Slate roof repair in Washington, DC

Slate came into wide use in Washington during the late 1800s, and it's still everywhere: steep front mansards on rowhouses in Logan Circle and LeDroit Park, turrets and conical towers on corner houses, and full gable roofs on the detached Victorians of Cleveland Park and Takoma. Many of those roofs carry Buckingham slate from central Virginia, a dark blue-black stone that barely changes color in a century.

The stone usually outlasts everything holding it. Original iron nails rust through, and the slate slides down until it hangs on the course below. Valleys and step flashing made of terne or early copper wear thin. Ice working into a cracked corner in January splits pieces that were sound in the fall.

So when someone says a slate roof is shot, ask what's actually failing. A roof with sound stone and bad fasteners is a repair candidate, sometimes for decades.

Slate is brittle, and every step on the field risks cracking another piece. We work from roof ladders hooked over the ridge, from scaffold on mansards, or from a lift when the street allows, so weight never lands directly on the stone.

To remove a broken slate, we slide a slate ripper up under it, hook the nails and cut them. The replacement is cut to size and holed on the bench. Then it's held one of two ways: with a copper or stainless slate hook nailed between the slates below, or with a copper nail set in the joint and covered by a copper bib slipped under the course above.

Hooks are quick and clean and show a small metal tab at the bottom edge. Nail-and-bib leaves nothing visible, which matters on a front mansard facing the street. We choose per location and tell you which you're getting.

We never face-nail a slate and smear it with roofing cement. That old shortcut cracks the stone and makes the next repair harder.

A good repair looks like nothing happened. We carry salvaged Buckingham and Pennsylvania slate in common sizes and thicknesses, and we source reclaimed Vermont gray, green and purple stock for roofs with patterned bands. Color, thickness and size all count: a thicker slate tilts the course above it and opens a gap.

Some Victorian roofs use fish-scale or diamond-cut shapes, especially on mansards and towers. We cut those shapes from rectangular stock when salvage pieces aren't on hand.

If the house is in a historic district, we stay with like-for-like stone. Swapping in a synthetic piece on a visible slope can bring the Historic Preservation Office into the picture.

On many older roofs the slate is fine and the metal is the problem. Valleys lined with terne or thin copper rust or wear through where water concentrates, and step flashing at a dormer cheek or chimney often failed decades ago and got painted with tar.

Rebuilding a valley means removing the slates along each side, pulling the old metal, laying underlayment and new 16 or 20 ounce copper, then setting the slates back with new fasteners. Chimneys get new step flashing and a cut-in counterflashing. If the masonry itself is loose, our chimney flashing repair work comes first.

Snow guards matter here too. Slate sheds snow in sheets, and on a rowhouse above a sidewalk that's dangerous. We add or replace copper snow guards where the old ones are missing.

There's a point where patching costs more than it saves. If more than about a fifth of the stone is soft, delaminating or flaking at the edges, or if nails are failing everywhere at once, we'll say so and price a slate roof replacement alongside the repair. Our article on repairing versus replacing slate lists the tests we use.

Even then, a new slate roof can often reuse sound stone from the old one on the most visible slopes, with new stone going on the rear.

Slate rewards small, regular attention. We suggest a look from the ground every spring and after any storm with strong gusts, plus a close check from roof ladders every few years. Catching one slipped piece early costs a fraction of what the rotted deck beneath it would.

Keep gutters and valleys clear, since standing water and wet leaves speed up the decay of nails and flashing. Ask cable, satellite and solar installers never to walk on the stone or screw mounts through it; we've repaired plenty of roofs cracked by a single service visit.

We also note every repair on a simple roof sketch with dates, so the next person on the roof knows which slates are new and where the copper work was done.