Common Slate Roof Installation Mistakes: What the Roof Opens Up to Show Me

I spend most of my working days standing on roofs that somebody else built. Not because I'm a builder — I'm the person who gets called in after the rain finds its way through. A slate roof that's been up for eight years and leaks is not a slate roof problem. It's usually a problem someone nailed into place, and then the weather took its time proving it.

Let me show you what I find when I peel open a failed roof. Most of it is not dramatic. No single mistake is a bang; it's a slow drip that finally reaches the living room ceiling on the night of a storm. The good news is that nearly all of it was avoidable — and the even better news is that if you're about to slate a roof, you can avoid the same errors by knowing exactly what to look for as you go.

The nail that rusts: wrong metal, wrong size, wrong place

I have pulled thousands of nails out of slate roofs. About one in four comes out with rust flaking off it. And of those, almost all were ordinary steel — the kind that belongs on a fence post, not on a roof that means to last a hundred years.

A slate roof should outlive its fixings. That's the rule. Copper nails were the traditional answer because copper doesn't betray the stone around it — it forms a protective patina, not a streak of rust. Stainless steel does the same job with a different chemistry. The iron or zinc nails that some contractors save money with simply disappear over time — a 3 mm shank rusts through, the head pops off, and one day the whole slate lifts and slides downhill by a few millimetres. Then the wind finds that gap, and the next storm does the rest.

Check the size, too. A nail too short doesn't reach the batten, so the slate hangs by a thread of timber. A nail too thick splits the batten or, worse, cracks the slate at the nib. And the place matters: a nail banged into the slate's exposed face — outside the headlap zone — sits in the open, where the weather reaches it. The fixing methods guide covers how to nail and where; the mistake I see on site is usually not the method, but the material. "It'll do," said the same contractor who signed a 30-year warranty. It didn't.

What I find on site: lift a course and look at the nail heads. Copper is still bright; iron is a lump of rust. The more rusted nails in a run, the more slates have shifted.
The chain of damage: corrosion → loose slate → wind lift → water entry → rotten battens → the whole slope re-done.
The fix and the prevention: on a new roof, replace every zinc or iron nail with copper or 316 stainless — the price difference spread over a century is noise. On an existing roof, hook the loose slates before they crack; if a rusted nail won't pull, leave it rather than prise it and split the stone from the root.

Vertical joints that line up like soldiers

The second most common thing I find when a roof leaks is a row of vertical joints stacked in a straight line, one above the other, course after course. In the trade we call that "straight joints." It looks tidy from the scaffold — the slater lined everything up on the chalk line — but what it has done is created a continuous gap in the armour.

Rain that hits a properly staggered roof runs off slate to slate, always landing on the middle of the tile below. When three or four joints line up, a sliver of water can run straight down through the whole layer and find the one place where the underlay is thin or the membrane's edge is uncovered. That's not a weatherproof failure, it's a geometry failure.

Most local regulations simply say: don't allow a vertical joint to be within a certain distance of the joint below. The practical version: keep the stagger — one slater says "a broken joint," and the joint spacing guide shows the pattern with the quarter bond. If you walk a roof and see three courses with their head joints in a straight line, that roof is only as good as the nearest drop of wind-driven rain.

What I find on site: from the ridge, straight joints read as pressed-flat lines running down the slope. Measure the gap between vertical joints on adjacent courses: anything under about 150 mm — half a tile width — is where the trouble concentrates.
The chain of damage: straight joint → water creeps down the line → local leak → damp battens → mould → frost spalls the face.
The fix and the prevention: on a new roof, re-lay the affected runs to a broken joint before you finish; on a finished roof, lift and re-lay three courses over the aligned section — cheaper than chasing the leak. Just never re-lay on the line.

Headlap that doesn't cover: the shortest lap on the roof

Every slate roof has a headlap — the amount one tile covers the tile two rows below it. It's the single most important number on a slate roof, and it's the one most often shaved. "A bit more exposure, looks cleaner, saves tiles" — I've heard that sentence from three different contractors, and I've seen what the third one's roof did to the room below.

The headlap is not about looks. It's about what happens when wind pushes rain up the slope — rain runs uphill between the laps, and the headlap is the barrier that stops it before it crosses into the underlay. Insufficient headlap is not a cosmetic thing. In exposed hills, a roof with short headlap leaks in sideways rain even when the sun is out.

The headlap for a standard 500×250 mm slate is 75 mm minimum on most roofs, rising with wind exposure and lower pitch. The headlap calculation guide runs the numbers properly. What I see on site is installers stealing 10–20 mm from it to "even out the courses" — and that is exactly the 10–20 mm that the driving rain finds two winters later.

What I find on site: lift a tile and the headlap's witness mark (the dressed edge where the courses cross) is level with, or already below, the course above.
The chain of damage: short headlap → wind-driven rain and melting snow forced up the lap → water behind the tile → drips onto the nail holes of the course below.
The fix and the prevention: new roofs follow the spec; on an old roof, any run under 10 mm of lap needs re-laying — you cannot patch more coverage on top. And listen for the "tick-tick" of drips on a rainy day; it finds the problem earlier than any paper calculation.

Underlayment: the hero nobody sees, until it's not there

People assume the slate is the waterproof layer. It is not. The slate is the armour; the underlayment is the raincoat. In a roof where slate overlaps and rain can be driven in, the membrane is the second line that actually catches the water and carries it to the eaves.

The biggest underlay mistakes I see on old and new roofs: the membrane installed with the printed side down (so water runs underneath), overlaps that are too narrow, torn and patched bits, and — the classic — the roll laid flat and nailed every few inches, which defeats the whole system. And in cold regions, the missing one: no ice-and-water shield at the eaves, the exact place where winter kills.

The deck, batten and underlayment guide shows the correct order. Here I only want you to remember the one principle I check on every roof: the membrane must run under the bottom edge of the tile above and over the top of the one below — like a roof-tile relay, not a fence.

What I find on site: lift a tile and the membrane shows a clear watermark line, or its edge curls away and the batten below already has a damp stain.
The chain of damage: pierced or wrongly-lapped membrane → water reaches the timber → battens blacken → mould and rot → structural strength lost.
The fix and the prevention: before slating, check the membrane's direction and its overlap (100–150 mm). On a finished roof, patch tears with a hot-air welded patch before the rainy season, or re-lay that one run — ten times cheaper than fixing the structure afterwards.

Eaves, ridges and hips: where the roof meets the sky

Every slate roof has a hundred hidden joints — the eaves where the water starts, the ridge where two planes meet, the hip, the valley. These are the places I have seen more failures than anywhere else, and the funny thing is they are also the places where the installer usually has the most confidence.

Three classics: the lead flashing at the ridge is nailed too early, so the water runs through the nail holes; the valley is made too narrow, and a blocked gutter sends the whole storm straight at the tiles; and the ridge cap is bedded on mortar with no breathing room, so frost pops it off. None of these is a slate problem — they are detail problems, which is why the eaves, ridges and valleys guide spends so many paragraphs on the metal and the bedding rather than the tile.

What I find on site: in winter, a line of stains on the gable under the ridge; in summer, the bedding compound cracks and the metal drips.
The chain of damage: cracked ridge tile → rain forced back into the timber → the top course of battens rots → rebuild.
The fix and the prevention: use a matching dry ridge or a proper mortar with a separation layer so the mortar never touches the slate. Each spring, walk the ridge and valleys and replace the metal before it fails.

The batten that never met the rafter

Roof battens are the skeleton of a slate roof. Each slate rests on a batten, and the batten rests on the rafters. If the batten is undersized, warped, or only nailed into the top 10 mm of timber, the whole slate field is floating on a faith.

I once took over a job where the previous crew had hung the battens with a nail every 60 cm instead of the spec's 30. It "looked fine" from below for a year, and then a damp summer made the whole field ruckle like a metal roof under a cat. The slate is the same — the fixing was the failure.

What I find on site: walk on the roof and a tile 'clicks' — the nail has not bitten the batten. Probe the batten with a screwdriver; if it moves, it was never fixed into the rafter.
The chain of damage: loose batten → tile drifts → joints open → leak → the whole slope destabilises.
The fix and the prevention: before laying, gauge the battens with a caliper and drive each one into the rafter; if a batten is found loose halfway up, lift that run and re-fix it — do not tell yourself 'later'.

A roof that can't breathe: the silent killer

Last one, and the one that surprises homeowners the most: a slate roof can be perfectly installed, with every nail and lap exact — and still fail from the inside out. Because the roof is not the problem; the moisture is. Slate roofs are dry and cool, but the timber and the membrane underneath need to breathe.

When a roof is sealed too tightly — underlayment wrapped right to the ridge with no gap, battens hugging the membrane, no counter-battens for an air channel — condensation builds up in winter. The wood rots at the bottom, the membrane grows a damp film, and one day the whole layer lets go. I have seen 15-year-old roofs fail because the ventilation was wrong, not because the slate was.

What I find on site: lift the top course and the battens below are uniformly blackened and spongy, and the membrane is beaded with condensation.
The chain of damage: moisture trapped in the system → condensation → batten and decking rot → even steel nails rust → the whole assembly loses support.
The fix and the prevention: on a new roof, always leave a ventilation channel (counter-batten or a breathable layer under the membrane); on an existing roof, check the loft boards for condensation each winter and open a few small vents in the blocked airway without puncturing the membrane.

Putting it together: the seven signs in one roof

Most failed roofs don't have just one of these — they have three or four at once. The water comes in through the short headlap, sits in the straight joint, and the rot starts at the poorly ventilated rafter. This is why I never point to a single "the" mistake. The roof is a system, and each part leans on the others.

Note: the table is a quick on-site check; the full prevention for each row is in the sections above and the linked guides.

Is your slate roof already telling you?

If you read this and think "mine has at least one of these" — it probably does, and the roof has been talking for a while. The good news: a slate roof gives years of warning before it fails. The bad news: most people don't listen until the ceiling drips. Walk around with a torch, look under the bottom course, and push a flat blade along a few joints. That five-minute inspection will tell you more than any warranty.

The other half of the equation — and the reason I'm writing this from a slate producer's point of view — is that the mistakes above are all fixable at the fixing stage, before a single slate is up. It's why at our yard we cut slates to a ±0.2 mm tolerance and inspect every single tile: so the only variable left on the roof is the hand that hangs them. If you're buying slate and want to hang it right the first time, the buying guide and the product range start you from the right place.

Questions installers ask about slate installation failures

What is the most common slate roof installation mistake?

In my experience, the wrong nail — an iron or zinc nail that rusts in a decade — is the single most common mistake, closely followed by vertical joints that line up. Both are invisible in a new roof and take years to become leaks.

Why does a new slate roof leak?

Nearly always it's not the slate. It's headlap shaved, a membrane that was laid wrong, straight joints, or details at the eaves and ridges. A slate roof with a leak within five years has an installation error, not a material one — assuming the slate itself passed the test.

Can you fix slate installation mistakes without re-roofing?

Early ones, yes. A slipped slate can be re-hooked, a single straight-joint run can be replaced, and a ventilated gap can be added at the eaves. If the underlay is bad or the battens are soft, that's a full re-roof — and that's why it's worth checking at the fixing stage.

How can I tell if my slate roof was installed correctly?

Use the signs above: check nails on the bottom edge of a few slates, measure headlap with a tape under the lap, look for straight joints, and verify the underlayment laps the right way under the eaves. A few minutes and a torch answer most questions.

Does the slate quality matter if the installation is wrong?

Yes, but in the opposite direction — good slate in bad hands lasts longer than bad slate in good hands, but both end the same way. The difference is the timeline. Quality slate (low absorption, dense, tight tolerance) gives a roof years of headroom before a mistake shows; it never cancels the mistake.

Can I trust a 30-year warranty?

Only if the installer is the one standing behind it, the materials are specified, and the warranty is detailed (what it covers, what voids it, who settles the dispute). A warranty does not fix a roof; good installation does. I've seen more warranties voided by a single straight-joint repair than by rain.

• Natural Slate Roof Installation Overview — the full order of work, where the mistakes of this page are prevented by sequence.
• Slate Headlap, Gauge and Exposure — the three measurements that stop the shortest-lap mistakes.
• Slate Layout and Vertical Joint Spacing — how to lay out joints so they never line up like soldiers.
• Roof Deck, Battens and Underlayment — the correct build of the layers that hide under the slate.
• Slate Roof Repair: Replace Broken Slates — when the mistake is already years old and you fix it slate by slate.