of orders, claims and roofs later taught me. When you buy roofing slate, you are not buying the output of a quarry. You're buying the output of a sorting floor. The quarry supplies the stone; the sorting floor decides what each tile becomes — and the difference between the piles is not always "good" versus "bad." It's fit for purpose, decided one tile at a time. A tile that would never pass as first choice on a manor roof can be the perfect second choice for a barn. A tile that fails both can be re-split into something entirely different. None of it is waste. All of it is sorting.

That's what this guide is about: what the grading floor actually does to every tile, in the order it happens, and what it means for the tiles you receive. Follow three tiers, and "graded and sorted" on a packing list will stop being a vague phrase.

Tier One — What the Quarry Decides Before Anyone Is Involved

The first sort happens in the quarry, and it never shows up on an inspection report. Slate beds are not one uniform sheet; they're layers. In our region, and in every slate region I've visited — Wales, Spain, Vermont — a quarry face shifts as you cut down through it. One bed splits clean and rings like a coin. The bed three metres below it yields a softer, grainier slate that takes water along its seams. The splitter reads the face the way a cook reads a cut of meat: every change in texture, every rusty line, every seam that wanders tells him what this block will be.

So the first "grade" of roofing slate — the geological grade — is decided before the tile exists. No sort, no test, no caliper can make a block from a poor bed into a first-choice tile. You can only split it well, tell the truth about it, and sell it for what it is. That's why "roofing slate from the same quarry" can arrive in three visibly different qualities: the quarry isn't one product, it's a sequence of layers.

What does this mean for you? Two things, both practical. First, when someone sends you a colour name and a price, ask which bed it came from — the answer tells you more than the photo does. Second, when you compare quality between suppliers, the honest question isn't just "what did the test say" but "which face did tonight's batch come off." A test report tells you the stone is good; it can't tell you the next block will be.

Tier Two — Size, Thickness and Weight (the Numbers Do the Sorting)

The second level happens once the block is split and trimmed. Here the factory has hard numbers to work with, and so do you. Slate is trimmed to size — at our line, the CNC cutter holds edge lengths to a tolerance we control at ±0.2 mm — but the machine checks size, it doesn't judge. Judgement starts after the cutting, and it starts with the two numbers that matter most: thickness and weight.

Thickness is the first split. A 5–7 mm tile and an 8–9 mm tile from the same bed are not a discount version of each other; they're different roof coverings with different thickness requirements, different fixings and different weights. The grade band (5–7 mm, 7–9 mm, 9–10 mm) is the first line drawn on the sorting floor, and it's the line the official grades S1/S2 and T1/T2/T3 are built around.

Weight is the second number, and it's more useful than it looks. Because good slate has a nearly constant density, a tile's weight is basically its thickness with a handle on it. On a 500×250 tile, the difference between 7 mm and 9 mm is around 0.5 kg per tile — a grader can feel that in the hand before a scale is needed. And weight has a hard commercial limit: roofing slate weight is what runs a container out before the volume does. A 20 ft box carries roughly 900–1,100 m² of 7–9 mm slate; go to 9–11 mm and the same box carries measurably fewer square metres. A factory that sorts by weight isn't being fussy — it's protecting the arithmetic of your shipment.

One more number matters at tier two: how a batch's thickness is distributed. A pack labelled "7–9 mm" can hide two different batches. The honest sort has its curve centred around 8 mm, with a few at each edge. The lazy sort — or the dishonest one — has a flat pile at 7 and a flat pile at 9, two grades tied in one bag. The first number to check when you open a crate isn't the grade name on the label. It's whether the thickness of ten tiles forms one pile or two.

Tier Three — The Bench (Sound, Split, Edge)

Tier three is what most buyers picture when they hear "grading," and it's where the grader's hands matter as much as their eyes. This is the sort, done one tile at a time. Three checks, and one honest trick that I'll get to at the end.

Check one — sound the tile

Pick a tile up and hold it flat on one hand. Tap it with the knuckle or the edge of a second tile. What you listen for is a clean ring — the way a good dinner plate says ping. A tile with a hairline crack, or a lamination that hasn't opened yet, answers with a dull thud, like a plate with a hidden crack. On a roof, that hidden split is exactly the tile that opens during a frost cycle — see our test results page for the freeze–thaw numbers that keep it off a roof.

This is the same sound test you'll read about in our common defects guide and our incoming inspection checklist. On the factory floor it's the same test, thousands of times a day. The grader's ear learns the sound of the current bed so well that one off-tile sticks out of the line like a wrong note in a crowd.

Check two — the split and the hand

Then the hand takes over. The grader balances the tile on a palm, thumbs across the split face, feels for the weight sitting unevenly. A tile that's heavy at one corner tells a story about the split; a tile that varies in thickness from edge to edge is a wedged tile — worth re-splitting if there's enough of it, demoted if there isn't. This is where the sorting floor's finest work lives: a seasoned grader's hands can split a batch into three or four weight bands without a scale in sight, and later the scale agrees.

Check three — the edge and the face

Then the eyes. The edge — clean or flecked. The split face — smooth or rough, and how the roughness sits. Some of what the eye finds is real defect — the things our defects guide lists: cracks, pyrite spots, carbon streaks. Some of it is merely degree: a small edge fleck on a tile that will sit in the lower, shadowed course of a roof costs the roof nothing. The same fleck on a gable-end tile is a visible fault. The grader doesn't throw the flecked tile away — the grader decides which roof it belongs to. That is sorting.

So the bench splits the day's output into two bands: the tiles that pass clean go to first choice (or grade 1, or "best"); the rest go to second choice (grade 2) or back to the splitter.

And it's worth saying plainly: a decent factory sorts almost everything — even its second choice. The truly dangerous stuff is the "ungraded" pile, the one that was never sorted at all, because with ungraded slate the sorting problem simply moves to your roof, tile by tile, for years.

The honest trick — the stack and the straight edge

Here's the check I promised. Sorting by eye and hand can drift — tired graders, a conversation at the wrong moment, a run of tiles that all look the same. So a well-run floor builds a check into the process. When a stack of tiles has grown, someone runs a straight edge over the top — or simply lays a tile from the stack on the stack and presses the corners. A tile that sits proud — even a fraction of a millimetre — comes out. It's a fast, physical check that catches thickness drift that the eyes stopped noticing an hour ago. You can run the same check in your own warehouse with any straight stick — it's the same receiving-side check our inspection checklist describes.

Where the Grade Names Come From (and Why They Don't Match)

Now for the part that confuses everyone, including people who've bought slate for years: the names. Around the world, the same set of tiles gets called three different sets of names.

• The market names. In the UK you'll hear "best," "second choice," and a few others in between. In China, "grade 1 / grade 2" is the common language; some factories add "rustic" as a deliberate third tier for a rugged roof look.
• The standard names. ASTM C406 (the US standard, read it in the test-report guide) calls its tiers S1, S2 and S3 — these describe the thickness and flexural strength of the slate, decided by test. EN 12326 sorts tiles into thickness classes T1, T2, T3 — the same set, renamed and decided by measurement (see our grades guide).
• The informal names. "Roofing slate" versus "riven slate," "seconds," "B-stock," "tile that has been graded," "ungraded."

Here's the key insight: these are three different telescopes pointed at the same pile of tiles. ASTM S1 says the slab is thick enough and strong enough. The market's "first choice" says the tile's face is clean enough. They are not the same axis — an S1 tile can carry an edge fleck that makes it "second choice" in the trade, and a "first choice" tile can still be a weak S2 by test. You have to ask which telescope the seller is reading through, and hold them to it. A quote that says "S1, first choice" is a good sentence; a quote that says "S1" alone is missing half the story.

And because the sorting floor is a human net with a tolerance, a real factory will give you its honest numbers: "first choice runs about 95% clean, the rest gets moved to second choice or back to the splitter." A supplier who claims a flat 100% isn't telling you about a better sort. It's telling you their labels and their roofs have never met.

The Last Sort — Packing, Where Grades Are Made or Mixed

The last sort is packing, and it's the one that decides what actually arrives at your door. A sorting floor can do a perfect day's work and lose it all in the packing line in an afternoon. Two failures to know about.

Failure one: mixed packs. A crate marked "first choice" with a band of second-choice tiles at the bottom. Sometimes it's deliberate — a trader offloading a size — sometimes it's a pallet mislabelled at the end of a day. Either way the crate is the problem, because a roof can't tell crates apart. The simple receiving check: open a crate, sound a sample across the top and the bottom, look at thickness spread, weigh the crate against the packing list. If the numbers tell two different stories, the grades got mixed somewhere.

Failure two: sorting the sample, not the delivery. Every factory can produce one perfect tile. The question is whether that tile came from the batch you're buying. Our sample approval guide covers this fully, but the rule fits here too: a sample is only evidence if it carries a batch number that ties it to your production run.

What good packing looks like, on the floor: graded batches that stay together. Each run gets a mark — a chalked grade letter, a run number, a weight band on the crate. When you open the crate and the mark says "G1," the roof is uniform, and the story matches. When the mark says "G1" and the roof shows two colours, something between the sorting floor and the container changed the story.

And a habit worth stealing: weigh the crate. Grade, size and m² are numbers; weight is the number that can't lie as easily. A 20 ft container of 7–9 mm slate has a weight band you can check in ten minutes with the weighbridge at any port. If the weight fits the m² and the thickness band — the sort is probably honest. If it's light, something got re-sorted between the label and the truck. Weight is the grade's witness, and it's free.

What the 70-20-10 Sort Looks Like

Here's a realistic day on a sorting floor, the kind you'd see if you toured a serious factory. A bed has been split, trimmed and cut. The day's tiles meet the graders. By the end of the shift the output divides — in our own work, roughly:

• About 70% — first choice. Clean sound, even thickness, clean edges. This is the tile that goes out the door as "grade 1" and becomes the visible roof.
• About 20% — second choice. A fleck here, a half-nib clipped there, a slightly heavy tile — none of it a defect on a roof that doesn't demand perfection. This is the "grade 2" that roofs barns, outbuildings, and the far slopes of a big roof without anyone ever noticing.
• About 10% — back to the splitter. Tiles worth re-cutting, tiles with a vein that could yield a smaller size, tiles that just missed. On the sorting floor, this isn't waste; it's the factory's future inventory.

Read those numbers again: the 70% isn't "the quality" — it's the average of what the bed gave this week plus the grader's standard. The 20% isn't defect — it's a different product with a different roof. The 10% isn't failure — it's next week's stock. A sorting floor isn't a quality filter; it's a sorter, and the sorting is the product.

For your own ordering, the practical version is this: a 1,000 m² order of first-choice 500×250 will contain a small honest percentage of off-grade tiles, because the sort is a human net with a tolerance. That's why we repeat our numbers openly: we inspect each tile, we grade each tile, and we quote our clean-out rate — and we'll show you the floor. A supplier who refuses to say how clean their "first" is, is asking you to buy the 70% but hoping you never count the 30% that got packed anyway.

FAQ

Related Reading

• Roofing Slate Inspection Checklist — the receiving-end checks that tell you whether the grade survived the journey
• Factory Quality Control from Quarry to Packing — the eight gates behind the grading, in order
• Roofing Slate Grades: S1, S2 & T1–T3 Explained — the official standard grades, decoded
• Samples and Sample Approval Process — tie your sample to the batch you're actually buying
• Roofing Slate Products — first and second choice, sorted and packed as separate batches