Why Do Slates Delaminate?

How a slate splits into layers

Natural slate is a metamorphic rock, formed from clay under pressure so that the minerals line up in thin, parallel sheets. That grain is what allows a slater to split a block into slates, and it is also the plane along which a slate will fail. Good slate is dense and tight, and water cannot get into it. Poor slate is more open. Moisture enters at the exposed edge or through hairline cracks, travels along the grain and sits there.

Once the water is between the layers, two things happen. In cold weather it freezes and expands, forcing the layers apart. In wet weather the stone softens and the layers lift. The surface may blister, then peel in flakes, and in the later stages the slate looks like a stack of pages with the corners curling. The flaking is often worst at the bottom edge and around the nail hole, where the slate is both exposed and under load.

The common causes

The first is the stone. Slate varies enormously by quarry and by seam. Cheaper imported slate and some modern stock have been judged to have a higher rate of problems than the best Welsh or Cornish stone, though the origin alone proves nothing about a given slate. Our guide to Welsh versus Spanish slate covers how to assess quality. The second is minerals. Iron pyrite in the rock oxidises and leaves rust stains, expanding as it does, while carbonate minerals dissolve in rain and open pores. White bloom on the face can be a clue, as we explain on our page on why slates turn white.

The third cause is the roof itself. Slates that sit on a damp, unventilated underlay stay wet for longer, and any roof that shades heavily from trees dries slowly. Nail holes punched too close to the edge, or nails driven too hard, start cracks that let water in. And walking on slates, or high-pressure cleaning, damages the surface and starts the process. Age is the last cause. Even good slate eventually weathers, and a roof that has served for generations may simply be tired.

Frost plays a bigger part than many owners expect. A slate that soaks up water on a mild day and then freezes overnight swells at the very layer where it is weakest, and each cycle widens the gap a little more. Roofs in exposed, open spots, and slopes that never see the winter sun, go through more of these cycles. That is why the north slope often shows flaking first, and why the edges and tails of the slates, which are the thinnest and wettest parts, are the first to go.

Telling a repair from a re-slate

The test is how much of the roof is affected. A scatter of flaking slates, perhaps on a north slope or near the eaves, can be replaced one at a time, using matching slates, and the rest left to carry on. Where delamination is spread across a slope, or the slates you lift are soft, thin and breaking at the nail holes, patching only buys months. Every slate you replace sits among neighbours that will fail next, and the repeat call-outs cost more than the roof would have.

Our page on the signs a slate roof needs replacing lists the other things that point the same way, such as widespread slipping, rusted nails and a worn underlay. If the roof does need to be re-covered, a sound set of slates can often be salvaged and reused on the front slope.

Man-made slates fail differently. Fibre cement and composite slates can lose their surface layer, curl or craze, but they do not split along a natural grain, so the word delamination is used loosely for them. If you have a newer roof with flaking slates, check what the slate is made of before you assume the stone is at fault, because a manufacturer may have a remedy for a faulty batch.

What we do on a survey

We lift a few slates at different points on the slope and check the thickness, the sound they make when tapped, the state of the nail holes and how the layers behave when pressed. Close-up photographs go into your report, and the verdict is plain. If a repair will do, we quote for the repair. If it will not, we say why and quote for a re-slate with a fixed price. You can read about the full job on our slate roofing in Basingstoke page.