Nail Sickness On An Old Slate Roof

Key takeaways

• The slates are usually fine. It is the nails that have failed, which is why the pattern is scattered rather than local.
• Slates slipping from several different parts of the roof at once is the signature.
• Iron and steel nails corrode. Copper and stainless do not, which is why the original nail choice decides the roof's lifespan.
• It cannot be repaired away, because every remaining nail is the same age as the ones that have already gone.
• Stripping and relaying with the original slates is often possible and much cheaper than new slate.

What is actually failing

Natural slate is one of the longest lived roofing materials there is, and good Welsh slate can outlast the building it was laid on. The nails holding it usually cannot. Historically slates were fixed with iron or mild steel nails, sometimes galvanised, and those corrode steadily in a damp roof space over decades until the shank is thin enough to snap under the weight of the slate. At that point the slate slides down the roof. Nothing has happened to the slate, which is why a roof suffering nail sickness produces a pile of perfectly reusable slates on the ground. The industry term is nail sickness or nail fatigue, and it is the single most common reason a slate roof reaches the end of its life.

The scattered pattern that identifies it

This is what separates nail sickness from ordinary damage, and it is visible from the street. Storm damage is local: one area, usually an edge, with the rest intact. Nail sickness is distributed: a slate missing near the ridge, another two thirds of the way down, a third over by the chimney, appearing over months or years with no relation to each other. If you find yourself calling somebody out to refix a slate two or three times in as many years, in different places each time, that is the diagnosis. Each individual repair is genuinely done properly and the next failure is somewhere else entirely, because what they have in common is age rather than location.

What it looks like from the ground

Photograph the roof with full zoom on a bright day and look for three things. First, gaps where a slate is missing, and note whether they are scattered rather than clustered. Second, slates that have slid partway and are sitting at an angle or resting on the course below, which are failures in progress. Third, and most usefully, look for a stepped or ragged line along the bottom edge of the courses, because slates that have slipped slightly without falling produce a subtly uneven shadow line that reads clearly at an angle. Also look in the gutters and on the ground for slate fragments, and for the rusty tips of old nails if any are visible where a slate has gone.

The other failure that looks the same

Battens fail too, and the symptoms overlap. If the timber battens have rotted, slates come away in the same scattered fashion, and the two often go together since the conditions that corrode nails also rot timber. There is a third possibility on some roofs: the slates themselves delaminating, which happens with poorer quality slate that has absorbed water and split into layers, and shows as slates that look flaky, swollen at the edges or that break when handled. A proper inspection distinguishes these because the remedy differs in cost. Nail failure with sound slate and sound battens is the cheapest outcome. Rotten battens mean more work. Delaminating slate means the material itself has to be replaced.

Why patching stops being sensible

There is a point at which individual repairs become false economy, and it arrives sooner than most owners expect. Every visit carries an access cost, usually a tower or a scaffold, which dwarfs the price of a slate and a nail. Three or four callouts across two winters will typically cost a meaningful fraction of a proper job, and at the end of it the roof is in exactly the same condition as before, with every remaining nail one winter older. The honest threshold is roughly this: once slates are coming off from multiple areas and the roof is beyond about eighty years old with its original fixings, further patching is buying time rather than solving anything, and it is worth pricing the alternative.

Stripping and relaying, the option people do not know about

Because the slates themselves are usually sound, they can often be taken off, sorted, and put back on new battens with new fixings and new underlay. This is called stripping and relaying, and it costs considerably less than a new slate roof because the most expensive material is already on site and paid for. Expect to lose a proportion of the slates to breakage and previous damage, typically somewhere between ten and twenty five percent, which is made up with reclaimed slates of matching size and colour. The result is a roof with its original character and appearance, on a modern breathable underlay with stainless or copper fixings that will not repeat the problem. On a period property, or in a conservation area, this is frequently both the cheaper and the more appropriate answer.

How long you have once it starts

This is not a sudden failure and there is usually no need to act within days. A roof entering nail sickness typically gives years of warning, with the rate of slipping gradually increasing. That said, each missing slate exposes the underlay beneath, and on a roof of that age the underlay is often the original bitumen felt which has become brittle and torn. Once the felt has gone, a missing slate is an open hole. The practical guidance is to stop paying for repeated patch repairs, get the roof properly assessed including a look at the felt and battens from inside the loft, and plan the work rather than being forced into it by a January storm.

Questions worth asking before committing

Ask what proportion of the existing slates the roofer expects to be reusable, and how that estimate was reached. Ask what will be used to make up the shortfall, and whether it will match in size, thickness and colour. Ask what fixings will be used, since stainless steel or copper is the whole point of doing the work. Ask whether the battens are being renewed and what grade. Ask about the underlay specification and how ventilation will be handled, because a breathable membrane in an unventilated roof brings its own problems. And ask to see the roof from inside the loft during the quote, because a roofer who has not looked at the underside is quoting on a guess. Ask too whether the existing slates will be sorted on site, since that sorting is where the reusable proportion is actually decided, and a generous estimate given before anything has come off the roof is worth treating with caution.

Frequently Asked Questions

What is nail sickness on a roof?

It is the corrosion of the nails holding slates onto the battens, to the point where they snap and the slate slides off. The slates themselves are usually undamaged. It is the most common reason a slate roof reaches the end of its serviceable life, and it typically shows up after roughly eighty to a hundred years with original iron or steel fixings.

How do I know if it is nail sickness or storm damage?

Look at the pattern. Storm damage is concentrated, usually at an edge, and happens on a known date. Nail sickness is scattered across different parts of the roof and appears gradually over months or years. Repeatedly refixing slates in different places each time is the clearest sign.

Can nail sickness be repaired?

Individual slates can be refixed, but it does not address the cause, because every remaining nail is the same age as the ones that have failed. Patching buys time. Once slates are coming away from several areas, stripping and relaying is usually the better use of money.

Do I need all new slates?

Usually not. Natural slate commonly outlasts its fixings, so the existing slates can often be lifted, sorted and relaid on new battens with new nails and underlay. Expect to lose roughly ten to twenty five percent to breakage, made up with matching reclaimed slate. It costs considerably less than a full new slate roof.

What nails should be used when the roof is relaid?

Stainless steel or copper, which do not corrode in the way the originals did. This is the entire point of the exercise: reusing the slates on modern fixings gives a roof with its original appearance and a service life measured against the slate rather than against the nails.

How urgent is nail sickness?

Not urgent in the sense of days, since it develops over years with gradually increasing frequency. What raises the priority is the underlay: on a roof of that age it is often original bitumen felt that has become brittle, so each missing slate may be an open hole rather than a covered one.