Why Do Slates Slip or Crack?

Every slate is held by nails passing into a batten, the timber lath running across the rafters, and those nails carry the load for decades. Over time poor or iron nails rust, snap or lose their grip in the timber. The slate then drops a little, wind lifts it and it slides out.

What does nail fatigue look like?

It starts with slates sitting slightly low, their tails out of line. Then slates begin to fall one at a time, often within the same patch of roof, and the pieces in the gutter are whole, not cracked. Roofers call the pattern nail sickness, and our answer on nail sickness on a slate roof goes into it.

The slates on one slope were all fixed together, with nails of one age, so replacing the slipped ones leaves the rest waiting their turn. When the pattern is widespread, a re-slate with better fixings can be the wiser spend than a string of call-outs. Copper and stainless nails resist corrosion far longer than plain iron, and the choice between them is worth discussing before any re-slate begins.

Battens matter here too. A rotted lath cannot grip a nail, however good the nail is, so the timber behind a slipped slate gets checked at the same time.

Water finds its way into tiny faults in a slate, and when it freezes it expands and opens the fault further. Over many winters a hairline becomes a split, and a split becomes a broken slate. Exposed roofs and slopes that face north, which stay wet longer, suffer most.

Is frost damage down to the roofer?

Not usually. It is the stone. A good slate with few faults resists frost well, and a poor one gives way. That is why quality counts when slate is bought, and why one slope can lose slates while the other stays sound. Our guide to frost, wind and slate covers the season in more detail.

A frost-cracked slate often breaks along its length or loses a corner, and the pieces stay near the roof. One that cracks but stays in place can leak long before you spot it, since the course above hides the break.

If your slates are also peeling in layers, the stone has a different fault, and our answer on flaking slates deals with it. Frost tends to attack the same slopes year after year, so the damage often repeats until the weakest slates are replaced.

Slate cracks under pressure. A boot, a ladder foot, a dropped tool or a branch in a gale can break a slate that was sound the day before. Fixing a satellite dish or clearing a gutter from the roof are common causes.

Can a ladder crack slates?

It can. A ladder leaned on the slates, instead of on a stand-off bracket or a firm gutter fixing, puts a point load on the eaves slates and may snap them. Roofers use a stand-off or tie the ladder to a suitable point, and work from a roof ladder when they must go higher.

This type of damage is the most avoidable of all. It ties in with our answer on walking on a slate roof, which explains why slate breaks and how to inspect without climbing.

If you suspect someone has been on your roof, look in the gutter below for pieces. A prompt look finds a cracked slate before it leaks, and the repair stays small.

Some slates are simply weaker. Thin slates, those with hidden faults and those that were poorly graded break more easily and slip more often. A roof of weak slate shows cracks everywhere, even where nobody has walked.

How do I find out which cause is at work?

A roofer looks at where the failures sit, whether the nails are rusted, whether the pieces are whole or broken and what the slates feel like in the hand. A few failures scattered over a roof of sound stone are bad luck. A cluster in one area is a pattern.

For scale, MyJobQuote gives around £215 as the average for replacing roof tiles (Oct 2026), a rough guide to small repairs, though slate depends on access and matching. Larger jobs follow the survey.

Our slate roofing service looks at the slope, shows you pictures of the nails and slates, and prices the repair or the re-slate in one written total. Your own count of how many slates have gone, with a picture of the gap, gives us a useful head start.

It also helps to note when each slate went. A cluster after one gale suggests wind found a weak spot, while slow, steady losses over a year or two suggest the fixings are giving up across the slope. That distinction changes the advice, and it costs nothing to record.