A leaking slate roof gets condemned more often than it deserves. A few slates slide off into the boxwoods, a stain shows up on a bedroom ceiling, and the first roofer through the door says the whole thing is shot. Sometimes that is true. Just as often the stone is fine and what has failed is a handful of nails, a valley, or a strip of flashing around the chimney, all of which can be fixed without touching the rest of the roof.
Slate outlasts almost everything attached to it
Good roofing slate is dense, splits into thin sheets, and absorbs very little water, which is why it can sit on a roof through a century or more of freezing and thawing. Not all slate is equal, though. The National Park Service, in its preservation brief on historic slate roofs, gives rough lifespans by source: a little over 60 years for Pennsylvania soft-vein slate, around 125 for Vermont and New York slate, 175 or more for Virginia's Buckingham slate, and at least 200 for Peach Bottom slate from the York County area along the Maryland line. The brief is careful to call those figures a general guide, and so should anyone else.
The rest of the roof does not last nearly as long. Slates were hung with iron or steel nails that rust through long before the stone wears out, and when a nail lets go, the slate slips down the roof or drops off entirely. Flashings, the metal pieces that seal valleys, chimneys, dormers, and the joints where one roof meets a wall, are usually the first thing to leak. A single pinhole in an old valley can soak the framing below while every slate on that slope looks perfect from the street.
So the age of a roof says less than it seems to. A 110-year-old roof of hard slate with tired flashings may have decades left after a proper repair. A 70-year-old roof of soft slate that is flaking apart may really be done.
What to look for, mostly from the ground
Binoculars and a dry afternoon tell a surprising amount. Missing slates leave dark gaps. Slipped slates hang lower than their neighbors, out of line with the rest of the row. Cracked slates often show a clean diagonal line, and a scatter of broken pieces in the gutters or the flower beds says the same thing.
The condition of the stone itself is the bigger question. Healthy slate looks like slate, a flat and even surface whether it is gray, green, purple, or close to black. Slate that is wearing out tends to flake and lose layers from its exposed face, a process called delamination, and the edges go soft and crumbly. Badly weathered slates sometimes turn a dull, washed-out white and ring with a dead thud instead of a clear note when tapped, though that test belongs to someone on a ladder, not someone on the roof. Weathered slate can also soak up water and pass it into the wood sheathing below without any visible leak, so a soft spot in the attic boards under a roof that seems dry is worth noticing.
Inside, look in the attic on a rainy day. Water stains along a chimney or under a valley point at flashing. Daylight through the boards is less alarming on a slate roof than it sounds, since slates overlap and a little light can show through gaps that never pass water, but wet wood around those spots is another matter. A ceiling stain on the floor below is the usual sign that something has been dripping for a while, and the guide on cracks in plaster walls covers what that kind of water does to old plaster.
Where the line between repair and replacement sits
The Park Service brief offers a rule of thumb: when roughly 20 percent or more of the slates on a roof, or on one slope, are broken, cracked, missing, or slipping, replacement usually makes more sense than repair. Below that, repair is the preferred route, as long as the slate itself is sound. The brief also points out that roofs rarely wear evenly. The south and west slopes take the most sun and weather, and it is common for one slope to need replacing while the others only need repair.
A roof that needs new flashing everywhere is still a candidate for repair. Replacing a valley or a chimney flashing means carefully pulling the slates around it, installing new metal, often copper, and putting the same slates back. That is skilled work, and it costs real money, but on a roof of good slate it is a fraction of a full replacement and keeps a material that would be expensive to buy again.
Where the slate is soft and flaking across the whole roof, or where the sheathing underneath has rotted from years of small leaks, patching gets more expensive every year and stops making sense.
How a proper slate repair is done
Replacing a single broken slate is a small job in the right hands. A slater slides a thin hooked blade called a slate ripper up under the broken piece, cuts or pulls its two nails, and removes it without disturbing the slates above. The new slate, matched in size, thickness, and color, goes in its place. Since its nail holes are now hidden under the course above, it gets held another way. One common method nails it through the gap between the two slates above and slips a strip of copper over the nail head to keep water off it. Another uses a metal hook that grabs the bottom edge of the slate. Slaters have their preferences, and the Park Service notes that the copper strip method can be pushed out of place by snow and ice in colder climates, so it is a fair thing to ask about.
Matching slate is often the hard part. Old roofs in this region were laid with slate from Pennsylvania, Vermont, and elsewhere, and the colors do not always pair well with what is sold new today. Salvaged slate from demolished buildings is a common source, and a slater who works on old houses will usually keep a stack of it.
The repairs that cause more trouble
Black roofing cement troweled over cracked slates, smeared along flashing, or spread across a whole valley is the most common sign that a slate roof has been worked on by the wrong person. It might stop a drip for a season or two. Then it dries, shrinks, and cracks, and the water runs underneath it. It also makes later repairs harder, since the tar has to be cut away first, and it can speed up corrosion of the metal around it.
Walking on the roof does its own damage. Old slate can crack under a person's weight, and a roofer who is not used to slate can break more pieces getting to a leak than the leak itself ever did. Slaters work from ladders hooked over the ridge and roof brackets, and keep foot traffic on the slate to a minimum. A contractor who climbs straight up the slope in work boots is telling you something.
Nailing new slates straight through the face, with exposed nail heads left to rust or sealed with a dab of caulk, is another shortcut that leaks within a few years.
If the roof does need replacing
When replacement is the honest answer, the choice of what goes back on matters. New natural slate is expensive up front, but much of the cost of any roof is labor, flashing, and tearing off the old one, and those costs are about the same whatever material goes on. Over decades, one good slate roof can outlast several roofs of something else. Synthetic slates made of rubber, plastic, or fiber cement are lighter and cheaper to buy, and some look convincing from the street. How they will look and perform after a few decades is less settled than the sales literature suggests.
Weight is a real concern in either direction. Slate is far heavier than asphalt shingles, and old roof framing was sized for it. A house that switched to asphalt at some point and wants to go back to slate should have its framing checked first, and so should a roof where rafters have been cut or notched over the years.
For houses in a local historic district, a roof replacement, and in some towns even a change of roofing material, can require review and approval before the work starts. The guide on historic district rules explains how to find out whether that applies.
Gutters, snow, and the rest of the system
A slate roof sheds a lot of water quickly, and what happens to it at the bottom matters as much as anything on the roof. Many older houses have built-in box gutters, metal-lined troughs hidden inside the cornice, and when their linings fail the water goes straight into the wood trim and wall framing. Clogged or broken downspouts send roof water against the foundation instead, which the guide on wet stone foundations covers in more detail.
Snow guards are worth a look too. Slate is slick, and a full slope of wet snow can let go all at once, taking gutters with it and landing hard on whatever sits below. Many old roofs had rows of small metal guards to hold snow in place, and replacing missing ones is a modest job while a slater is already up there.
Hiring for it
Plenty of roofing companies install asphalt shingles well and know very little about slate. The difference shows up in the first conversation. A slater will want to look closely before naming a price, will talk about flashing and nails before talking about replacement, and will have a source for matching slate. Some useful questions:
- How much of your work is on slate, and can you show a recent repair rather than a new roof?
- What do you think is causing the leak, and how did you figure that out?
- How will you reach the problem area without walking on the slate?
- How will replacement slates be fastened, and where will you get slate that matches?
- What metal will you use for new flashing, and will the existing slates go back on afterward?
- If you recommend replacement, what share of the slate do you think has failed, and on which slopes?
A second opinion is cheap compared with tearing off a roof that had another fifty years in it.