Walk round a street of older houses after a windy week and you will see the same thing on several of them: a slate hanging down a few centimetres, a dark gap in a course, a corner snapped off and lying in the gutter. Slipped and cracked slates are the commonest fault on any slate roof, and they range from a harmless one-off to the first sign that a whole roof is failing. This guide covers what causes slates to slip and crack, how to read the pattern, what a proper repair involves, and when a handful of repairs should become a bigger plan.
How a slate stays put, and how it lets go
Each slate is held by two nails through holes near its top, driven into the batten behind. The slate hangs from those nails and is held flat by the weight of the courses above overlapping it. Nothing else fixes it. That is a simple arrangement and a very durable one, but it depends on the nails being sound, the holes being in the right place, and the slate being strong enough to carry its own weight across the two points.
Slates let go in one of two ways. In a slip, the slate stays whole but moves down the roof because the nails have failed or the holes have torn, and it hangs from whatever is left or drops into the gutter. In a crack, the slate itself breaks, across the nail line, across a corner or along a flaw, and the lower half goes while the top half stays nailed. A slip is a fixing problem. A crack is a problem with the stone or the way it was loaded. The two have different causes and different cures, and they are worth telling apart.
The common causes of slipped slates
Corroded nails
This is the main one. Older roofs were often nailed with iron or galvanised steel, which rusts in damp air. A rusted nail swells in its hole, cracks the slate around it, and eventually snaps. Sometimes only the head rusts through, and the slate drops with the shank still in the batten. Roofers call this nail sickness. When it appears, slipped slates tend to show up across the whole roof, not in a single patch, and they come back after repair. We discuss the fixings in our comparison of slate hooks and nails.
Holes in the wrong place or torn through
If the nail holes were punched too close to the top edge or the sides, the slate can tear out round the nail under wind load. Slates re-holed by hand during a repair are particularly at risk if the hole is made carelessly.
Rotten or loose battens
A nail is only as secure as the timber it sits in. Battens that are soft with rot, split along the grain or loose on the rafters will not hold the nail, and the slate slips even though the nail looks sound. Water from earlier leaks and condensation in the roof space are the usual culprits.
Wind uplift
Strong gales lift the tails of slates and work them back and forth on their nails. A roof in an exposed position, or one with eaves and verges that lack extra fixings, can lose slates in storms even if the fixings are otherwise healthy. Tail clips or hooks help here.
Frost and movement
Water in a porous slate or behind the laps freezes and expands, lifting slates slightly. Repeated thaw and freeze cycles loosen fixings and open up gaps. Settlement of the structure can also open the courses.
The common causes of cracked slates
Impact and foot traffic
Slate is hard but brittle. A footstep, a ladder leaning on the roof, a dropped tool, a falling branch or a hailstone can crack one. Many cracked slates are the result of someone walking the roof to clean the gutter, put up an aerial or inspect a chimney, which is why we advise against it; our guide on whether you can walk on a slate roof explains the risk.
Nails driven too hard
A nail hammered right home pinches the slate against the batten. As the timber shrinks and swells with the seasons, the slate cannot move and cracks around the hole. This is a laying fault and shows up in the first few years after a new roof.
Flaws and delamination
Slate is layered rock, and some stone has weaknesses along its bedding. A slate with a hidden flaw may crack along it under normal load. If the surface is lifting in thin plates, the slate is delaminating, and cracks will keep appearing; our guide on why slates delaminate covers that failure.
Thermal stress and uneven support
Slates over an uneven batten, or over a dip in the underlay, are not supported evenly and can snap under load. Heating by sun, followed by a sudden cold downpour, adds stress to a flawed slate.
Reading the pattern on your roof
One slipped or cracked slate, close to a place where someone might have stood or something might have fallen, is an isolated event. Repair it and move on. A cluster of slipped slates in one corner often marks a local problem, such as a rotten batten, a blocked valley, or a leak that has soaked the timber. A scatter across every slope, especially of slates that have simply slid down, is the pattern of failing nails, and spot repairs only buy time.
Look at the cracks themselves. Cracks that run across the nail line suggest nails driven too tight or holes too close to the edge. Cracks that follow a straight line from the foot of the slate suggest an impact. Edges that peel in layers suggest delamination. You can check from the ground with binoculars, and from the loft by looking for daylight and for slate fragments lying on the insulation.
Photographs make the pattern easier to judge. We send them with every survey report, so you can see which slates are affected, and where, on your own roof.
How slipped and cracked slates are repaired
The repair method depends on the cause, and it is rarely just pushing the slate back up.
For an isolated slip, the slate is lifted, the old nail or its remains are cut with a tool called a slate ripper, and the slate is either re-fixed or replaced. A copper strip known as a tingle, or a stainless hook, holds the new slate from below; our page on what a slate tingle is explains that fix, and how a single slate is replaced describes the whole job. For a cracked slate, the broken slate is cut out and a matching one slipped in and secured the same way.
The new slate should match the old in thickness, size and colour. A bright new slate stands out on an old roof, and a thicker one will not lie flat. Roofers keep a stock of reclaimed slate for this reason. Where the batten beneath is soft, it is replaced or a section is fixed alongside it.
Published guides give typical UK costs of £100 to £250 for the labour of replacing one slate, with the replacement slate costing £5 to £25, and £300 to £800 for a tower or scaffold when access needs it. The figures are for orientation; the firm price follows the survey. Access dominates the cost, so batching repairs makes sense.
When repairs add up to something bigger
A roof can take repeated small repairs, but there is a point at which they stop working. If slates keep slipping in new places after each repair, the nails are failing across the roof. If the cracked slates are delaminating, the stone is the issue. If the battens crumble when touched, the roof needs stripping to fix them. In each case, the better plan is to strip and re-fix the affected slope or the whole roof, reusing the good slates with new fixings, as weighed up in our repair-or-replace guide, is it worth repairing a slate roof.
The regulatory side is worth a thought. Small repairs fall below the thresholds that need a building control application, but a large re-fix or re-cover can pass them. Your building control body confirms what applies to your job, and we handle the paperwork when the work is notifiable.
Preventing the next one
Most slipped and cracked slates are preventable with a little attention. Keep people off the roof; if the gutter, chimney or an aerial needs work, hire someone who will use a roof ladder or crawling boards that spread the load. Trim branches that overhang the slope, since they drop twigs and the odd limb in a gale. Keep gutters clear so that water does not back up under the eaves courses and soak the battens. Check the roof after severe weather from the ground, with binoculars, and look for gaps in the courses, bright new lines where a slate has gone, and slates sitting proud of their neighbours.
When a repair is made, ask for the right fixing. A slate replaced with a tingle or hook, and with new copper or stainless nails, is held more securely than one pushed up and jammed with sealant. Ask for the old slates to be checked for delamination before they are reused. And ask for the battens to be looked at wherever a slate has slipped, because a rotten batten is often the real fault and the slate only the symptom. A roof inspected every few years from the ridge, with a short written report, costs little and catches most problems while they are small.
What to do if you spot one
If a slate is hanging loose, treat it as a safety problem. It can drop onto a path or a car, and a loose slate in a gale can travel a long way. Keep people away from the area below. Do not climb up to fix it yourself; ladders resting on slates crack them and a slipping foot is a long way from the ground. Take a photograph from the ground, note the position, and call a roofer.
A leaking roof is a priority, and we prioritise calls about active leaks and will make the roof safe first. Then we plan the lasting repair for a dry day. In the loft, a torch and a bucket under any drip will limit the damage until the repair is made. We photograph what we find, quote in writing at a fixed price, and the repair carries our ten-year written guarantee on workmanship. For the wider service, see slate roofing in Basingstoke, or call 01256 213956 or send a WhatsApp message.
