Why are my slates flaking?

What delamination is

Slate is a metamorphic rock that formed under pressure, and it splits along flat planes, which is exactly why it makes a good roofing material. A good slate holds those planes tightly together. A delaminating slate lets them open, so the surface peels or flakes in thin leaves, and the edges soften and turn powdery.

You see it from the ground as slates with ragged lower edges, as flakes lying in the gutter, or as a roof that looks patchy where some slates have gone pale. From the loft you may find fragments on the underlay, on the insulation or along the wall plate. Binoculars from the pavement are a safe way to look, and no one should climb a slate roof to inspect it.

Delamination is not the same as a slate that is cracked by a knock or a footstep, and neither is the same as a slate that has simply slipped because its nail has gone. A cracked slate breaks across its layers in a line. A delaminating slate peels along them. The difference matters, because one is an accident and the other is the stone ageing.

The common causes

There is rarely one cause. The usual ones are these, and more than one can act together on the same slate.

• Slate quality. Not all slate is equal. MyBuilder notes that Chinese slate has a considerably shorter lifespan, and lower grades of any origin can split sooner than a dense stone. A roof laid with the cheapest slate can show flaking far sooner than Welsh slate, which is reported to last over a century.
• Water and frost. Water seeps between the layers, and ice formed there takes up more room than the water did, opening the layers further. Slate that has taken up a lot of moisture is the most vulnerable.
• Iron and rust. Some slates carry iron minerals. When these rust, they stain and swell and can push the layers apart. Stirling City Heritage Trust describes how iron nails corrode too, and the swelling around a nail hole can start a flake there.
• Over-driven nails. A nail driven too hard cracks the slate around the hole, which is where flaking often begins.
• Age. Even good slate does eventually soften at the surface. On a roof that is old, a thin layer of flaking at the exposed lower edge of each slate is part of normal weathering, and it only becomes a worry when the flakes are deep enough to expose the nail hole or the headlap.

Is the roof failing?

The question is how much, and where. Look at the pattern, not at a single slate.

A few flaking slates among sound ones are a repair. They can be replaced one at a time, as our answer on replacing a broken slate explains, with a matching slate fixed by tingle or hook.

Flaking across a whole slope, on many slates at once, is a different message. The slate has reached the end of its own life, not just its fixings. Our answer on how long slate lasts draws the same line: soft, flaking or crumbling slate points to the stone and not only the nails. Patching it will not hold, because the slates beside each repair will go next.

If the flaking is limited to one elevation, particularly the one most exposed to wind-driven rain, a part re-slate of that slope may be enough. The survey should say which, and it should also check the lead and the gutters on that side, because they are often failing alongside the slate.

What can be done

Repair is the first answer when just a handful of slates are involved. Replace the failed ones, check the nails and battens around them, and tidy the gutters so water is not pooling at the eaves.

Where flaking is widespread, the realistic options are a part or full re-slate. Sound slates are stripped, graded and reused where they pass, and the flaking ones go on the scrap pile, and sound new or reclaimed slate of the right size fills their places. On older roofs, that also means new battens, underlay and fixings. The matching question is covered in our answer on matching slate colour.

Do not coat or paint flaking slate, and do not fill the splits with sealant. Neither fixes the cause. A coating hides the problem and traps moisture. Pressure washing makes it worse, and the cost guide we cite for cleaning warns the method may damage delicate coverings.

A full re-roof usually passes the point where Building Regulations approval applies, and the insulation can then need improving. Building control paperwork, where the work is notifiable, is ours to file.

Getting a clear answer on your roof

Telling patchy failure from general failure takes a close look. Our survey is free: we photograph the slopes and the loft, and note where the flaking sits. You receive a written account of the roof's condition, plus one fixed price for whichever option suits it.

If repair is enough, repair is what we quote. Where it will not, the photographs show why. Pick up any fragments that have fallen into the gutter or on the path, since a slate that flakes in layers in your hand tells us more than a view from the ground.

For smaller faults, see slate roof repairs, and for the wider picture of the service, the slate roofing page sets out how we work on natural slate.