Slate roof: repair it or replace it?

Good slate from Vermont, Pennsylvania or Virginia quarries can last a century or more. What gives out first on houses around Cleveland Park, Chevy Chase and Takoma is the steel nail that holds each piece, the felt underneath, and the copper or galvanized flashing at valleys, chimneys and dormers.

When a nail rusts through, the slate slides down and leaves a gap. Rain gets under the next course, wets the old felt, and drips along a rafter until it shows up on a bedroom ceiling far from where it started. One loose piece can look like a big leak.

So before anyone says the roof is shot, the first job is figuring out which layer has actually failed. Stone, fasteners and metal all age differently, and the answer to repair or replace depends on that breakdown.

Stand across the street on a bright morning, ideally with binoculars or a phone set to full zoom. You're looking for patterns rather than single pieces. A few slipped or missing slates scattered over a big roof are normal maintenance. Dozens across one face tell a different story.

Look at the color too. Slate that has turned chalky, flaky or rust-stained at the edges is weathering in layers, a process called delamination, and soft slate breaks under a roofer's weight. Hard slate keeps a crisp edge and a steady color even after 90 years.

• Slipped slates hanging below the line of their course
• Tin or copper strips showing between slates, a sign of earlier repairs
• Black tar smeared over valleys, ridges or chimney bases
• Rust or green streaks running down from valleys and step flashing
• Sagging along the ridge or between rafters

If your attic has a pull-down stair and a floor to stand on, go up on a dry afternoon with a flashlight. Daylight showing through the boards around a chimney or along a valley is useful information. So are water stains running down rafters or dark rings on the sheathing boards.

Pay attention to rusty nail tips poking through the roof boards. If most of them look orange and swollen, the fasteners across the roof are near the end, and that points toward a larger job even if the stone looks fine.

Don't step between joists on an unfinished attic floor, and skip the trip if the access is a tight hatch above a closet shelf. A photo from the hatch opening is enough for us to start the conversation.

On the top floor, note where stains appear relative to outside features. A ring under a dormer valley, a damp patch beside the chimney breast, or a streak along the wall below a box gutter each narrows the search before a roofer ever sets a ladder.

Repair makes sense when the slate itself is sound, the leaks trace to specific spots, and the flashing problems are local. On many DC houses, a crew can replace broken pieces, reset slipped ones with copper nails and slate hooks, and rebuild one valley or one chimney's flashing in a day or two.

We carry salvaged slate in common sizes and colors, sorted by thickness, so new pieces sit flush with the old. A replaced slate is slid up under the course above, fastened through the joint with a copper nail or hook, and set so it sheds water like its neighbors.

Flashing usually decides how long a repair holds. Step flashing along a dormer wall, counterflashing let into a chimney's mortar joints, and the open valley metal between two slopes all wear thin long before the slate does. Replacing worn galvanized pieces with 16-ounce copper during a repair visit often adds decades to that part of the roof.

Typical slate roof repair visits run about $465-$1,380 for scattered pieces, and a full valley or chimney flashing rebuild often lands between $1,640 and $4,290. Height, scaffolding, access from the street and how hard it is to find a close color move the number. We'll price it after we've seen the roof.

Replacement starts to win when the slate is soft across most of the roof, when nails are failing everywhere at once, or when years of tar patches have glued pieces together so they can't be repaired without breaking more. At that point, every visit costs money and fixes one spot while the next one starts dripping.

A slate tear-off lets the crew replace rotted sheathing, lay new underlayment, install new copper flashing at every penetration, and either relay sound salvaged slate or put down new stone. Replacing a slate roof around Washington usually runs $28,700-$61,500 or more for a typical detached house, driven by area, pitch, dormers, chimney count and the slate chosen.

Some owners choose a synthetic slate or a quality architectural shingle instead to control cost. Each option has trade-offs in weight, look and life span, and historic district rules may limit what can go on a front-facing slope.

There's also a middle path that many owners overlook. When one slope faces south and takes the brunt of summer sun and winter freeze cycles, it can wear out decades before the shaded north slope. Replacing that one face and salvaging good stone from it to repair the others can stretch the budget and keep the house's original look.

Houses in DC historic districts such as Capitol Hill, Georgetown, Mount Pleasant and Cleveland Park often face rules about changing roofing material on visible slopes. Repairs in kind with matching stone are generally simple. Swapping slate for another material on a street-facing slope usually needs the Historic Preservation Office to sign off first.

Our post on DC roof permits and historic districts walks through how that process tends to go and what to have ready.

Call (202) 771-7443 and describe what you're seeing, even if it's one stain on one ceiling. We answer every day, around the clock, and can usually tell from a short talk whether it sounds urgent.

On site, we photograph the slopes, test a few slates for soundness by tapping and flexing, check fasteners and flashing, and show you what we found. You'll know if the roof needs a few pieces, a section, or a full replacement before anyone picks up a tool.