Nail sickness is the corrosion of iron roofing nails over 80 to 130 years, causing slates to slip as the fixings fail. It is the single most common roofing problem on Southampton's Victorian terraces in Portswood, St Denys and Bevois Valley. Once 30% of slates are affected, full re-slating with stainless fixings is the economical fix.
If you own a Victorian or Edwardian property in Southampton — particularly in Portswood, St Denys, Bevois Valley, Highfield or the streets around Southampton Common — nail sickness is the roofing issue most likely to affect your home. This guide explains exactly what it is, how to recognise it, and what the fix costs.
What is Nail Sickness?
Welsh slate, which was the standard roofing material on Victorian and Edwardian properties built between roughly 1860 and 1920, was fixed to the roof using iron nails. Over 80 to 120 years, those iron nails corrode. As they corrode they expand within the hole drilled through the slate. This expansion eventually cracks the slate at the nail hole — which is why slipping slates on old roofs often have a characteristic crack at the top rather than breaking randomly. Once the fixing fails, the slate slides down and either exposes the underlay or falls off entirely.
Iron cut nails used in Victorian construction have a typical service life of 80–120 years before corrosion-driven failure, per Lead Sheet Association and heritage roofing guidance.
Source: Lead Sheet Association / heritage guidance
Why is it so Common in Southampton?
Southampton's coastal climate — persistent humidity from the Solent, above-average rainfall, and salt-laden air in waterside areas — accelerates the corrosion of iron fixings compared to inland towns. The Victorian housing stock in Portswood, St Denys and Bevois Valley was built at the height of the Welsh slate era and has now been standing for 100 to 140 years. The iron nails used in 1890 were always going to fail eventually — they are simply reaching that point now across a significant portion of Southampton's older housing.
How to Recognise Nail Sickness from the Ground
You can get a good indication of nail sickness without going on the roof. Stand back and look at the roof slope. Signs to look for include: slates that have dropped below the line of the surrounding slates, slates that are sitting at an angle rather than flat, gaps where slates have fallen off completely, and a general unevenness in the roof surface that was not there a few years ago. If you are seeing one or two slipped slates, the problem may be contained. If you are seeing displaced slates across multiple sections, nail sickness is likely widespread.
How Much Nail Sickness Can Be Repaired?
A roofing survey will give you a percentage of affected slates. The decision point is typically around 25 to 35 percent affected. Below that level, targeted re-fixing of affected slates using modern aluminium or stainless steel clips — which do not require drilling a new hole — can extend the roof's life cost-effectively. Above that level, the number of remaining sound fixings is insufficient to hold the roof together over the next 10 to 15 years, and re-slating becomes the right call financially.
What Does Re-Slating Cost in Southampton?
Re-Slating Costs — Southampton Victorian Terraces
These costs include new Welsh slate (or equivalent), breathable underlay, all new stainless fixings, replacement of any damaged battens, and new ridge tiles in flexible bedding. They do not include scaffolding erection costs, which typically add £500 to £1,200 depending on access.
What Happens if Nail Sickness is Left Untreated?
Progressive slate loss exposes the underlay beneath. Modern breathable underlay handles short-term exposure reasonably well. Original bitumen felt — still present on many Southampton properties — degrades rapidly once exposed. Once the underlay fails, water enters the roof space directly. Sustained water ingress damages ceiling joists, rafters, plasterboard and insulation. What starts as a re-slating job becomes a re-slating job plus internal repairs.
Worried About Nail Sickness?
We carry out free roof inspections across Southampton. We'll tell you honestly what percentage of your slates are affected and whether repair or re-slating is the better financial decision for your property.
Call 0238 184 9628Should I Use Original Welsh Slate?
Where the property is in a conservation area — parts of Portswood and Highfield — or where matching the existing roof is important, we recommend sourcing genuine Welsh slate. Alternatives such as Spanish slate or fibre cement slates are available at lower cost, but the colour and texture differences are noticeable on period properties. On a Victorian terrace in a conservation area, using cheap imported slate can create planning issues.
Last reviewed: 20 July 2026 by Southampton Roofing Experts