The short answer
Tiles slip because whatever holds them has let go. The usual causes are corroded nails, broken nibs on the tile's back, softened battens and frost cracking, with wind often giving the final push. One slipped tile after a storm is a small job. Tiles that keep dropping across one slope point to failed fixings beneath, which need a different repair.
The four common reasons a tile or slate slides out of line, and how to tell a one-off slip from a roof that is giving way.
Nails that have given up
Slates and plain tiles hang from the battens on nails, and nails do not last forever. Where an old iron or steel nail rusts through, the slate it holds loses its grip and slides down until it catches on the course below. Roofers call this nail sickness. The nails look fine from the surface, because the damage is at the head and shank, hidden under the overlapping slate.
A typical sign is a scatter of slates dropping out across the same slope over a few seasons, often with a gap that fills in with a bit of daylight in the loft. One slipped slate can be put back on a tingle, a small strap of metal that hooks over the batten and holds the slate's tail. If the nails are failing everywhere, strapping them one by one is a short-term answer, and the real fix is re-fixing the slope on new nails. Our answer on nail sickness in a slate roof shows what that involves.
Check whether fallen slates or tiles lie in a gutter or on a path. Photographs of the pieces, and of the gap they left, help a roofer see whether the fault is at the fixing or at the tile.
Broken nibs and cracked tiles
Most concrete and clay tiles hang from the batten by nibs, small projections underneath that catch over the timber. If a nib snaps, the tile has nothing to hold it except friction and a nail or clip, if one was used. A tile with a broken nib may stay put for years in calm weather and then drop in the first gale.
Nibs break for several reasons: a hard knock when the tile was laid, someone walking on the roof, frost working into a hairline crack, or ageing of the tile itself. The tile looks whole from the ground, so the fault is only found when the tile is lifted. That is why a roofer who finds one slipped tile often lifts its neighbours to see whether more are cracked.
Replacing the tile is straightforward, as covered in our answer on broken tile nibs, and a nail or clip can be added to hold the new tile against the wind. Tiles that keep slipping even though their nibs are intact usually point to the next cause, which is the timber they hang on.
Soft battens under the tiles
The battens are the thin timber laths nailed across the rafters. Everything above them depends on those timbers holding a nail and carrying the tile's weight. If a leak has let water into the timber for years, the batten softens, the nail pulls loose and the tile slides, no matter how new the tile is.
This is why a replacement tile on rotten timber will slip in its turn. When a roofer lifts a slipped tile and the batten crumbles under a screwdriver or springs away from the nails, the repair is to strip that patch, fit new battens and replace any damaged underlay below. Our guide on when roof battens need replacing sets out the signs.
A related cause is the underlay itself. Torn or sagging sheet lets water sit on the batten, which speeds up the rot. If you notice damp patches near the eaves, or wet insulation, mention them in your enquiry. They tell the roofer to look at the timber and not only at the tile that came down. Fixing the cause matters more than the number of tiles missing.
Frost, wind and fixings that were never enough
Water soaks into porous tiles. If a hard frost then turns that water to ice, the swelling can crack a tile or flake its surface, and a cracked tile loses its hold on the batten. That is why slipped tiles are often found after a cold snap, even when the wind was modest.
Wind is the last push. It lifts the edges of tiles, and if the only thing resisting it is a tile's own weight, the tile can walk down the slope or blow off. Many roofs laid a generation ago used few clips or nails, and a modern code of practice, BS 5534, covers slating and tiling and recommends mechanical fixing. Marley notes that complying with it is not a legal requirement, but that mortar alone is no longer enough to hold ridge and hip tiles. Slipped ridge tiles are often a sign of failing mortar bedding.
Ask the roofer for photos of the underside of what slipped, the fixings and the batten. They separate a one-off from a roof in decline. Our answers on whether roof tiles need clips and roof tiles lifting in wind give the next step, and our pitched roofing page covers the wider repairs.
More on this in Pitched Roofing Services in Crawley
Two more slipped tile questions
Should I climb up and push a slipped tile back?
No. Tiles break underfoot, ladders slip, and a tile pushed back with no fixing will drop again. Any roof, however low, carries a fall risk. Note where the tile sits, photograph it from the lawn, and keep people off the path beneath. A roofer can refix it properly and check the battens that caused the slip.
Is a slipped tile covered by the roof's guarantee?
That depends on who laid the roof, what the guarantee covers and how old it is. Many exclude storm damage or wear. Find the paperwork if you have it, and read what it says about fixings and weather. Photos of the slipped tile and the roof around it help whoever you approach to judge the cause.