Mansard roof repair in Washington, DC

A mansard is really two roofs. The front face rises from the cornice at a steep pitch, often with one or two dormers, and is usually covered in slate laid in rows of rectangles, fish scales or hexagons. Behind and above it sits a low-slope roof covering most of the house, hidden from the street. You see this form across Capitol Hill, Shaw, LeDroit Park and the Logan Circle blocks.

Where those two roofs meet is the weakest point. A metal or membrane transition lies at the top of the mansard, and if that joint opens, water runs straight down behind the slate onto the front wall. Many leaks blamed on the slate actually start there, at the flat roof edge.

At the bottom, most mansards drain into a built-in or box gutter set into the cornice. When that gutter lining cracks, water soaks into the cornice and the wall below. We look at the gutter with every mansard repair and handle failures through our box gutter work.

Slate rarely wears out before the nails holding it. Old iron nails rust through at the head, and the slate slides down until it rests on the course below or falls into the gutter. On a mansard, the pitch is so steep that a loose slate drops quickly and can hit the sidewalk.

To replace a slate, we use a slate ripper to reach under the course above and cut the old nails. A new slate of matching size, thickness and color goes in, held with a copper slate hook or a copper nail through the joint, covered by a copper bib. We do not face-nail slates with a dab of sealant, which is the shortcut that fails within a few winters.

Matching matters on a visible front. We keep salvaged slate in common DC sizes and colors, including the soft gray Pennsylvania and purple and green Vermont slates used on many local blocks. On patterned mansards we match the shape of the piece, not just the color. Our slate roof repair page covers the same methods on pitched main roofs.

Dormers make a mansard work as living space, but each one adds side walls, a small roof and a window with its own flashing. The side walls, called cheeks, are often covered in slate or metal and flashed into the main slope with step pieces. When that flashing rusts or the cheek wood rots, water shows up around the inside window frame.

We open the area around the dormer, replace rotted sheathing or trim, install new copper step and apron flashing, and reset the slate. The small dormer roof on top is often a tiny flat or low-slope surface, and it gets the same attention as any other flat roof, with membrane turned up properly at the wall.

Window trim on dormers is wood, painted, and exposed to every storm. Peeling paint and soft sills are the first sign of trouble. Catching that early keeps the repair to a few boards of trim rather than a full dormer rebuild.

Mansard repairs usually happen above a public sidewalk on a busy street. Depending on the scope, we use ladders with stand-offs, roof brackets set from the top, or a pipe scaffold on the front, and we protect pedestrians below. Larger jobs that need scaffolding or a sidewalk closure may need a public space permit from the District, which we plan around.

Many mansard streets sit in historic districts, including Capitol Hill, Shaw, Mount Pleasant and LeDroit Park. Like-for-like slate repair generally keeps you clear of extra steps, while switching materials or colors on a visible front can need approval from the Historic Preservation Office. We stick with slate, copper and matching details on the street face.

If more than about a fifth of the slates are slipping, the nails are failing across the whole face, or the underlayment and sheathing are rotted in several places, a piece-by-piece approach becomes expensive and never ends. At that point, stripping the face, replacing damaged boards and relaying the good slate with new copper nails gives the front another long run.

We count the slipped and broken slates, check the sheathing from inside where we can, and show you photos before recommending anything bigger than a repair. A full relay is covered under slate roof replacement, and good original slate is often reused on the front to keep the historic look.