With simple spot repairs, a slate roof can last a lifetime — and more

If you've spent your life in the building trades, you may have done a job or two that you look back on with regret. I still remember one from about 30 years ago. A contractor I worked for had us tear off a 20-square Vermont slate roof because there were a few leaks. We replaced the slate with asphalt shingles.

Some of those slates were almost too heavy to lift — more than an inch thick, in various shades of green, gray, purple, and red. The roof was laid out in the old "graduated" style, with the large, heavy slates applied in wide courses near the eaves, and the reveal getting smaller with each course so that the lightest slates would be used in narrow courses near the ridge. Some of the heavy slates we took from the lower courses were reused afterward as paving stones — they were that big and rugged.

That roof wasn't worn out; it just had some corroded flashings and fasteners. The leaks would have been simple to fix, and those existing slates were good for another hundred years or more. The asphalt shingles we replaced them with are most likely shot by now.

Since that time I've replaced and repaired more slate roofs than I can remember, but I've never removed a slate roof that didn't need replacing. Even the lowest grades of slate have a service life of 75 years, and many kinds can last 200 years or more. Odds are that any slate roof you encounter is still going strong (see Figure 1).

Figure 1.The main roof of this turn-of-the-century Victorian still has most of its original slates, while the asphalt shingles installed on the building's lower porch roof in mid-century are at the end of their service life.

There are plenty of reasons a slate roof might need work. Tree limbs, snow, and ice can damage slates, for example. Leaks occur when flashings corrode, and slates can come loose because fasteners rust through. Or maybe the roof is affected by a remodel or addition, or just needs a small opening for a plumbing vent. Looking at some of the patches that have been done to slate roofs by people who don't have the basic skills, I can understand why someone might think the only solution is a whole new roof. Poor workmanship, unfortunately, is common in slate repair. But with good technique and matching materials, repairs can be long-lasting and undetectable. And the techniques are no harder than any other roofing skills.

Inspection: Repair or Replace?

A slate roof is worth fixing only if the existing slates are going to last. You can determine this with a visual inspection: Slate with life left in it will show no signs of delamination or flaking. If the edges or the field of the individual slates are beginning to look like the flaky edge of a dinner roll, they're terminal.

Like ripe watermelons, serviceable slates have a distinctive sound. Hold a piece of the slate in your hand and tap it with the rubber grip of your hammer, or lightly with the business end (Figure 2). If the slate emits a crisp, sharp ring, it has serviceable life left. If you hear a clunky, dull sort of thud, you've got an old slate that is almost gone. Too many of those and it's time to think about replacing the roof.

Figure 2.Sound slates make a crisp ringing sound when struck with a hammer. This piece, salvaged from an old Vermont barn, could provide another hundred years of service on a roof.

Choosing and Matching Slate

When you work on an existing roof, it's always worth the effort to match color, size, and texture as closely as possible. Slate is a natural material — it's just stone that is quarried from the ground and split into sheets. Slate from different regions and different quarries varies in color, hardness, and durability.

Experienced slaters come to recognize many slate types (Figure 3). Pennsylvania slate from the quarries north of Allentown and Bethlehem is grayish black in color; you still see it on thousands of homes and buildings in that area. Another Pennsylvania slate comes from York County; these black slates are more durable than the more common Pennsylvania gray slate. Slate from New York and Vermont comes in various shades of green, red, purple, and gray. Slate from Virginia is bluish gray and very durable. The quarry that produced the hundred-year-old slate you're trying to match may not still be operating, but finding at least a visual match from a different quarry should be possible. There may also be second-hand slate available that matches.

Figure 3.Slate's natural color varies from a subtle range of blacks and blue-grays to striking shades of green, purple, and red. Quarries still in operation can often supply a close or exact match, while some colors are available only as salvage from old buildings.

Finding slate. There are plenty of good slate suppliers who can help you get the right material. Some of the bigger slate roofing outfits keep a large inventory on hand, much of it recovered as salvage from old barns, churches, and industrial buildings that have been re-roofed or torn down. Salvaged slate is available in today's market for around $300 a square. New slate prices depend on the type; red Vermont or New York slate may run $1,000 and up per square, while unfading green or black from the same region could go for $350, or even $250 in small sizes. Thickness and size will also affect price. You can expect to pay more for 24-inch slate 12 to 16 inches wide than for 10-inch pieces 6 to 10 inches wide.

If you can't find a close enough match for a highly visible repair, it's easy to shift slates around. Remove slates from a less obvious area of the roof, do the repair, and replace the removed slate with new material (Figure 4). Even slates that are a totally different color can be made to work if you use some imagination — instead of laying the slate in haphazard patches, create a pattern. You can make up your own shape, or imitate one of the classic patterns woven into roofs by the old-time slaters. Many of those old roofs are still around to provide examples.

Figure 4.This repair with mismatched slates stands out like a sore thumb. If matching shingles were not available, the roofer could have repositioned existing slates while installing the new ones in a decorative pattern or band.