Where the price of the stone comes from
Hard slate is dense, absorbs very little water and, on a well-built roof, outlasts the people who paid for it. The traditional American sources are quarries in Vermont, New York and Virginia. Soft slate, historically quarried in Pennsylvania, is easier to work and cheaper, and still lasts many decades, though not as long. Imported slate, much of it from Spain, spans the whole range, so origin tells you less than grade.
The grade comes from ASTM C406, which sorts roofing slate into S1, S2 and S3 by expected service life. S1 is the one to ask for on a new roof. Color moves the price as well: unfading slates keep their color, weathering slates drift toward buff and brown over their first years, and rare colors such as red sit at the top of the range. A quote that says only "slate", with no source, grade or thickness, is not finished.
Can the house carry it?
Slate weighs three to four times as much as asphalt shingles. A house built with slate, as many older Northeastern houses were, was framed for that load, and re-slating it usually means repairs rather than reinforcement. A house built for asphalt was not, and putting stone on it starts with a structural check of the rafters, their spacing and span, and the snow they already carry; the snow load calculator shows how much of the framing winter already claims.
Undersized rafters are sistered with new lumber or propped partway along the span, often with an engineer signing off. Neither the engineering nor the reinforcement is in this estimate. Nor is new decking: every slate hangs on nails driven into the deck, and those nails have to hold for as long as the stone lasts, so soft or thin sheathing found at tear-off gets replaced, priced at roof decking installation.
Slope is the other limit. IRC chapter 9 sets 4:12 as the minimum for slate, and on pitches just above that, slaters lengthen the overlap between courses so wind-driven rain cannot reach the nail holes, which takes more slate per square. Check your roof with the roof pitch calculator.
The nails and flashing have to last as long as the stone
Old slate roofs seldom fail because the slate wore out. They fail because steel nails rusted through, or the valley and chimney flashing gave up, decades before the stone did. That is why the estimate carries copper nails ($159 to $319) and copper flashing ($116 to $164). Both cost several times their steel equivalents and are still small lines next to the slate.
Each slate is punched with two holes and hung on two nails, driven just far enough that the head touches the stone. Driven harder, the nail cracks the slate; left proud, the head wears through the slate laid over it.
Two lines in the estimate are economy choices, and a slater may quote differently. The edge metal is galvanized steel drip edge, where many slaters fit copper. The hip and ridge line is a slate-gray asphalt ridge shingle ($158 to $224), the cheapest way to close a ridge; a slate ridge or a formed copper cap costs more and lasts as long as the field. The copper roll priced here also suits a simple roof. Valleys, dormers and a chimney are usually lined in heavier sheet copper formed on site, and that work is quoted roof by roof.
Synthetic slate, the option not priced here
Synthetic slate is molded from rubber or plastic and colored to look like stone. It weighs a fraction of what slate does, so it rarely needs structural work, and an ordinary roofing crew can cut and nail it. That removes the two biggest costs of a real slate roof: the stone and the specialist labor.
The trade is lifespan and character. Synthetic products carry warranties measured in decades rather than the century a good slate roof can reach, and up close they neither look nor sound like stone. A historic district review board may not accept them on a listed house. Luxury asphalt shingles imitate slate for less again; the shingle types guide compares them. Real slate makes most sense when the house was built for it or you plan to stay for a long time.
Who to hire, if you need a new roof at all
Slate is its own trade. A roofer who is excellent with shingles may never have hung a slate, and the mistakes show up years later as cracked and sliding stones. Ask for the addresses of slate roofs the contractor installed several years ago, and go and look. Slaters work from hook ladders and roof brackets rather than walking on finished slate, and replace single slates with a slate ripper and a copper hook or strip. Anyone who proposes roofing cement on cracked slates is the wrong contractor.
If you already have a slate roof that leaks, ask a slater whether it can be restored before pricing a new one. Sound slate rings when tapped; soft, flaking slate sounds dull. Sound stone with failed flashing and a scatter of broken slates can usually be repaired for a fraction of a re-slate.