Nail sickness is the trade name for a slate roof whose fixings have given out while the slates themselves are still good. A slate is pinned at its head, with a nail or two passing through into the batten beneath. Over a long life those nails corrode, thin and snap, or the hole around them wears, and the slate drops a little at a time until it slides out. The first slipped slate looks like bad luck. A second and third in different places is a pattern. By then the cause is in every nail on the roof, hidden under the slates above, and patching one slate at a time stops working. This guide explains how to recognise the condition, what a repair can and cannot do, how to tell nail sickness from other causes of slipping, and why it so often ends in a re-slate. It suits anyone with an older slate roof who has started finding pieces of slate on the lawn.
What nail sickness is, and why it happens
A slate roof is held by very little. Every slate is pinned by two nails through holes near the head, and the weight and overlap of the courses above keep the water out. The nails do all the work of keeping the slate up, and they do it in the dampest place on the roof, in the narrow gap between slate and batten where moisture lingers.
On older roofs the nails were commonly iron or steel, sometimes galvanised and sometimes not. Damp, condensation and the slow action of rainwater attack them. The shank thins, rust swells inside the hole and can crack the slate around it, and eventually the nail snaps or lets go. Timber battens add to the problem: when a batten is wet for long periods it holds moisture against the metal, and old battens can be too soft to grip a nail that is still sound.
The slate often has plenty of life left. Welsh slate in particular can outlast several sets of fixings, so a roof can seem fine from the pavement while quietly coming apart. The phrase nail sickness points at the metal, not the stone.
Why do the slates not simply fall at once? Because each one is held by friction and by the slate above it. A slate with a broken nail may stay in position for a long time, then slide in a storm or a hard frost. That is why the first sign often comes after bad weather. It also means that a roof with many failed nails can still look intact, with its trouble spread across the whole area rather than concentrated in one spot.
Warning signs outside and in the loft
Looking for the pattern needs no ladder. These are the signs we find most telling, and the view from inside the roof space can say more than the garden does.
Outside
- Slipped slates. A slate sitting a few centimetres lower than its neighbours leaves a gap at the tail. A row of them, or slates slipped in different places on several slopes, points to fixings rather than a single knock.
- Fallen slate. Whole slates or broken pieces beneath the eaves, among the plants or lying in the gutter are the commonest first warning.
- Patches of tingles. Small metal strips showing at the tails of slates mean someone has already repaired slipped slates. Lots of them is evidence that the nails are failing generally, not that the roof has been unlucky.
- Rust streaks. Orange-brown staining below the nail position can come from corroding nails. It can also come from iron inclusions in the slate itself, so look at where the streak starts. The difference is explained under rust stains on slate.
- An uneven roof line. Courses that look wavy, with the tails no longer in straight lines, suggest slates have moved.
In the loft
- Daylight showing through the roof where a slate has gone.
- Water staining on rafters, battens or the felt.
- Rust-coloured dust or small broken nail shanks on the ceiling insulation below, which suggests nails are disintegrating above.
- Soft, dark battens that crumble when pressed with a screwdriver.
None of these signs alone is proof. Together they build a picture, and a survey from the roof itself settles it.
Telling nail sickness from other causes
Slates slip for several reasons, and the remedy depends on which one you have. Getting the diagnosis right avoids both over-spending and under-fixing.
A single impact. A fallen branch, a ladder against the roof or a person walking on it can crack one slate or knock a few loose. Everything else stays firm. Putting right the affected slates is all that is needed, using the methods in slate roof repairs.
Wind. A storm can lift slates on an exposed edge, around a hip or along a verge where the fixings are weaker, while the main slopes stay put. That is usually a localised repair.
Delamination. If the slates are flaking into layers and breaking up, the slate itself is failing, which is a different problem and a different conclusion. It generally argues for replacing the slate and not just the nails.
Frost and moisture in the batten. Rotten battens let go of sound nails. The cure is new battens, and it goes together with re-fixing.
Nail sickness. The slates are in reasonable condition, the nails are rusted or snapped, and the problem recurs across several slopes. Here a repair that treats one slate at a time is only a delay.
A simple field test helps. We lift a slate carefully in a few areas, not just where one has slipped, and examine the nails and holes. If sound nails are found everywhere, localised repair is reasonable. If most of the nails we expose are thin, snapped or rusted through, the condition is general. Sampling several slopes is important, because a south-facing slope in the sun and a damp north slope can age very differently, and the answer on one does not carry to the others.
What a repair can achieve
If nail sickness is caught while most of the roof is still sound, repair buys time and costs little. It is worth understanding both what it does and where it stops.
The standard fix for a single slipped slate is a tingle. You cannot nail a replacement because the nail position is hidden under the course above, so the replacement is pushed up into its course and held by a copper or lead tongue screwed or nailed to the batten below and folded up to catch the tail. Done neatly, it is nearly invisible. The cost is modest. Typical UK ranges from the MyBuilder repair guide (2026) put a minor job, a few slates for instance, at £100 to £300, with a moderate job at £500 to £1,000. A fixed price follows the survey.
Tingles are a stop-gap by nature. They hold the slate, but they do nothing for the nails in the courses around it. Each repair is a small admission that more will follow. Where a few slates have gone in one area, repairing them while checking the neighbouring nails is sensible. Where slates have gone in many areas, the cost of repeated visits and scaffold begins to exceed the cost of a proper solution.
Some roofs can have the nails renewed without a full strip. A slater can lift a section, re-fix with a new nail or a hook, and relay the slates on the same battens. This works for a limited area, where the battens are sound and the slates are in good order. It also needs care, because lifting slates can crack the ones next to them.
The strength of the repair depends on the fixing. A tingle holds a slate by the tail, and hooks or clips hold it at the bottom edge, while a new copper or stainless nail holds the head. Our page on slate fixing methods compares them.
Why it so often ends in re-slating
The reason is mechanical, and it is worth setting out so that a recommendation to re-slate does not sound like a sales line.
Every nail on a slate roof sits under the slate above. To renew it you must lift the courses, and lifting slates breaks some of them. On a roof where all the fixings are weak, you would be lifting, re-fixing and relaying everything, which is the work of a re-slate under another name. At that point it makes sense to strip the roof, sort the slates, renew the battens and underlay as needed, fix the slates properly, and re-lay the roof.
Re-slating does not mean losing the slate. Much of the old covering can usually return to the roof once it has been sorted. We separate sound slates from cracked, soft or flaking ones, and make up the shortfall with matching reclaimed slate, as explained in our guide to reclaimed slate, or with new slate where it is appropriate.
Size of job also matters for the paperwork. A re-slate reaches well past the small-job thresholds, so Building Regulations approval comes into play and the thermal performance of the roof structure may need improving. Weight can trigger approval too: the line is a 15 per cent increase, and beyond it a structural check is added. The Planning Portal sets out those rules at re-roofing and Building Regulations, and where work is notifiable we handle it.
If your home is listed or lies within the conservation area, settle the consent position before booking anything, using our notes on listed slate roofs.
Replacement fixings, and the alternatives to relaying slate
The way a roof is re-fixed decides how long it will hold, so the choice of nail and method deserves attention.
The standard replacement is a corrosion-resistant nail, usually copper or stainless steel. Each slate is nailed twice, through holes punched at the correct distance from the head. Copper nails are the traditional choice and stainless is common now. Whichever is used, it should outlast the slate, which is the purpose of the exercise. A cheap or galvanised nail on a new roof can repeat the problem.
Hooks and clips offer another route. They secure the slate at its lower edge, so the slate does not depend on a hole at all, and they are often used where the head of an old slate is damaged. On a protected roof, though, visible fittings may not be acceptable, and nailing is normally preferred.
Another option homeowners raise is a man-made slate. It is lighter and cheaper to buy, but it comes with pitch limits, a different appearance and, on a protected building, the need for consent. The trade-offs sit in our man-made slate guide. For a roof with sound natural slate and failed fixings, keeping the slate is nearly always the better answer, because the slate is the part that has not worn out.
The survey is how this is decided. We go up, lift slates in several places, photograph the nails and battens we find, and show you the evidence. Either route, repair or re-slate, comes back as a written fixed price, so the two can be set side by side. Our slate roofing page describes the rest of the work, and the guarantee on our workmanship is written and lasts ten years either way.