Slate roof replacement in Washington, DC

Good slate outlasts almost everything around it. Pennsylvania and Vermont slate on Capitol Hill and in LeDroit Park has often been on the roof since the 1890s and can keep going. What usually gives out first are the iron nails holding each slate, the copper or tin valleys and the wood lath behind the mansard face.

So the first step is a hard look at the stone. We pull a few slates from different slopes and tap them. Sound slate rings; slate that thuds, flakes or shows white powder at the edges is delaminating. If the stone is good and only the nails failed, the answer may be stripping and relaying the same slates on new underlayment, which keeps the historic material and saves money.

If the stone itself is soft, a full replacement with new slate is the honest recommendation. Our article on repairing versus replacing slate covers how we draw that line, and smaller problems belong with our slate repair crew.

A DC Victorian mansard is a steep, nearly vertical slope across the front of the house, usually with dormer windows set into it and a flat roof behind. The slate is often laid in decorative bands of fishscale, hexagon or diamond shapes, sometimes in two colors. Matching that pattern is part of the job, not an extra.

Turrets and conical bay roofs are harder. Each course gets narrower toward the top, so every slate is cut and holed by hand to taper. We take photos and measurements of the original pattern before stripping, then lay it back course by course.

Where the mansard meets the flat roof above, a copper or terne-coated cap usually covers the transition. That top edge leaks more than any other spot on these houses. We rebuild it with new soldered copper, lapped down over the top slate course and up under the flat roof membrane.

• Hand-cut slates for turret and cone roofs
• Fishscale, hexagon and banded patterns matched
• Soldered copper at the mansard top and dormer cheeks
• Salvaged slate sourced to match faded color

Many slate houses sit inside historic districts such as Capitol Hill, Mount Pleasant, Cleveland Park, Shaw, LeDroit Park, Anacostia, Woodley Park and Takoma DC. In Georgetown, the Old Georgetown Board reviews visible exterior changes. Replacing slate with slate of similar size, color and pattern is generally the path of least resistance through the Historic Preservation Office.

Changing material on a visible slope is harder. Swapping a slate mansard for asphalt shingles is often denied on contributing buildings, and even synthetic slate may need a conversation. We assemble photos, product samples and drawings so the Historic Preservation Office has what it needs. Our permits and historic review guide covers the process.

Outside the District, Old Town Alexandria and Takoma Park MD have their own historic district rules. The DC Department of Buildings permit, or the county permit office outside DC, comes once design questions are settled.

We strip slate by hand, saving any sound pieces for reuse on the same house or for future repairs. The deck gets inspected board by board, and rotten lath on mansards is replaced with new boards. Over that we lay heavy felt or a synthetic underlayment rated for slate, with ice barrier at the eaves and in valleys.

Every slate gets two copper nails, set so the head sits flush without cracking the stone. Each course follows a headlap of about three inches, the overlap that keeps water out. Valleys and step flashings are 16-ounce or 20-ounce copper, so the metal lasts as long as the slate.

Snow guards go on above doorways and walkways, especially on steep Victorian slopes over the front stoop. Sliding snow off a slate roof can bend gutters and hurt someone walking below.

Slate is not only a rowhouse material. Tudor revivals and center-hall colonials in Chevy Chase DC, Cleveland Park, Bethesda and older parts of Arlington often carry full slate roofs with steep gables, hips and cross gables. Those roofs run larger and more complex than a mansard, with long valleys, multiple chimneys and built-in gutters at the eaves.

On these houses, the slate is often fine while the built-in gutters rot out behind the cornice. Replacing slate without fixing that eave detail sends water straight back into the wall. We open the cornice, rebuild the gutter liner in copper, and tie the bottom slate courses into it properly; our box gutter repair work handles the cornice side.

Staging differs too. Detached homes allow roof jacks and planks or a lift on the lawn, and we protect boxwoods and walks under every drop zone. Slate chips are sharp, so we sweep beds and lawns by hand at the end of every day rather than just running a magnet.

Owners who face a full slate tear-off on a large house often ask about alternatives. Outside historic districts, synthetic slate or standing seam can cost less, and we price those side by side when the owner wants to compare.