Question answered
What is nail sickness on a slate roof?
The short answer
Nail sickness is the failure of the nails that hold slates to their battens, usually through corrosion. The slates themselves can still be sound, but they slip because the fixing at the top has rusted or snapped. A few slipped slates can be repaired. Once slates are slipping across a whole slope, the roof normally needs stripping and re-slating.
Slates often outlast the nails that hold them. Nail sickness is what happens when those fixings rust through, and it is a common reason for an old slate roof to start shedding sound slates.
Why a good slate falls off a sound roof
A slate is simply hung. One nail is driven through a hole close to the head of the slate and into the batten, while the courses above and below overlap it. Gravity does the rest, and the nail carries the load day after day, through every gale and every frost.
Slate itself is a dense, long-lived stone, and the fixing is the weaker part. When a nail rusts, its shank thins, then snaps at the head or just below the slate. The slate may stay where it is for a while, held by friction and the courses around it, then slides down a few centimetres in a blow. From the ground it simply looks like a sound slate that has dropped out of line.
So a roof whose covering is good can still need attention. The material is fine, the fixing is not. Our guide to slate tingles explains the repair for a few such slates.
What causes the nails to fail
Corrosion is the usual cause, and it comes down to the metal used and the damp around it. Older roofs can carry plain iron nails that were never made to last for a century, and a nail wets and dries every time it rains. Condensation in the roof space can work from underneath, especially where the underlay is old or the loft is poorly ventilated.
The size and kind of nail also matter. Some roofs carry nails that were too short or too thin from the day they were laid. Where a roof was re-slated in the past with a cheaper fixing, that choice shows up years later.
Movement does harm too. A slate that rocks in the wind works the nail loose in its hole, and a nail loose in rotting timber has nothing to grip. How fast a given metal corrodes depends on the nail and its surroundings, so we pull a few and look at the actual nails on your roof.
Signs on the ground and in the loft
From the pavement or garden, look for slates that have dropped below the ones beside them, breaks in the straight lines of the courses, and pieces of slate lying in a gutter or on an extension roof. After a gale, a few on the path are a warning, not just bad luck.
In the loft, with a torch, check the underside of the slope. Rust-coloured nail points, short stubs of nail in the batten and daylight through the covering are all clues. Fallen slates leave dark, wet patches on the underlay and stained timber below. You can also find broken nail heads on the loft floor, small and rusty.
We would rather you stayed off the roof and used binoculars or a phone camera from the ground. Our guide to slate roofs in conservation areas shows why older roofs on protected streets often fall into this pattern.
Patch it or strip it: how the call is made
The decision is about extent, not age. If the failure is confined to a few slates, perhaps near a gutter or a damp valley, repairs with tingles or replacement slates may be enough. The roof is then kept, and the work is quick.
If slipped slates appear in many places, or when we lift a few slates and the nails are thin and rusted throughout, patching will cost more over time than it saves. Every repair disturbs neighbouring slates, and each disturbed slate is another weak fixing. At that point the sound move is to strip the slope, check the battens, fit new fixings and relay the same slates where they are in good order.
We show you photographs of the nails we pull out, so you see the evidence, and a written quote then follows from what we have seen. The slate stays, if it can, because it is often worth more than the nails. Our broader slate work is set out on the slate roofing page.
Fixings for a re-slate, and what to ask any roofer
When a roof is relaid, the fixings should be chosen deliberately. Ask what material and size of nail or hook is being used, whether the battens are being renewed, and what the underlay is. If you want to compare approaches, the discussion of slate hooks or nails sets out when each is used.
Pitch and exposure feed into the fixing choice as well. A shallow roof needs a generous headlap, and the limits are on our page about the minimum pitch for slate. Cupa Pizarras says hooks are unsuitable on any pitch under 25 degrees.
Ask for the fixings in writing as part of the quote. Someone who is happy to name them has planned for the long term, not just for this winter.
Nail sickness: two more questions
Can nail sickness be cured by treating the roof?
No coating or treatment restores a rusted fixing. The nail has to be replaced, either slate by slate or by relaying the slope. Cleaning moss off does help a roof stay drier, but it does nothing for nails that have already thinned.
Are slates worth keeping if the nails have failed?
Often yes. The stone is usually the best part of an old roof, so sound slates are lifted, sorted and relaid with new fixings, and replacements match any that are cracked or flaking.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.