Where this starts
A tile or a slate is held by a nail, a clip, or a hook, and by the units around and above it. When one moves, the question is whether what held it failed by itself or whether everything holding everything else is the same age and the same material.
COUNT AND MAP is the whole first pass, and it can be done from the ground with binoculars. One slipped unit, or several in one small area, is local: an impact, a gust catching a unit that was already loose, or a defect where somebody walked on the roof. A SCATTER across the slope — two here, one there, another on the far side — with no pattern and no obvious cause, is the roof telling you its fixings are going.
NAIL FATIGUE is what that scatter usually means, and the mechanism is simple. Ferrous nails corrode in a roof that gets damp; the nail thins until the weight of the unit shears it, and the unit slides. Because every nail on the roof was the same material, driven on the same day, into the same conditions, they arrive at that point together — so the failures start slowly and then accelerate, which is exactly what people describe when they say it was fine for thirty years and then went in two.
THE SECOND FAMILY is the batten rather than the nail. A batten that has rotted or split releases whatever is nailed to it, and that shows up as units slipping in a LINE along a course rather than scattered. That distinction — line against scatter — is worth making before anybody goes up.
WHY IT MATTERS BEYOND THE TILE: a slipped unit exposes the underlay, which is not a weatherproof layer on its own and degrades quickly in sunlight. And a unit that has slipped is a unit that can fall, which on a roof over a pavement, an entrance or a conservatory is a safety question rather than a maintenance one.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Nail fatigue across the roof
Most commonSeveral units slipped at scattered points with no common cause, on a roof of a certain age, and the rate increasing year on year.
How to check: Map where they are. Scattered with no pattern is the fixings ageing together rather than a series of accidents.
A single failed fixing or an impact
Most commonOne unit, or two adjacent, often after a storm, a falling branch or work on the roof, with everything else sound.
How to check: Whether it is one place. A single event leaves a single mark.
A rotted or split batten
CommonUnits slipping in a LINE along one course rather than scattered, sometimes with a visible dip in that course, often below a leak or a valley.
How to check: Whether the slipped units follow a horizontal line. A line is the thing they were all nailed to.
Wind uplift at an edge or a corner
CommonLosses concentrated at the verge, the eaves, the ridge or a corner, after high winds, on a roof whose perimeter fixing looks the same as its field.
How to check: Whether the losses are at the edges. Uplift is far higher there, which is why fixing specifications tighten in those zones.
Damage from foot traffic
Less commonCracked as well as slipped units, often in a path from an access point to an aerial, a flue or a solar array, and dating from when that work was done.
How to check: Whether the damage traces a route across the roof. Somebody walked it.
Original fixing inadequate for the exposure
Less commonRepeated losses since the roof was new or recovered, particularly on an exposed or coastal site, with a fixing pattern that does not match what the exposure requires.
How to check: Whether it has ever been settled. A roof that has slipped units from the start was fixed for a milder site than it stands on.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
One is an event, a line is a batten, a scatter is the fixings ageing together, and an edge concentration is wind. Four different answers from one look through binoculars.
- 2
Nail fatigue is an age and a material question, and a roof approaching the life of its ferrous nails behaves very differently from one halfway through it.
- 3
Fixings fail together, so the useful signal is acceleration: one a year becoming four a year is the roof changing state, not bad luck.
- 4
A slipped unit over a pavement, a door or a conservatory is a safety question with a different timescale from one over a lawn.
- 5
The underlay is the second line of defence and is not weatherproof in sunlight. Perished or torn underlay behind a slipped unit changes a repair into a larger question.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where units have slipped at several scattered points, since that indicates the fixings are failing together and a patch repair is a recurring cost rather than a solution.
- Where anything could fall onto a pavement, an entrance, a conservatory or a neighbouring property, which is a safety matter with its own timescale.
- Where the underlay visible through the gap is perished, torn or absent, because the roof is then relying on the covering alone.
- Where slipped units follow a line, suggesting a batten has failed and the problem is beneath the covering rather than on it.
- In every case where access is required: a pitched roof is not a place to learn, and the access usually costs more than the tiles.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.