Roof drain and scupper repair in Washington, DC

Most DC rowhouse roofs slope gently from front to back. Water runs toward the rear wall and leaves through a scupper, a rectangular opening through the back parapet, into a conductor head bolted to the brick. From there a leader pipe carries it down to the yard, the alley or an underground line. On houses without a rear parapet, the roof edge drains straight into a hanging gutter instead.

Larger rowhouses, corner buildings and apartment buildings often have internal drains instead: a cast iron drain bowl set into the roof with a clamping ring and a strainer dome, connected to a pipe that runs down inside the building. Churches and small commercial roofs may have both, with overflow scuppers set slightly higher than the main drains as a backup.

Every one of these outlets is a hole through the waterproofing. That's why they leak more often than the open field of the roof, and why we look at them first on any flat roof repair call.

A scupper is a metal box lined through the wall, with flanges that lap onto the roof membrane and onto the face of the brick. Old scuppers were often galvanized steel or lead-coated copper set in tar, and decades later the solder joints have opened and the tar has cracked. Water running through the box drops into the wall cavity instead of out the other side.

We also see scuppers that are simply too small. A 4-inch opening may have been fine for the original roof, but after a deck or a rear addition was added, more roof area drains through it. In a July cloudburst it can't keep up, and water rises over the membrane flashing at the parapet base. Widening the opening or adding a second scupper solves that.

Brick around the scupper suffers too. Water that misses the conductor head runs down the wall, washes out mortar and stains the brick white. When the masonry is that far gone, we coordinate with parapet and coping repair so the wall and the outlet are fixed together.

Internal drains fail in a few predictable ways. The clamping ring that squeezes the membrane against the drain bowl loosens or rusts, letting water slip under the roof. The strainer dome goes missing, and leaves and gravel wash into the pipe. Or the roof around the drain has sagged, leaving the drain sitting higher than the puddle it's supposed to empty.

When we replace a membrane at a drain, we lower the bowl area with tapered insulation, called a sump, so water flows toward it from every side. We set the new membrane in sealant on the bowl flange, torque the clamping ring evenly and add a cast aluminum or iron strainer dome. Retrofit drain inserts with their own built-in clamp work well when the old bowl is cracked and can't be removed without opening the ceiling below.

If water backs up out of the drain rather than draining down, the problem is in the pipe below the roof. We'll scope what we can reach from the top and tell you if a plumber needs to clear the line from inside the building.

The conductor head is the box on the outside of the wall that catches water from the scupper. Rust, a clogged outlet or a head that's too shallow will spill water over its sides and down the brick. We replace rusted heads with heavier-gauge aluminum or copper boxes sized to the scupper, with a mesh screen to keep leaves out of the leader.

Leaders on DC rowhouses often run into cast iron or clay pipe at ground level. If that pipe is blocked, the head fills and overflows no matter how new it is, so we flush the line and check it during the repair. The same tie-in issues come up with rear gutters, which we cover on our gutter repair page.

Leaves blown from street trees, tennis balls, roof deck debris and loose gravel pile up at the low point of a flat roof, which is right where the outlet sits. A clogged scupper on a roof with a high parapet can hold several inches of water in a hard rain, and that weight sits on joists that weren't sized for it.

A quick clearing in late fall and early spring prevents most emergency calls. We include drain and scupper checks in every scheduled maintenance visit. If water is ponding on your roof right now, call (202) 771-7443 and we'll come clear the outlet and find the source of the leak.