Flat roof replacement in Washington, DC

Most DC rowhouse roofs are not truly flat. They pitch gently from the front parapet toward the rear wall, where water exits through a scupper, an internal drain or a box gutter. Over a century, joists settle, boards cup and the slope flattens out, so water collects in shallow ponds that cook the membrane every summer.

Leaks usually start at the edges, not the middle. The base flashing running up the parapet, the joint along the party wall shared with the house next door, and the outlet at the rear are where old roofs open up. Our article on why party walls leak explains the details if you've been chasing a stain on the side wall.

Layer stacking makes it worse. Many roofs in Shaw, Eckington and Trinidad carry an original built-up roof, a later roll roofing layer and several coats of aluminum paint. Each layer traps moisture against the deck, and the added weight sits on joists that were never sized for it.

A new membrane laid over the same dips will pond in the same places. During tear-off we map the low spots with a level and string, then design a tapered polyiso insulation layout that moves water toward the outlet. Crickets go behind chimneys and between drains, where water would otherwise stall.

The outlet gets as much attention as the field. A rowhouse scupper that's too small, set too high or rusted through will back water up behind the parapet no matter what membrane you choose. We rebuild or enlarge scuppers, fit new conductor heads and downspouts, and clear internal drains down to the stack.

Insulation helps twice. Rowhouse top floors are hot in July because the roof deck sits right above the ceiling, often with nothing between. Adding polyiso board under the membrane cuts heat gain, and the taper solves ponding, which means the whole roof works harder for the same tear-off.

There is no single right membrane for every rowhouse. White TPO is our most common choice for open roofs because it meets DC cool roof expectations and its heat-welded seams resist ponded water. EPDM suits roofs with lots of penetrations and odd shapes, and a white EPDM is available where reflectivity is needed. Two-ply modified bitumen with a reflective cap handles foot traffic and rooftop units well.

We look at what will sit on the roof before choosing. A future deck, solar racking, condenser stands and a lot of maintenance traffic all push toward tougher surfaces or added walkway pads. Our comparison of TPO, EPDM and modified bitumen goes deeper into the tradeoffs.

Whatever membrane goes on, the wall details get the same care. New flashing runs up the parapet, under the coping or into a reglet, and every pipe, vent and hatch is flashed with a matching boot or curb wrap.

• TPO: reflective, heat-welded seams, strong under ponding
• EPDM: flexible, simple to repair, handles odd shapes
• Modified bitumen: tough surface, good for traffic and units

Replacing a flat roof in the District usually needs a DC Department of Buildings permit, and DC energy code rules push low-slope replacements toward reflective cool roof surfaces. In practice, that means white or light gray membranes on most rowhouses. We pull the permit and note the membrane's reflectance on the paperwork.

Owners sometimes ask about green roofs while the deck is open. DOEE's RiverSmart Rooftops program has offered rebates for vegetated roofs in the past, and terms change, so check what's current before planning around it. We can prepare the deck and membrane for a later green roof if the structure supports it.

Historic districts rarely care about a flat roof hidden behind a parapet, but rooftop additions, decks and visible equipment can trigger Historic Preservation Office input. We flag that early so the schedule doesn't slip.

Flat roof jobs in the city happen in tight quarters. Material usually comes up from the alley by ladder hoist, and on blocks without alley access we lift through the front with a small crane or carry rolls through the roof hatch. We tell your neighbors which days we'll be loading, since shared alleys get crowded fast.

Old built-up roofing is heavy and messy, so it goes down a covered chute into a dumpster or straight onto a truck. We cover adjoining skylights and the roof surfaces next door, and we sweep the alley for nails and gravel at the end of every shift.

Most single rowhouse roofs are stripped and dried in on the first day. If a storm cell builds in the afternoon, we stop tearing off and seal what's open, then pick up the next morning. The house is never left uncovered overnight.