The short answer
Nail sickness is the roofer's name for the failure of the nails holding slates to the battens. The nails corrode and weaken, then snap or pull through, and slates slide out of their rows one after another. The slate itself may be perfectly sound. Once the trouble spreads across a slope, re-slating with new fixings often costs less over the years than patching again and again.
The slates on a sick roof are often still good. It is the small fixings behind them that have run out of life.
How a nail fails
Each slate hangs on one or two nails driven through it near the head and into a batten, one of the timber laths that run across the roof. The row above covers the nail head. That overlap keeps weather off the nail, but not for ever. Damp creeps in at the laps, and iron nails rust, thin down and break.
Why do slates outlast their nails?
Slate is hard stone, and the Confederation of Roofing Contractors says it can last 100+ years if well maintained. A small nail sitting in a damp joint has very little metal to lose. When it goes, nothing holds the slate in place except friction and its neighbours, and the next gale shoves it out. Fixings in other metals do not rust in the same way, a point explored in our answer on copper nails for slate.
That mismatch explains why old roofs can look handsome from the road while their fixings are close to the end. The stone is not the weak link, and that is good news for the owner, since the slates can often be saved even when the nails cannot. A roof survey can sort one from the other, and the sorting is where the saving lies.
Signs you can see from the ground
A roof with nail sickness gives warnings before a large patch lets go. Slates slip a few millimetres down their rows, so the neat line of bottom edges turns wavy. Single slates turn up in the gutter, on the lawn or on a shed roof. Rust-brown streaks can show under the laps, and on a damp day a loft may show daylight or water stains.
Is one slipped slate nail sickness?
Not by itself. One slate lost after a storm is a one-slate job, and our answer on why slates slip covers other causes, such as a broken batten or a branch strike. The pattern gives the game away: several slates from the same slope, over a short space of time, each with a rusted or snapped nail at the hole. Photograph what you find from the ground and send it to us, with a note of the date each one fell. Keep any slate that has come down, because the broken edge shows how the nail let go and whether it rusted or simply pulled through.
Patching or re-slating
A roofer can mend a slipped slate with a tingle, a narrow lead or copper strip, or with a hook, and a handful of repairs can keep a sound roof going for years. The difficulty is that every slate on a slope hangs from nails of one age. Working on one disturbs its neighbours, and a few more drop as you go. A tingle holds the new slate at its tail with a folded strip, and a hook does much the same job, but neither does anything for the nails in the slates around it.
When does re-slating become the better buy?
When repairs keep returning to the same slope, or when the survey finds corroded nails in several places. The choice is then between paying for call-outs one at a time and stripping the slope, sorting the sound slates and fixing them again onto fresh battens with new nails. If the slates are in good order, that keeps the roof's appearance and reuses stone you already own. Our guide to re-slating a roof shows what the work involves.
How we test a roof for it
A few lifted slates reveal a great deal. At the survey we raise a sample from each slope, usually near the eaves, the middle and the ridge, and examine the nails and the batten behind them. You see the photos. Where the nails are sound and the damage is a one-off, we recommend repairs. Where the nails crumble in the hand, we explain what re-slating would involve and why. Sometimes the answer is mixed: a sound slope stays, and the worst one is re-fixed first, so the work can be spread over more than one season.
What do I get in writing?
It is a single total in writing, given after the survey, for the work we have agreed with you. Read about the full service on our slate roofing page, then send a photo of the slipped slates so we can arrange a survey that tests the nails properly. Keep any fallen slates safe, because a broken edge tells us how the nail let go.
More on failing nails
Can nail sickness be fixed without re-slating?
Only if it is caught early and limited to a small area, such as one corner of a slope where a downpipe overflowed for years. A few repairs on an otherwise sound slope are sensible. If nails across the slope are failing, repairs only delay the point at which the slope needs stripping, and the cost of repeated visits can add up.
Will my insurance cover slipped slates?
The wording of your policy and the reason the slates fell decide that. If water is coming in, ring the insurer before any permanent work, put a temporary cover on, and hold on to dated photographs and every receipt. Wear and tear is often treated differently from storm damage, so read your policy wording or ask your insurer how it sees a slipped slate.
The full picture: Slate roofing in Taunton ยท More short answers