What Is Nail Sickness in a Roof — And What Do You Do About It?

What Is Nail Sickness?

Nail sickness is the corrosion of the metal fixings used to secure roofing slates and tiles. When roofs were built before the mid-20th century, slates were typically fixed with iron or steel nails — iron earlier, steel later. Both metals corrode over time, and as they do, the nails lose their holding power. Slates begin to slip — first at the tail, hanging at an angle, then eventually falling completely.

On clay tile roofs, the problem manifests slightly differently — the original nails corrode and the tiles begin to slip or rock in high winds rather than staying properly bedded.

BS 5534, the British Standard code of practice for slating and tiling, recommends non-ferrous fixings such as copper or aluminium nails specifically to avoid the corrosion failure known as nail sickness.Source: BS 5534

How Common Is It in Farnham?

Very common. Farnham has a significant stock of Victorian and Edwardian slate properties — particularly in Hale, Upper Hale and the town centre — where original iron fixings installed 100+ years ago are reaching the end of their service life. It's one of the most frequent conditions we encounter on period Farnham properties.

How to Identify It

From ground level, look for slates that are slipping or hanging at an angle. They may be hanging below the course above, or you might see them at varying angles rather than lying flat. On clay tile roofs, look for tiles rocking or displaced in the tile course. In the loft, you may be able to see daylight through gaps where slates have slipped.

If you have slipping slates on an older property and the slates themselves aren't broken or cracked, nail sickness is the overwhelmingly likely cause.

What's the Solution?

There are two options: re-nailing (strip and re-nail) or full replacement. The right choice depends on the condition of the slates themselves.

Re-nailing is the correct solution when the slates themselves are sound — they're not delaminating, porous, or crumbling. The process involves stripping the roof, inspecting each slate, re-nailing with copper fixings (which don't corrode), and re-laying. This extends the roof's life by another 40–60 years at a fraction of full replacement cost.

Full replacement is warranted only when the slates themselves are failing — widespread delamination, spalling, or porosity that means the slates won't provide adequate weatherproofing even with new fixings.

Watch Out for This Advice

Nail sickness is sometimes used as a reason to recommend full roof replacement when re-nailing is actually the appropriate solution. If you're told your roof needs complete replacement due to nail sickness, ask specifically whether the slates themselves are sound or failing. If the answer is that the slates are mostly sound, re-nailing is worth considering seriously before committing to a full replacement.

We'll always give you an honest assessment. If re-nailing is the right answer for your roof, that's what we'll tell you.

Last reviewed: 19 July 2026