The short answer
Slates slip mainly because the nail holding them has rusted through or the batten around it has softened. Wind, frost, a blocked gutter or a knock from a branch can start a slip, but sound fixings resist all of these. One slipped slate is a simple repair, while more appearing each winter point to fixings failing across the roof.
What holds a slate up
Nothing glues or bolts a natural slate down. One or two nails pass through holes close to its top edge and bite into a batten, a lath of timber that runs over the rafters. The slates above cover the nail heads, and nails plus overlap keep everything in place. Our slate roofing page describes the whole build-up.
So when a slate slips, the cause is almost always in that small assembly. Either the nail has failed, the batten has failed, or the slate has broken around the hole. The stone itself, if it is a decent grade, is not what lets go.
Below the battens sits the underlay, the sheet that catches water blown past the slates. A slipped slate on a roof with sound underlay is an inconvenience. The same slip on a roof with no underlay can put rain straight into the loft.
Rusted nails and nail sickness
The most common cause on older roofs is the nail. Many were iron or poorly galvanised steel. Moisture reaches them through the slate, and over decades they corrode, thin and snap. Stirling City Heritage Trust describes iron or poor galvanised nails corroding over time and allowing the slate to slip, which the trade calls nail sickness. For a lasting job the British Standard recommends fixings in copper, aluminium or silicon bronze.
Nail sickness tends to spread. Nails of the same age and type, driven into the same sort of timber, fail at similar times. You see one slate go, then another a few courses away, and each winter's wind and rain brings a few more down. The detail of how it develops is on our nail sickness page.
Modern roofs use copper or stainless steel nails, which do not corrode in the same way. That is the main reason a properly re-slated roof stops shedding slates.
Timber, water and weather
Batten condition is the second big cause. Timber that has been wet for years, from condensation in the loft or a slow leak, loses its grip on the nail. A nail in soft wood slides out under the weight of the slate. This often shows near eaves and valleys, where water collects.
Gutters matter here. An overflowing gutter sends water back beneath the lowest course and soaks the timbers at the eaves. A line of slipped slates along the bottom edge can often be traced to a blocked gutter.
Wind and frost start slips on roofs that were already weak. A gust lifts a slate against a worn nail, or frozen water in a crack splits the slate across the nail hole. The weather is the trigger, but the weakness was there beforehand.
Finally, there is physical damage: a falling branch, a ladder leant against the slope, or someone walking on the roof. See walking on a slate roof for why slate gives way under a boot.
What a growing number of slips means
The number and the pattern both carry information. One slate slipping after a storm, with firm slate all around it, is a repair. Slates slipping in several places, or the same area failing again after repair, means the fixings or timber there are the problem.
Metal straps dotted over the slope show earlier repairs, and our slate tingle page explains how to read them. A roof with straps in many places and new slips every winter is past the point where each slate can be treated as a separate event.
Look in the loft as well. Wet or stained battens, daylight through the slope, and slates lying in the insulation tell you more than the view from the ground. If you find a slipped slate in the garden, keep it, because its condition and the nail hole help show what failed.
What to do about a slipped slate
Do not climb up to put it back. Slate is slippery, brittle and unforgiving, and the fall risk is real. A roofer can reach the slope with a roof ladder or from scaffold, replace the slate on a tingle or a nail, and check the ones beside it.
Where rain is already getting in, the first job is to stop it. A temporary cover or a quick replacement holds the damage back while the wider picture is assessed, and leaks and storm damage are looked at before routine work.
For a single slate, the repair is small. For a pattern, the right answer may be a larger one, and our re-slating guide sets out how a full re-slate is done. After the survey you see photographs of your own roof and get a written fixed price for whichever route suits it.
If a slate has come down in the garden, keep clear of any others hanging loose above it, and keep children and pets away from the ground under the damaged slope until it has been checked.
Keep a note of dates and places as slips happen. A roofer reading that history can tell a weather event from a roof that is wearing out, and an insurer may want the same record if you later make a claim. A note such as "two slates off the rear slope, after the January gale, nail stubs rusty" is worth more than a general worry, and it is exactly what we want to hear when you ring.
