A slipped slate that arrives on the ground undamaged has told you exactly what went wrong, and it was not the slate.
Two nails. That is it. Each slate is nailed through two holes and the courses above cover those nails, which is what keeps the weather off them and what makes the roof watertight.
On any roof laid before roughly the middle of the last century those nails are iron or mild steel. Some are galvanised, which buys time; the earliest are plain iron, which does not. Copper and aluminium nails are the modern answer and they are the reason a properly reslated roof outlasts the original.
The slate itself is close to permanent. A good Welsh slate lifted off a roof laid in 1890 is usually still a serviceable slate, and reclaim yards trade in exactly that. So when a roof starts shedding slates, the material has not failed. The ironmongery has.
A nail in a roof is not sitting in dry air. It is in the gap between the covering and the batten, where humidity cycles daily, condensation forms on cold nights and any water that gets past the covering runs across it.
Steel in those conditions rusts, and rust occupies more volume than the metal it came from. The nail swells, splits the timber around itself slightly, thins in the middle where the corrosion concentrates, and eventually the head parts from the shank or the shank simply fails.
It is gradual and it is invisible. The slate stays exactly where it is until the day the second nail lets go, and then it slides down the roof and stops on whatever is below it. Nothing about the roof looked different the day before.
The trade name for this is nail sickness, and the important thing about it is that it is a population effect. Every nail on that elevation went in on the same day, from the same box, and has had the same weather since. When one reaches the end, the rest are close behind.
Salt is an electrolyte. Airborne salt deposited on and around metal fixings accelerates the electrochemical process that corrodes them, which is why coastal and estuary properties everywhere in Britain have shorter fixing lives than inland ones.
Along the Dee that effect is real and it is directional. The weather face of a building takes the salt-laden wind and the sheltered face does not, so it is entirely normal here to find one elevation shedding slates while the other, on the same house, laid at the same time, has lost nothing.
That asymmetry is useful diagnostically. If slates are coming off one side only, the answer is almost certainly fixings and exposure rather than anything wrong with the roof as a whole.
It also changes what a repair should use. Refixing on a coastal roof with the same specification that would be perfectly adequate ten miles inland is how a repair here turns into a return visit.
Not every slipped slate is nail sickness, and the difference is visible in the slate that came down.
Broken in two, roughly across the nail line. Impact or foot traffic. Somebody has been on the roof, or something struck it.
Broken with the nail still through the hole. The slate has failed around the fixing, usually because it was nailed too tight or the hole was punched badly when the roof was laid. That is an original workmanship issue.
Delaminated - splitting into layers like a book. Poor-quality slate, or slate that has been through many freeze-thaw cycles while saturated. Rare in good Welsh slate, commoner in some imported material.
Intact, no nail, no damage. Nail sickness. The commonest of the four and the one that matters.
One slipped slate a year on a large roof is normal ageing and always will be. The question is whether the number is climbing.
One in 2023, two in 2024, five last winter is a curve. It is not five separate pieces of bad luck; it is the same population of nails reaching the same point together, and the next winter will be worse than the last.
That is the honest test, and it is arithmetic rather than opinion. It is also why we ask how many have come down and over what period rather than only looking at the one that came down last week.
Where the rate is climbing, stripping the slope and relaying the existing slates on new battens with non-ferrous nails resets the clock on the part that actually failed, at considerably less than the cost of new material. Ask about that before accepting a quotation for a new covering.
The rest of the series, on the questions people ask most.
Every town has its own page, written around what the roofs there are actually like. Start with the main roof repairs page.
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We cover Chester, Cheshire, the Wirral and Flintshire from Elton, just north of Chester.
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