Rowhouse roofing in Washington, DC

Most rowhouses built between the 1880s and the 1930s in neighborhoods like Petworth, Bloomingdale and Capitol Hill carry a low-slope roof hidden behind a brick front parapet or a decorative cornice. From the street it looks flat. In reality the joists are cut or shimmed to fall a few inches toward the alley, so water runs back to a scupper, a collector box and a downspout on the rear wall.

That design works well until one piece stops doing its job. The roof shares a brick wall on each side with the houses next door, and each wall rises a little above the roof as a parapet capped with terra cotta, stone or sheet metal coping. Every one of those joints, plus the hatch, the plumbing vent and any old chimney, is a place where the membrane has to turn up and be flashed.

Then there's the rear ell, the narrower two- or three-story wing behind the main block that holds the kitchen and back bedrooms. Its roof usually sits lower and drains separately, sometimes onto the main roof's water path and sometimes into a gutter of its own. We treat it as a second roof with its own details, not an afterthought.

The seam between your roof and the next house is where most rowhouse leaks start. Sometimes your membrane laps over a shared parapet; sometimes there's no parapet at all and the two roofs simply meet along the wall line. Before we tear anything off, we walk that line and work out which roofing sits on top, how old it is, and whether it's stuck to yours.

When the parapet is there, we run the new membrane up the inside face and terminate it under the coping or behind a counterflashing set into a cut reglet in the brick. Where the parapet is short or crumbling, we point the brick first and raise the base flashing to at least 8 inches wherever the wall height allows.

When there's no parapet, the tie-in has to bridge two roofs. We cut back to clean, sound material on the neighbor's side, prime it, and lap our new sheet across with a target strip wide enough to cover the joint's movement. We talk to the neighbor before we touch their side, and we photograph the finished edge for both households.

Three systems cover nearly every rowhouse we replace. Two-ply modified bitumen with a white granule cap is forgiving around odd parapets and old brick. A fastened or adhered TPO sheet, usually 60 mil, gives a bright white surface with heat-welded seams. EPDM rubber works on simple, open roofs with few penetrations.

The right pick depends on what's up there. A roof crowded with condenser stands, a hatch, solar racking and three plumbing vents favors modified bitumen or TPO, since both take flashing details well. If you plan a roof deck within a few years, we size the protection layers now so the deck won't damage a new membrane later. Our article on choosing between TPO, EPDM and modified bitumen walks through the trade-offs in more depth.

Whatever the cap sheet, DC's energy rules push low-slope replacements toward reflective surfaces, so a black roof is rarely the answer anymore. We also add tapered polyiso insulation where there's room under the parapet height, which cures ponding and helps the top floor stay cooler.

Rowhouse jobs are mostly logistics. Material comes up by ladder hoist from the alley, or over the front when there's no rear access, and debris goes down a chute into a dumpster that may need a public space permit from DDOT when it sits on the street. We plan the staging with you a week ahead so parking and trash days don't collide with the work.

We strip the roof down to the wood deck, often finding two or three older layers: a built-up roof with silver coating, a roll roofing patch layer and sometimes a rubber sheet on top. Each layer adds weight and hides rot, so we don't recover over them. Soft boards come out and new plywood or plank goes in, fastened to the joists before insulation starts.

The front edge gets attention too. Behind many cornices there's a built-in box gutter that the front strip of roof drains into, and on some blocks the roof meets a slate mansard or a tin bay roof. We tie the new membrane into those edges so water has one clean path off the house.

A full roof replacement in the District generally needs a permit from the DC Department of Buildings, and we pull it as part of the job. If your house sits in a historic district such as Capitol Hill, Shaw or LeDroit Park, any change visible from the street also goes through the Historic Preservation Office. A low-slope roof hidden behind a parapet usually isn't visible, but cornices, front mansards and bay roofs often are.

On the permit drawings we note what stays the same and what changes, and we keep the front profile unchanged unless you've asked for something different. Our guide to DC roof permits and historic districts explains which items tend to need extra steps.