Slate roof cost

A natural slate roof costs $1,785 to $4,200 per square installed, so a 20 square roof is $36,000 to $84,000. It is the most expensive roof most houses will ever carry, and the only one that routinely outlives the person who paid for it.

A properly laid slate roof lasts 75 to 150 years. That is not marketing; there are slate roofs in Pennsylvania and New England still on their original slates after a century. The question is almost never whether it lasts. It is whether the house can take it and whether you can find someone who can lay it.

What it costs

Quarry matters. Vermont, Virginia and Pennsylvania slate are the domestic standards and are priced accordingly. Imported Spanish slate is cheaper and, at the better grades, entirely sound; at the cheaper grades it delaminates in thirty years, which on a roof sold as a century roof is a serious failure.

Ask which quarry and which grade. "Slate" on a quote is not a specification.

Weight, and the sentence a good contractor says first

Slate weighs 800 to 1,500 pounds per square depending on thickness. Asphalt is 250 to 400.

If the house was built with a slate roof, the structure is sized for it and nothing needs doing. If it was not, an engineer has to look before the number means anything, and the reinforcement can run $10,000 to $25,000.

A contractor who quotes slate on a shingle house without raising this is either inexperienced or hoping you will find out later.

Finding someone who can lay it

This is the real constraint, and it is not price.

Slate is a separate trade. Each slate is hung on two nails driven to a specific tightness — too tight and the slate cracks under thermal movement, too loose and it rattles. Headlap is calculated from the roof pitch. Valleys are lined in copper because a slate roof will outlast anything else you put there. Nothing about it transfers from asphalt roofing.

Practical filters:

• How many slate roofs in the last five years, and can you visit one
• Copper or stainless flashing, never galvanized, since the flashing must last as long as the slate
• What headlap they calculated for your pitch and why
• Whether they carry spare slates from your batch afterwards

A cheap slate quote usually means an asphalt crew, and a slate roof laid by an asphalt crew fails in a decade rather than a century.

Repair, not replacement, is the usual job

Most slate work is not a new roof. It is a hundred-year-old roof whose flashing has failed while the slates are perfectly sound.

• Replacing individual broken slates: $200 to $600 for a small batch
• Rebuilding a valley in copper: $1,500 to $4,000
• Chimney flashing rebuild: $800 to $2,500
• Full re-flashing of an otherwise sound roof: $5,000 to $15,000

Compared with $50,000 for replacement, that is usually the right answer. A slate roof that leaks is far more often a flashing problem than a slate problem, and any contractor who leads with full replacement on a sound roof should be asked to justify it slate by slate.

Synthetic slate

Composite slate imitates the look at 300 to 500 pounds a square and $700 to $1,400 installed. It solves the weight, the cost and the trade-scarcity problems at once, and on a house that wants the appearance rather than the century it is a sensible choice.

What it does not solve is authenticity on a historic house, where a review board may simply not permit it.

Frequently asked questions

Is slate worth it?

Over a hundred years, yes, and by a wide margin: one slate roof against four asphalt ones. Over ten years, no. Almost nobody buys slate on the arithmetic. They buy it because the house has it, or because they want it.

Can slate be walked on?

By people who know how, using hook ladders and staging that never rests on the slates. Everyone else cracks them. This is why routine work like chimney sweeping and satellite installation should be arranged with someone who has done it on slate.

What is a "soft" slate?

Lower-grade slate that absorbs water and delaminates. It looks identical on day one. The ASTM grading system — S1, S2, S3 — exists precisely for this, and S1 is the 75 year and up material. Ask for the grade in writing.

Does insurance treat it differently?

Often, yes, and not always well. Some carriers cap the payout at asphalt replacement cost unless slate is specifically scheduled. Check your policy before you need it, not after a storm.

How do I know if my slates are still good?

Tap one. A sound slate rings; a delaminating one thuds. A slate roofer will do this across the roof as part of an inspection, and it is the single most useful thing that inspection produces.