The short answer
A slate slips when the nail holding it fails, most often because it has rusted through at the head. Wind, frost and a knock from a ladder or a falling branch can start it. A lone slate among sound neighbours is normally a small repair: the roofer slides a lead or copper tingle under the gap and secures the slate. A cluster of slipped slates points to nail sickness across the roof.
How a slate is held in place
A slate roof does not rely on glue or mortar. Each slate hangs from one or two nails driven through holes near its head, or sits on a hook, and the slates in the course above overlap it and press it flat. The overlap is what keeps water out and the nail is what keeps the slate where it is.
Nails are small and spend decades in damp air. Once a nail corrodes through, the slate has nothing to hold it except friction with its neighbours, and it creeps down the roof a centimetre at a time. Eventually it hangs by its lower edge or drops into the gutter. By then it has usually been letting water in for a while, which is why a slate found in the garden is worth taking seriously.
You may notice it first as a dark gap in a straight row, or as a slate that sits proud of the others. From the road it can look like a small shadow, easy to miss until you know to look for it.
Common reasons a single slate slips
The most frequent reason is a failed nail. Copper nails last far longer than iron, and the original fixings on an old roof may be neither. Next come knocks: a ladder leant on the edge, someone walking across the slates, or a bough dropped by the wind. Our guide to walking on a slate roof explains why even a careful step can crack a slate across.
Wind lifts slates that were laid with too little lap or fixed too lightly, and exposed corners and gable ends take the worst of it. Frost does its own harm, working water into hairline cracks so a slate gradually splits. If the slate looks intact but has slid, the nail is almost certainly the culprit. If it is broken in two, impact or ice is the likelier cause. Either way, the repair starts by working out which, and by checking the slates around it for the same signs.
The usual fix: a slate tingle
A tingle is a thin tongue of soft metal, lead or copper, fixed to the batten below, which carries the replacement slate. The roofer slides the strip under the course above the gap, sets the slate on it and folds the metal up over the lower edge. It is the standard method when a slate cannot be nailed in the normal way, because the neighbouring slates lock the new one in.
The job is quick, costs little and does not disturb the slates around it. Because the strip shows, it should be the right metal in a dark finish that blends with the roof. Our page on slate tingles covers the technique and when a tingle is the wrong answer. Where the slate itself is broken, the new piece should match the old for colour, thickness and size, so that the repair blends in.
When one slipped slate is a warning
One slate that has slid on a roof laid properly may be just that. Several in a patch mean something else. Look for slates sitting lower than the rest of their row, a line of small dark gaps and chips of slate on the ground. Those are the marks of nail sickness, where the fixings are failing across the roof and not at a single point.
Roofs at that stage often give years more service if the sound slates are lifted and refixed with new nails, a job that costs more than a tingle but far less than reslating. Age matters too, and our guide to how long a slate roof lasts sets out typical lifespans, bearing in mind that every one depends on the weather the roof has faced and the care it has had.
A roofer will often check the nearby slates by tapping and lifting gently, which tells them more in a minute than a long stare from the ground. A sound slate rings; a cracked one sounds dull and flat. They also look at the nail holes and the battens, because a slate that slipped once may be sitting on a rotten batten that will let go again next winter. Catching that early decides whether the fix lasts or has to be redone.
What to do while you wait for a repair
Resist the urge to climb up and nudge the slate back yourself. A slipped slate is loose, others around it may be loose too, and slate is slippery in the dry and treacherous when wet. Photograph the roof from the ground or an upstairs window, then check the loft for damp patches directly underneath.
If water is already coming in, we treat it as urgent, prioritising leaks and storm damage, and secure the roof first. Pictures sent to us often show whether the job looks like a single tingle or something wider. The survey then records your own roof in photographs, and a written quote comes after it. We would sooner recommend a modest repair that works than a large job that is not needed.
Workmanship is covered by a 10-year guarantee, given in writing. A slate repaired with the right fixing should hold for as long as the slates around it do. For the full service, see slate roofing.