Flat roof repair in Washington, DC

A flat roof repair only works if the new material bonds to the old one, so the first job is knowing what is up there. Many DC rowhouses carry granulated modified bitumen, a tar-like sheet torched or self-adhered in two plies. Others have black EPDM rubber, white or gray TPO, or older built-up roofing and roll roofing coated silver over the years.

We check by looking at seams, cutting a small core where needed, and noting how many layers sit on the deck. Some rowhouses we open have three roofs stacked on top of each other, each one installed over the last. That matters, because a wet lower layer can feed a leak no matter how good the top patch is.

Once the membrane is clear, we pick the matching repair: torch-down or cold-applied mod bit for mod bit, EPDM with the right primer and cover tape for rubber, heat-welded TPO for TPO. Our guide to the three main membranes explains how they differ.

Flat roofs are not truly flat; they should slope a little toward a scupper or drain at the rear of the house. Over decades, joists sag, decking deflects and new layers change the pitch, and water starts to sit in low spots for days after rain.

Standing water breaks down coatings, collects dirt and leaves, and freezes in winter, working seams open. When we see ponding, we look at why: a sagging joist bay, a drain set too high, or a scupper partly blocked by flashing buildup. Small low spots can be built up with tapered insulation or extra plies when we repair. Larger structural sags need framing work below.

Drain and scupper fixes often solve ponding without touching the field. On rowhouses in Eckington and nearby blocks, we regularly find rear scuppers that were half covered by successive roof layers, leaving water trapped an inch below the outlet.

The weakest point on a rowhouse roof is where the horizontal membrane turns up the brick. Base flashing has to stretch up the wall at least eight inches, adhere fully, and be capped by metal counterflashing or the parapet coping. Heat cycles pull it away from the wall over time, and once a gap opens, water runs straight down behind the sheet.

We strip loose base flashing, clean and prime the brick, install new flashing ply fully adhered, and terminate it with a bar or a new counterflashing reglet cut into the mortar. If the parapet itself is cracked or the coping is loose, that work goes on our parapet repair list, because new flashing against a failing wall will not last.

Rowhouse roofs carry more than rain. Condensers on wood stands, satellite dish mounts, solar racking, old TV antenna bases and the roof hatch all sit on or pierce the membrane. Each stand leg that was bolted straight through the roof is a small leak waiting for its turn, and we often find rusted lag screws sitting in a smear of sealant.

When we repair around equipment, we set the legs on pitch pockets or prefabricated flashed supports, or on rubber blocks that sit on a protection pad without piercing the sheet. Roof hatches get new curb flashing up the sides and a fresh gasket so the lid seals when it closes.

Foot traffic wears paths too. The route from the hatch to the condenser, or across to a neighbor's roof, gets scuffed down to the reinforcing scrim on older membranes. Walkway pads laid along that route cost little and stop service techs from wearing a new leak into the roof every spring.

We repair when the membrane is still flexible, still stuck tight to the deck, and the damage is limited. If we find widespread blistering, brittle felts that crack when bent, soft decking in several bays or more than two existing layers, a repair is money that will not come back.

In that case we give you a repair price for the immediate leak and a separate price for a full low-slope replacement, so you can compare. DC energy rules push reflective membranes on low-slope roof replacements, which also cuts top-floor heat in July and August.

Roof decks and rooftop HVAC equipment change the math too. If a deck or condenser stand needs to come off for proper repair, the extra labor sometimes tips the decision toward replacing the section under it while access is open.