The short answer
Roof tiles slip because whatever holds them has failed. The most common cause is nail sickness, where the fixings rust through and the tile slides down. Other causes are worn nibs, rotten or split battens, a roof structure that has moved, frost and wind lifting loose tiles, and impact damage. A slipped tile leaves a gap in the covering, so rain reaches the underlay, which is why it should be dealt with soon.
How tiles are held
A roof tile hangs on a timber batten by one or two nibs, small lugs on the back of the tile, and many are also nailed or clipped. On a slope, gravity pulls each tile down, and the nibs and nails resist it. Where all of those hold, the roof stays put. When one gives way, the tile can slide down until it rests on the one below, leaving a visible gap above it.
Plain tiles are thin, light and hung from nibs. Interlocking tiles are bigger and heavier, with side locks that hold neighbours together. Either type can slip if the fixing system underneath fails.
Nail sickness
The leading cause is corrosion of the nails. Older roofs were often fixed with mild steel nails, which rust, and the shank thins until it can no longer hold the tile. This is called nail sickness, and it affects whole areas of a roof at once, not just one tile. You will notice tiles dropping in a pattern, often along a row. Our page on nail sickness has the detail.
Modern roofs use stainless steel, copper or properly coated nails that resist rust. If your roof has been re-nailed in stainless steel, this cause is far less likely.
Battens, nibs and structure
The batten is the weak point in many roofs. A batten that has rotted from damp, split along the grain or been poorly fixed to the rafter can no longer hold the nibs or nails. Tiles then drop. Damp battens are often the result of an earlier leak, so slipped tiles can be a clue to a wider moisture problem.
Nibs can also wear or break. A cracked nib on an old clay tile lets the tile ride down. If the roof structure has moved, with a sagging ridge or a bowed rafter, the tiles can shift out of line and slip. Our answer on why a roof sags covers that.
Weather and impact
Frost works on tiles that have absorbed water. The water freezes, expands and breaks the bond at the nib, and thaw lets the tile ease down. Wind lifts loose tiles and drops them back slightly out of place, repeating the movement until the tile slips. Our page on roof tiles lifting in the wind goes into that.
Impact is the simplest cause. A fallen branch, a ladder leaning against the tiles, a TV aerial fitter or a clumsy sweep can knock a tile out of place or crack it, and the damage may only show in the next gale.
Where slipped tiles tend to show up
Slippage is not spread evenly. It tends to appear on the steepest slopes, where gravity pulls hardest, and on the windward side, where uplift works the fixings loose. Tiles beside the verge, along the ridge and at the eaves course are the most likely to move, because they sit at the edges where the load changes. A south-facing slope that bakes in summer and freezes in winter, with large temperature swings, also works fixings loose.
Roofs that have had work done by others are worth a second look. A dropped tile near a chimney, a solar panel or an aerial bracket can be a sign that fixings were disturbed during installation. Tell us who has been on the roof recently, because it helps narrow down the cause.
What a slipped tile does
A slipped tile opens a gap. Rain then falls onto the underlay, which is designed to be a second line of defence and not the main one. Wind-driven rain can get past it, wet the battens and drip into the loft. Over time the batten rots, which causes more slipping. Birds and squirrels may use the gap. In winter, ice forming in a gap can lift the neighbouring tiles.
Checking from the ground
Binoculars show most slipped tiles: a tile sitting lower than its row, a dark gap above it, or a broken edge. Keep a note of the date and a photograph, so a change is easy to spot later.
How slipped tiles are fixed
For one or two, the fix is quick: lift the tiles above, slide the slipped tile back or fit a new one, and secure it with a stainless nail or clip. See our guide to replacing a single roof tile for the method. For a pattern of slipped tiles, re-fixing a whole area is more effective than chasing them one by one. We lift a few tiles to inspect the nails and battens and show you what we find. More about our work is on the pitched roofing page.
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