What a Slate Roof Costs, and What Slope Does to It

Angi puts the national average at $13,348 from 255 verified projects, with most homeowners paying between $4,389 and $22,377 and installation running $10 to $30 per square foot. This Old House gives the same $10 to $30 for natural slate and averages it at $20. On a 2,000 square foot roof that is $20,000 to $40,000 on Angi's size table and $20,000 to $60,000 on This Old House's. Synthetic slate is a different price altogether, $4 to $15 per square foot depending on which publisher you read.

On cost and weight, clearly. Synthetic runs $4 to $15 per square foot against $10 to $30 for natural, it does not need the framing upgrade that This Old House prices at $1,000 to $10,000 for most homes, and it installs like a shingle roof. What you give up is published lifespan, and that is where the numbers get soft: Angi says synthetic composite can last up to 100 years on one page and that synthetic falls short of natural slate's century on another. Treat the warranty as the real answer.

Nobody agrees, and the disagreement is worth knowing about before a salesman quotes you a century. One This Old House article gives four different answers in four places, from 50 to 100 years up to 150 to 200. Angi says up to 100. The one figure here that comes from an engineering firm rather than a cost aggregator is Wiss, Janney, Elstner's: Pennsylvania Hard-Vein slate has an expected service life of about one hundred years, and that assumes a roof slope between 8:12 and 14:12.

More than the brochures suggest. WJE's primer on the subject puts the same Pennsylvania Hard-Vein slate at about a hundred years at a normal 8:12 to 14:12 slope and at a hundred and twenty years and counting on a 25:12 mansard installed around 1895. The reasons are physical. Lower slopes hold snow and ice longer, the slates stay wet longer, and water that gets into the keyways spreads further sideways between the overlapping pieces.

It is allowed and it is a bad trade. WJE notes the International Building Code permits slate down to a 4:12 slope with a four-inch headlap, and then says plainly what that costs you: slate laid between 4:12 and 6:12 leans on the underlayment, which has to be punctured by fasteners, and will not reach the full expected service life of the slate. If your roof is below 6:12, the published century is not the number that applies to your house.

Angi's slate page puts the average slate repair between $800 and $3,200, and its roof repair page gives slate an average of $1,800, the highest of any material it lists. The labor rate is the reason: slate work runs $75 to $150 an hour or $5 to $12 per square foot, because it is specialized and because walking a slate roof badly breaks it.

Usually not the slate. Inspectors and the engineering literature both point at the flashings and the fasteners, which means a roof with sound slate can still be leaking and can often be saved rather than replaced. That is the question to settle before anyone quotes you a tear-off.

Ask us, and start with the free inspection. From the ground, real slate and a good synthetic are hard to tell apart, and the answer changes what the roof is worth, what it costs to work on and what your insurer needs to know. We will get on it, photograph it and tell you in writing which of the two you have and what condition it is in. Then we can talk through the options for your house, whether that is slate, synthetic slate or metal, and what each one would mean for the roof you already have.