Slate

Typical lifespan

75–150 years

Installed cost per square

$1500–$4000estimate

Weight

800–1,500 lb per square

Fire class

Class A

Impact class

Hard but brittle; walking on slate breaks it

How does slate actually work?

Slate is quarried stone split into thin sheets and hung on nails over a solid deck. It is not manufactured, it is extracted, and its lifespan is a property of the rock rather than of any coating or treatment — which is why a well-laid slate roof outlives the people who commissioned it. The nails are the weak point, not the stone. A slate roof normally fails because the fasteners corrode and individual slates begin slipping, at which point the roof needs re-nailing rather than replacing, and the original slates are usually still perfectly serviceable. Hard slate and soft slate are genuinely different products with different service lives, and knowing which one is on a house changes every decision about it.

What drives the cost?

Slate is the most expensive common covering and the reasons compound. The material is heavy and quarried. The structure usually needs evaluating and sometimes strengthening. The trade is genuinely specialist — laying slate is not a skill an asphalt crew picks up on the job — and the pool of competent crews is small enough that geography affects price sharply. Copper flashings, which are what you want under a hundred-year covering, are their own line item. Against all that, the roof may never need replacing in your ownership, which changes the arithmetic more than it changes the invoice.

What separates a good installation from a bad one?

Correct slate work is about the fasteners and the hang. Nails must be non-corroding — copper or stainless — and driven so the slate hangs rather than being clamped, because a slate pinned tight will crack as the deck moves. Headlap has to suit the pitch. Flashings should be a metal with a life comparable to the slate, which in practice means copper; putting a twenty-year flashing under a hundred-year roof guarantees the roof gets opened up in twenty years. Valleys are formed in metal, generously. None of this is negotiable detail — it is the difference between a covering that lasts a century and one that starts sliding in fifteen years.

Where does slate work well?

Best climates: Cold and temperate; historic northeastern and mid-Atlantic housing stock

Worst climates: Anywhere the roof will see regular foot traffic

What maintenance does it need?

Do not walk on it, and do not let anyone else walk on it either. Slate breaks underfoot and the damage is cumulative and invisible from the ground until pieces start arriving in the garden. Have it inspected periodically by someone who works in slate, replace slipped or broken pieces individually as they appear, and watch the flashings rather than the field — on a slate roof the flashings and the nails are what age, and both are repairable without touching the covering.

What fails first

The nails and the flashing, not the slate. A slate roof is usually lost to corroded fasteners while every tile is still sound

When should you choose it?

Choose slate on a historic house where it is the correct covering and anything else looks wrong, on a structure already built to carry it, and when the intention is to solve the roof permanently rather than for a generation. It is also the right call when a local authority or historic district requires it, which is more common than people expect in older northeastern and mid-Atlantic housing stock.

When should you think twice?

Avoid slate if the structure has not been evaluated, if the budget is genuinely tight, or if the roof needs regular foot traffic. Be very careful about hiring a general roofing contractor for slate work: the trade is different enough that a competent asphalt crew can do real damage in good faith, and the repair bill for a badly re-nailed slate roof is worse than the original job.

What ratings does it carry?

Common mistakes

• Hiring a general roofing crew for slate work. The trade is different enough that competent asphalt roofers can do real damage in good faith.
• Using corrodible nails. The stone outlasts everything; a slate roof normally fails at the fasteners, so the nail choice sets the real lifespan.
• Putting short-life flashings under a long-life covering. A twenty-year flashing guarantees the roof gets opened up in twenty years.
• Walking on it. Damage is cumulative and invisible from the ground until pieces start arriving in the garden.
• Replacing rather than re-nailing. A roof that is slipping usually has perfectly sound slates and needs refastening, not a new covering.