Question answered
Delaminating slates: what causes it
The short answer
Slates delaminate when the stone splits along its natural layers, flaking into thin sheets and crumbling at the edges. Poor-quality slate is the usual cause, with repeated wetting and frost making it worse. Delaminated slates cannot be repaired or sealed, so they are replaced. If many have gone, the whole slope may need re-covering.
Some slates start to peel into thin layers, flake at the edges and go soft. That is delamination, and it is a fault in the slate itself, not just in the way it was fixed.
What delamination looks like
Slate is formed in layers, and a good slate splits cleanly along them only when it is cut at the quarry. A delaminating slate does it by itself, on the roof. The surface begins to lift in thin flakes. The lower edge frays, corners crumble, and the slate takes on a soft, dull, sometimes rusty-brown look.
You may find little flakes of slate in the gutter or on a lower roof, and slates that have thinned and bowed. A slate that has gone soft will often feel chalky or crumble when handled, which a sound slate never does. Pressing a fingernail along the edge of a fallen slate can peel the layers apart.
This is a different fault from a cracked slate, which breaks across the stone. A cracked slate is usually the result of a knock or a foot. A delaminating slate fails from within, and its neighbours may do the same.
Why some slates split
Slate quality varies a great deal. A hard, dense slate with a tight grain will shed water for generations. A softer or more porous one lets water in between its layers, and once it has taken in water, it is on its way to failing.
Frost is a regular culprit. Water that has soaked into the layers freezes and expands, forcing them apart, and the next thaw lets more water in. Repeated cycles open the slate bit by bit. Poorly finished edges and thin slates suffer first.
Some mineral content can also weaken a slate, with iron-bearing patches that rust and swell. How far this goes depends on the stone, and we judge it from the roof itself. The cause lies in the slate itself, which is why changing the fixings alone will not stop it. Our wider slate roofing page sets this against the other ways a slate roof ages.
Why you cannot repair or seal a delaminating slate
Painting or coating a flaking slate may hide the surface for a season, but it does not rejoin the layers, and it can trap water in the stone. A coating that cracks lets moisture in, then keeps it there.
A tingle can hold a slipped slate, but it cannot make a soft slate sound. The same goes for stitching or bedding a failing slate with mortar or sealant, which does nothing for the stone and makes the next repair harder.
Once a slate has begun to delaminate, the right job is to replace it. We lift it, check the batten and the underlay beneath for damage from the water that got through, and fit a slate of similar size, thickness and tone, taking care that its headlap matches the courses around it. Where the old slates came from a quarry that has closed, a reclaimed slate is often the closer match.
How many bad slates before the roof needs more
A small number of failed slates, scattered across a roof that is otherwise good, can simply be replaced one by one. The courses around them stay as they are, and one visit usually covers it. Reclaimed slates are often a fair match for an old roof, and reclaimed slate can suit a repair, provided they are sound.
If delamination is widespread, it shows up all over the slope, often in the same pattern, because the slates came from the same batch. At that point, replacing one at a time becomes a loop. Each repair disturbs the roof and lifts more soft slates, and the next shower finds another.
Where a slope is failing across the board, a strip and re-slate may be the sound decision. We take photographs of the flaking slates and of the battens, and explain what we found before the quote is written. You receive a single written figure after the survey.
Staying safe and avoiding more damage
Do not walk on a roof with flaking slates. Soft slates break under weight, and a fall from a roof is far worse than a leak. Slate that has started to flake is also slippery underfoot when damp. Our page on walking on slate explains why even a sound slate roof is a poor place to stand.
Watch the roof from the garden or pavement, with a pair of binoculars if one is handy, and photograph anything that looks wrong. Send the pictures to us on WhatsApp or the enquiry form. They help us decide how urgent the job is before we visit.
Where slates have fallen and rain is entering, the first visit is about making the roof safe, with the permanent repair planned afterwards. In the meantime, keep gutters clear, because flaked slate collects in them and holds water against the eaves. Delamination does not reverse by itself. A short wait for a proper repair costs far less than leaving the slope to fail slate by slate.
Flaking slate, two more questions
Does frost alone make good slate delaminate?
A dense, well-made slate stands up to frost for generations. Frost does the damage where a slate has already taken water in between its layers, so the underlying quality of the stone is where our inspection begins.
Can delaminating slates be reused elsewhere on the roof?
No. A slate that has started to flake will not recover, and moving it only relocates the weak spot. Sound slates from the same roof can be reused on a better-protected slope, but the flaking ones are replaced.
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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