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What is a slate roof? How it works and how to check one from the ground

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Slate is stone, and it can outlast the house. What fails first is rarely the slate itself, and you can read most of the warning signs without leaving the yard.

A slate roof is a roof covered in thin pieces of natural stone, split from rock and hung in overlapping rows. The stone barely wears out. The nails and metal around it do. That is why a slate roof can look perfect and still leak.

How a slate roof sheds water

Each slate hangs from a nail or two and overlaps the courses below it. Water runs down the face and over the joints, never through them. Nothing is glued. The system works by gravity and overlap, which is also why it needs a decent slope.

Under the slate sits a felt or synthetic layer, and at every chimney, valley and wall there is metal flashing. Those parts do the hard sealing work.

What fails first is not the stone

Good slate can last a very long time. The fasteners and flashing rarely do. Nails rust and let slates slip. Metal in valleys and around chimneys corrodes or splits. You get a leak while the roof still looks handsome.

Softer, thinner slate does age faster, and it can flake or delaminate. Quarry origin matters. A roofer who knows slate can tell you which kind you have.

Signs you can read from the ground

Stand back with binoculars and look for:

  • Slipped slates, which show as a gap or a tilted piece in the row.
  • Broken or missing slates and fresh, light-colored patches.
  • Rust streaks running down from the nail lines.
  • Flaking or crumbling edges on the surface.
  • Sagging in the roof line or wavy courses.

Pieces on the ground or in the gutter are a strong clue. A handful of small flakes is normal aging. Whole slates on the lawn are not.

Check the attic for daylight and damp

On a dry day, take a flashlight into the attic. Look at the underside of the roof for pinpoints of daylight, dark stains on boards and damp insulation. Water often travels along a rafter before it shows. Note where the marks line up with a chimney, valley or wall.

Why you should never walk a slate roof

Slate is brittle and slick. One step can crack a slate you cannot see, and the crack becomes the next leak. It is also easy to slide. We do not give on-roof steps for this reason. Read working at height safely before you consider anything else, and leave the roof to a licensed roofer with slate experience.

Repair is often worth it, replacement rarely is

A slate roof with sound stone and tired fasteners is usually a repair job. Replacing the whole roof throws away the long life of the material. If a roofer proposes a full tear-off because of a few slipped slates, ask for photos and a second opinion. See repair or replace for the questions that decide it.

Questions to ask a slate roofer

  • Is the stone sound, and what is failing: nails, flashing or slate?
  • Do you have matching slate for the repair?
  • Can you show photos before and after?
  • Have you worked on roofs of this age and type?

Slate work is a specialty. Use our guide to hiring a roofer. We do not publish prices.

Frequently asked questions

What is a slate roof made of?

Natural stone split into thin pieces and hung in overlapping rows. It is fixed with nails, with metal flashing at the joints.

Why does my slate roof leak if the slate looks fine?

The nails and flashing usually fail before the stone does. Rust lets slates slip and corroded metal lets water in.

Can I walk on a slate roof to check it?

No. Slate cracks under weight and is slippery. Use binoculars from the ground and the attic, and let a slate roofer go up.

How do I check a slate roof from the ground?

Look for slipped, broken or missing slates, rust streaks, flaking edges and a wavy roof line, and check the gutter for fragments.

Should I replace a slate roof?

Rarely, if the stone is sound. Most problems come from fasteners and flashing, which a skilled roofer can repair.

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.

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