Nail Sickness

Nail sickness is a long-established British and Irish term for the failure of the fixings on a slate or tile roof. Iron or poorly protected steel nails corrode, their heads rust off, or the shank thins just above the batten or deck, so slates slide out of position or fall. It can occur while the slates themselves still have many years of life left.

Causes

• Nail material. Plain iron, steel cut nails, steel wire nails and lightly zinc-coated nails tend to corrode. Copper, stainless steel and some high-quality alloy nails usually outlast the slate; US slate specialists report that copper and stainless steel slating nails are typically found in good condition on older roofs.
• Moisture concentration. Corrosion is often worst where water collects: below valleys, around dormers, or where a low-slope roof drains onto the slates.
• Condensation under unventilated roofs.
• Ageing timber battens that lose their grip on the nail.

Signs

The most visible symptom is individual slates slipping down the roof or lying loose in gutters. Rows of slipped slates, rust staining, and slates that lift easily in wind all suggest widespread nail failure.

Repair options

• Isolated slips can be refixed with metal strips known as tingles or slate hooks, typically copper, lead or stainless steel, nailed between the slates of the course below and bent up to hold the slate. These are a recognised repair but are visible and are generally treated as a short- to medium-term measure on heritage roofs.
• Widespread nail sickness usually calls for stripping the roof, salvaging the sound slates, replacing battens or underlay as needed, and relaying the slates with copper or stainless fixings.

When reinstalling, slaters keep exposure and headlap consistent. Slate specialists note that reducing headlap to gain exposure is not acceptable because it weakens weather resistance. Heritage authorities generally advise like-for-like repair and expert advice on listed or historic buildings.