Nail sickness is the corrosion of the iron nails holding Welsh slate to the roof, common on Bath's Victorian terraces built between 1850 and 1910. When nails fail, slates slip even though the slate itself is sound. Once failure passes roughly 30 percent across the roof, full re-slating becomes more economical than ongoing repair.
The Slow-Motion Emergency
Nail sickness is the most common roofing problem on Victorian and Edwardian properties across Bath, and it has one characteristic that makes it particularly costly: it progresses gradually and almost invisibly until the point of failure, when multiple slates begin to slip simultaneously. At that point, what could have been a planned, scheduled re-slating job becomes an emergency repair, often combined with internal water damage that would not have occurred if the problem had been caught six months earlier.
The good news is that nail sickness gives warning signs, and a homeowner who knows what to look for can identify the problem while it is still in the “repair by appointment” category rather than the “call on a Sunday evening” category.
NFRC technical guidance: NFRC technical guidance identifies nail fatigue as the primary failure mode on slate roofs over 80 years old — placing most of Bath's Victorian housing stock within the at-risk window.
What Causes Nail Sickness
Victorian Welsh slate roofs across Oldfield Park, Bear Flat, Widcombe, Bathwick, and the surrounding suburbs were fixed using wrought iron cut nails driven through two holes near the head of each slate. These nails had a service life of approximately 80 to 120 years in typical UK conditions. Bath's above-average rainfall and the higher ambient moisture in the Avon valley mean the lower end of that range is more common here than in drier parts of the country.
As the iron corrodes, it expands slightly within the slate hole. The expansion widens the hole, reducing the nail's grip on the slate. At first, the slate is held partly by the nail and partly by the weight of slates above bearing down. As more nails fail, the distribution of load shifts entirely to the remaining intact nails, which then fail faster under the increased stress. This is why nail sickness accelerates: it is self-reinforcing.
Early Warning Signs
The following can all be assessed from ground level with a pair of binoculars:
1. Slates That Have Slipped but Not Fallen
Look for slates that are visibly displaced — sitting lower than their correct position, or at a slight angle to the surrounding slates. A slate in this position has lost nail grip but is still being partially held by adjacent slates or its own weight. It is a reliable indicator that the nails on surrounding slates are also at or near the end of their life.
2. Irregular Gaps in the Tile Pattern
Look along each course of slates for any gap or irregularity in the pattern. Slates that are visibly darker in colour than their neighbours may indicate water has been entering the gap during rain events even if the slate itself has not fully displaced.
3. Rust Staining Below Fixing Points
On properties where the nail holes are on the upper third of each slate (head nailing), look for faint vertical rust staining tracks running down the face of individual slates from the nail position. This indicates the nail is already corroding and the oxidised iron is being washed down the slate surface in rainfall.
4. Slates Found on the Ground After Wind
Any slate discovered on the ground or in the garden after a period of strong wind was sitting on failing nails. A single fallen slate is not necessarily alarming; two or more from the same elevation in the same weather event indicates the nail bed across that section is in advanced failure.
5. Moss Growing in a Line Along a Course
Heavy moss growth in a regular horizontal line across a roof slope, rather than general surface growth, often indicates a gap between slate courses where moisture is accumulating. This can be caused by slates that have displaced fractionally — not enough to be obviously visible from the ground, but enough to create a moisture trap that feeds persistent moss growth.
What to Do if You Spot These Signs
A condition assessment from a roofer is the next step. This should involve a physical inspection from scaffold or access tower — not just a walk around the garden. The roofer should assess the proportion of nails that are failing across each elevation and give you a recommendation on whether targeted repair or full re-slating of the affected elevation is the right approach.
The key question to ask is: “What percentage of the nails on this elevation are failing?” If the answer is below 20 percent, targeted repair with new slates and copper nails is viable. If the answer is above 30 percent, full re-slating of the elevation is almost certainly better value over a 10-year horizon.
Timing matters: A re-slating job planned and scheduled in spring or summer costs significantly less and is completed faster than the same job carried out as an emergency in November. The scaffolding is the same, the slates are the same — the only difference is the urgency premium and the internal damage that may have accumulated if the problem was allowed to develop further.
What to Do If You Suspect Nail Sickness
The first step is a physical inspection by a roofer who knows what they are looking for. Ground-level binocular inspection can identify displaced slates but cannot assess the percentage of failing nails across an elevation — that requires someone on the roof surface, probing nail holes and testing slate grip across a representative sample. We carry out nail sickness assessments as a standalone service, providing a written report that quantifies the percentage of failing nails on each elevation and gives a clear recommendation on whether targeted repair or full re-slating offers better long-term value.
The False Economy of Continued Repair
The most common mistake on nail sickness-affected Bath properties is years of incremental repair: a few slates replaced each year as they slip, at 150 to 300 pounds per visit. This approach feels economical year by year, but the cumulative cost over a 5-year period on a progressing nail sickness elevation typically exceeds the cost of re-slating the elevation in one job. Worse, each repair visit disturbs surrounding slates and accelerates the nail failure in adjacent tiles. A roofer who recommends continued repair on a 40 percent nail sickness elevation rather than re-slating is not giving honest advice.
Nail Sickness in Bath's Conservation Areas
For properties within Bath's conservation areas and World Heritage Site, nail sickness re-slating must use matching Welsh slate from appropriate sources. The planning authority requires like-for-like replacement on visible elevations — modern alternatives such as fibre cement or synthetic slate are not acceptable on period properties within the conservation zone. We source matching Welsh slate for all Bath conservation area re-slating work and can provide material sourcing documentation for listed building files.
Last reviewed: 19 July 2026