Question answered
Why do slates slip?
The short answer
Slates slip because whatever holds them has failed. The commonest causes are corroded nails that snap, a slate that has cracked across its nail holes, and softened or rotten battens that no longer grip. Wind and a thaw can then push a loose slate down. One slipped slate is usually a warning that others on the same slope are about to follow.
A slate that slips has almost always lost its fixing. The stone rarely fails first, so the cause lies in the nail, the batten beneath it or a break in the slate itself.
How a slate is held on
Each slate is carried on a batten, a thin timber strip running across the rafters. Two nails go through holes near the top of the slate (or a single nail through the middle on some older roofs), and the slate in the course above overlaps it by a set distance, the headlap. The overlap does the waterproofing and the nails do the holding.
That gives a roof two ways to go wrong. If the overlap is reduced, because a slate has slid down, rain can reach the batten and the underlay. If the nails fail, the slate hangs on its neighbours alone, and any gust or vibration can push it loose.
A slate is not meant to be held by friction or by the tight fit of its neighbours, though that is what keeps many failing roofs in place for years after the fixings have gone. The first slip is often the moment the roof reveals a problem that has been building for a long time.
Nail sickness and broken slates
Nail sickness is the old trade name for corroded fixings. Iron and steel nails rust where they pass through the slate and into the batten, thin down and snap. A durable non-ferrous nail avoids the problem, which is why the choice of fixing matters on a new job. The slate stays put until the nail gives way, then drops a few centimetres and catches on the next course, showing as a crooked line.
Our page on nail sickness describes the pattern in more depth. A roof with the problem may need many slates refixed, and the work is better done as a slope than slate by slate.
A broken slate is a second route. A slate cracked across the nail holes, by frost, an impact or somebody's footstep, has nothing to hang from, and the lower half falls away. Check the gutters and the ground below the eaves for fragments.
Battens that no longer grip
The batten matters as much as the nail. A softwood batten that has been wet for years, through a leaking valley, a failed underlay or condensation in the loft, softens and rots. A nail in rotten timber has nothing to hold, and a slate fixed to it will come loose even when the nail itself is sound.
This makes the fault harder to see from outside. The slates may be sound and the nails intact, yet the whole slope is working loose as the batten crumbles. The loft is the place to look: daylight between slates, dark and damp timber and soft battens that give under a screwdriver.
When battens are the cause, replacing a few slates leaves the problem in place. The slope has to be stripped, new battens fixed, and the sound slates put back with fresh fixings. This is a larger task than a patch repair, yet it need not mean a new roof. Our guide to slate roof repairs covers the range of work.
Wind, pitch and fixing methods
On the Sussex coast the wind does the final work. A gale lifts the tail of a slate that is only held by a failing nail, and slopes facing the Channel get the strongest gusts. The wind seldom starts the failure, but it decides when a weak slate finally goes.
Pitch plays a part. Slate supplier Cupa Pizarras reads BS 5534:2014+A2:2018 as setting a 20 degree floor for natural slate in moderate exposure, with hook fixings ruled out below 25 degrees. A roof laid below those limits, or fixed with the wrong method, will tend to give trouble sooner.
Poor original workmanship also shows up here. Slates nailed too high, too few nails or an undersized headlap each leave the roof short of margin. These faults come to light when something else weakens the fixing, and a storm is then blamed for what was already wrong.
What to do about a slipped slate
Look from the ground, then from the loft if you can reach it safely. Count the slipped slates, note which slope they are on and check for damp stains on the ceiling beneath. Do not climb on the roof, because slate breaks underfoot and the fall can be serious.
One loose slate can often be secured with a slate tingle, a metal strip that holds the slate in place without disturbing the others. Several slips on one slope call for a proper look at the nails and battens. We trace the cause first, then send pictures from your roof with a written quote.
Typical costs for small repairs are on the page about roof slate prices. If you would like a general check on the roof, our slate roofing page describes what we do. Until it is repaired, a loose slate is a risk to anybody below, so keep the area clear.
Slipping slates: two more questions
Can I push a slipped slate back myself?
It is not advisable. Pushing it back does not restore the fixing, and reaching it means working at height on a brittle surface. A roofer can fix it so that it stays, and can check the neighbouring slates at the same time.
Is the whole roof at fault if one slate has slipped?
Not always. One slate may simply have cracked from an impact. When several have gone on the same slope, fixings or battens are more probable, and a survey shows how far it runs.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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