Common Causes of Broken Roofing Slates

Quick answer: a roofing slate breaks for one of six reasons — impact (hail, tree, a dropped tool), frost (ice in the crack), fixing (nail or hook stress), foot traffic, wind fatigue, or a hidden problem in the structure beneath. About 80 percent of the time the crack itself tells you which one it was, if you know what the break looks like. This article is the lesson: read the fracture, then repair the cause, not just the slate.

Let me start the way I start every inspection. I pick up the broken piece, turn it over, and let the stone tell me what happened. Slate is a poor liar — it can't hide how it cracked. The crack is the autopsy, the witness statement, and the repair instruction in one. If you learn to read it, you stop replacing the same slate twice a year and start fixing whatever keeps breaking it.

How to read a broken slate, in three looks

You don't need a lab for this. You need to look at three things, in order:

• 1. The shape of the crack. Is it shell-shaped (impact), straight across the middle (foot pressure), radiating from a hole (nail), or meandering with a whitened edge (frost)?
• 2. Where it sits on the slate. The middle of a large surface has one story; a crack right at the nail line tells a different one.
• 3. What the roof around it says. North slope or south? Under a tree? Any storm last winter? Who has been on the roof? The crack answers "how", the roof answers "why".

In thirty years of walking roofs, I can count on one hand the broken slate where the surrounding context didn't back up the crack pattern. If they match, you can fix. If they don't match, keep looking — that's the case that's still breathing.

Death 1 — the blow that breaks it

Shell-shaped fracture, small impact point. This is the signature of a knock: a hailstone, a wind-blown branch, a dropped ladder, a tile that flew off the ridge above. The slate takes a hit, the shock runs out from that point, and the stone sheds a round, scalloped piece. The crack edges are usually sharp and bright — there is no erosion, no staining, because the break is fresh.

Hail is the silent one. In the UK and central Europe, a storm with stones over about 25 mm starts to damage natural slate, and the typical result is not a hole but a hairline crack running from the impact point — barely visible from the ground, and it will happily sit there for months before the frost or the wind turns it into a real leak. That's why the standard after any hailstorm is to walk the roof edge and the gutter line looking for slate fragments: a few small pieces in the gutter is the roof telling you it took a beating. Every storm-triggered inspection starts with this walk.

Impact is the easiest cause to fix, because it is the least about the slate: replace the victim, check the neighbours for the same small crater, and move on. It is also the one where people replace a tile and forget the branch that did it — the branch is still over the roof, and next storm, same crack.

Death 2 — frost

Meandering, whitened edges, peeled scales. Frost does not blow the slate apart like an impact. It works from the inside out, and it takes years. Water seeps into a crack or a hairline fault — often where the slate meets its neighbour or around a nail — freezes, and ice takes about nine percent more space than the water it came from. It is the most patient killer on a roof. Over winters, the crack widens, the edge begins to feather and flake, and the surface whitens as tiny layers peel away. The crack is never a clean line; it wanders, because it follows the path of least resistance through the stone.

You can't do much about the ice itself, but you can absolutely do something about the water that feeds it. The freeze–thaw resistance of a slate is a measured number: a stone with 0.2% water absorption shrugs off 120 freeze–thaw cycles in a lab with almost zero weight loss, while a more absorbent slate can start shedding scales in a fraction of that. On a live roof, that same difference is what you see when a north-facing slope keeps its slates crisp and the south-facing slope sheds its feathers after twenty winters. The usual cure is not a new slate but a check on what keeps the roof wet: blocked gutters, moss holding water against the stone, a batten holding snow. And once a slate has started to feather, replace it; it will not heal.

Death 3 — the nail that came back to bite

Cracks that radiate from the nail hole like spokes. This is the most common fracture I find in modern slate work, and the most preventable. It has three versions:

• Nailed too hard. The nailhead presses the slate down like a finger on glass; the stone can't give, so it cracks outwards from the hole. The crack starts at the hole and runs to the edge.
• Nail too loose. The slate hangs free and works back and forth; the hole becomes a drill point, and the crack follows. This one moves, and you can often feel the slate rock under your hand.
• Rust. The old steel or zinc-coated nails that were everywhere before copper and stainless steel became standard — they swell with rust, and swelling inside a hole the stone was not designed for pries it open. I have picked up slates where the nail hole is a rust-coloured crater and the crack starts a few millimetres out from the hole. A 30-year-old zinc nail has more in common with a pickaxe than with a fixing.

Every slate has a physical limit: the nail should land in the upper third of the slate, in the headlap zone, and the hole should be drilled, not punched, with the nail diameter matched to it — about 2 mm bigger than the nail shaft. And the fixing material should live as long as the stone: copper or stainless steel nails are the only fair companions for a slate that expects to be there for a century. If your cracks all radiate from nail holes, the roof is telling you its fixings are the wrong age for the stone.

Death 4 — a foot, where a foot shouldn't be

One long crack across the middle, like a fracture line. Slate is a stone that loves compression and hates tension. When you step in the middle of a slate, the underside is in tension, and the stone parts company with itself along a straight, bored crack that runs from one edge to the other. It's the signature of a foot, a knee, a dropped bundle of slates, or a ladder-foot resting on a single tile.

The arithmetic: a 500×250×8 mm slate under the test rig takes a breaking load of roughly 2–3 kN — about 200 to 300 kg — and even that number assumes the load is spread evenly across the rig. But a walking man puts his full weight, close to 1 kN, down in a heel-sized spot. The leverage of the overhang plus that point load cracks the stone at a fraction of its rated strength. That's why the old roofers' rule is: never stand on the middle of a slate. Step on the headlap zone (where the slate is doubled by the one beneath) or on the crossing of two battens — and even then, step gently. Moss-covered slate is the trap of traps: it looks dry and holds your weight, and cracks under you.

If you've ever had a roofer up there to replace a chimney and found three new broken tiles in the following week — that's this cause, almost guaranteed. Access is the roof's one weakness: every time someone gets up there, a little something pays for it.

Death 5 — the wind that never goes away

A hairline that starts at the nail and sneaks towards the tail. This is the only crack on the list that makes no sense as a one-off. It is fatigue: the wind lifts the tail of the slate a few millimetres, a few thousand times a year, and each lift flexes the stone at the nail. The crack develops without any impact and without any frost, as a pale hairline, and one day the tail simply drops off. Roofs on exposed ridges, coastlines and the windward gable of an open valley are the clients of this cause.

The cure is not a thicker slate but a roofing system that does not flap: proper fixing methods — double nailing or the hook system — hold the slate down, and short slates on exposed roofs break less simply because there is less tail for the wind to lift. If you inspect and find the same hairline running to the tail on several slates on one exposed pitch, don't replace one tile: refix the whole pitch with the wind in mind.

Death 6 — the hidden ones beneath the roof

Sometimes the fracture pattern looks like a mystery because the cause is not in the slate at all. It is in the roof that holds it.

• A rafter that sagged. The slates follow the deck. If the rafter moves, a slate is pinched between its neighbours — the crack will follow the deck structure line, and the neighbours will show the same pattern.
• A rotted or kicked batten. The batten holds the slate at two points; if it shrinks, the slate loses support at one of them and twists its own weight — cracks that appear "for no reason" near a batten line.
• Growth and settlement. A building that moves a few millimetres, even once, transfers that movement to the most brittle member of the roof.
• The legacy of an old repair. A slate fixed with too large a nail head, or a vent flashing that pinches the two slates beside it. The stone is not the guilty party.

If you find broken slates on a brand-new roof, or a cluster of cracks along one straight line, look underneath before you re-slite. A slate that lives on a warped roof is just the messenger.

The crack: an identification table

Print this, fold it into your phone case, and you'll be ahead of 95% of call-outs. One look, one column, and you know what to ask next.

If two columns match at once — say, a shell-shaped crack and rust around the nail — treat the older of the two as the root cause. The slate only broke once; the history of the roof is in the rust.

One roof, eight broken slates, four causes

Let me show you the method on a real job, because it's how this stops being theory. A house on the north slope of a valley, roof about 30 years old, the owner said exactly what you'd expect: "the slates keep breaking, must be the batch."

I counted eight broken slates. Reading the cracks:

• Four had radial cracks from the nail holes — all four were on zinc nails, all the holes were cratered with rust. That's one cause: fixings that died before the stone.
• Two were on the north pitch, under a chestnut, edges peeled and whitened. That's frost — and the moss holding water against a slate that was never the problem.
• One was a shell-shaped break on the weather-facing slope, no neighbours damaged. That's a hail hit — three years ago, actually, from a storm nobody remembers.
• One was a straight crack across the middle, under the chimney, at the exact spot where the chimney had been repointed the spring before. That's foot traffic.

Four causes, one roof, one "bad batch" that wasn't bad. The fix was: refix the whole north slope with stainless nails, clear the moss and trim the branch, replace the two hail- and one frost-damaged slates, and put a walkboard over the chimney next time. The owner got an eight-slates problem and a one-roof plan. That is what reading the cracks buys you — it separates the roof's disease from the roof's symptoms.

Is the slate ever the guilty one?

Honestly — yes, sometimes. A natural slate can arrive with a hidden weakness: a barely visible sheen, a calcite streak, or a patch of pyrite that will never behave. It's rare in a tested slate, and rarer in one that's been individually inspected, but it exists. The rule of thumb: one random break, the slate takes the blame and the neighbours are clean; three of the same pattern, the slate is a witness, not the criminal — look at the roof. If a whole area of a batch fails with the same crack around the nail, that's the batch's report card, and you read it in the defects guide and the test data behind it.

This is where the factory test results do their job. Water absorption tells you how frost-tolerant the stone is, flexural strength tells you how much of a beating it takes before the line breaks, and a freeze–thaw certificate tells you how many winters it has been designed for. A slate with 0.2% water absorption and 120 clean freeze–thaw cycles under its belt does not fail from the first frost — if it's cracking, something else is doing the breaking. The stone is usually the last thing to die on its own roof.

What to do with the verdict

So you've read the crack and you know the cause. What now?

• Impact: replace the victim, and if it was a branch, deal with the branch. One tile, not a campaign.
• Frost: remove moss and improve drainage at the gutter; if it's in a few, replace them; if it's a slope's worth, consider a slate with better freeze–thaw numbers.
• Fixings: this is the one that pays to fix big — the nail bank on a whole roof is usually the reason for recurring "random" breaks. Copper or stainless refix is a job, but it's the fix that ends the series.
• Foot traffic: fix the access. A slate on a repair route will be broken again. This is the whole story of chimney and valley roofs.
• Wind: if one tail has gone, its neighbours have the same fatigue — fix the whole exposure in one pass.
• Structure: the roof underneath gets the inspection first, then the slates on top. A slate on a sagging rafter will break its whole replacement.

And when it comes to the replacement itself, don't guess on size, thickness or colour: match the neighbour, because the roof will be judged from the street for the next hundred years. If you are ordering roofing slate for a repair or a re-roof, buy from a supplier who can show the test sheet for the batch — a slate that knows its own numbers is a slate you can build a decade on.

The last piece of the method

A slate roof doesn't break one day — it breaks the day the right cause meets the right weakness. A crack is the only message the roof sends you before it lets the rain in. Read it. Then you'll know whether to buy one slate, one batch of nails, one branch cut, or one visit from the structure engineer.

That's the whole job, really. The roof talks. Most people just listen for the leak. Listen for the crack instead.

FAQ — the questions you'll ask when a slate breaks

Related Reading

• Slate Roof Repair: Replace Broken Slates — once you've read the crack, here's the safe step-by-step swap of a single broken slate.
• Freeze–Thaw Resistance of Roofing Slate — the frost mechanism behind the meandering crack, explained with the test that measures it.
• Roofing Slate Fixing Methods: Nails and Hooks — the right fixing is the best insurance against nail-hole fractures.
• Copper Nails vs Stainless Steel Nails — which fixing material will outlive the slate, and what the rust of the old nails means.
• How Often Should a Slate Roof Be Inspected? — when to climb up (or look up) so a hairline never becomes a ceiling stain.