Skylight repair and replacement in Washington, DC

Many DC rowhouses have skylights over the main stair, set on a wood curb built up from the low-slope roof. The membrane runs up the sides of that curb, and the skylight frame sits on top. Over years of reroofing, the curb often ends up too short, with the roof surface creeping up within an inch or two of the frame.

When the curb is short, water that ponds on the roof can reach the top of the flashing and spill over. Snow piled against it does the same. We rebuild short curbs to at least eight inches above the finished roof, wrap them with new base flashing and set the skylight back on top, or fit a new unit sized to the curb.

Old wired-glass or single-pane hatch skylights from the mid-1900s still turn up on blocks around Columbia Heights and Shaw. Their putty and metal frames are long past saving, and swapping them for a modern insulated unit cuts heat loss and condensation as well as the leak.

On shingle or slate roofs, skylights are usually deck-mounted with a manufacturer flashing kit: a sill piece at the bottom, step flashing up the sides and a head flashing at the top. Problems start when the kit was skipped and the unit was sealed in with roof cement, or when shingles were trimmed so tight that water runs into the side channels.

We remove the roofing around the unit, install a proper flashing kit or custom bent copper on slate, and weave the shingles or slates back in. If the unit itself is old, it usually makes sense to replace it during the same visit, because the cost of opening the roof is already spent.

Leaks at the top of the unit are often from debris packed against the head flashing. Skylights under big oaks and maples in places like Cleveland Park catch leaves on their upslope side, and the wet pile works water under the head piece.

We also check the drywall or plaster return inside the shaft. If a slow leak has soaked it for years, the framing around the opening may be stained or soft, and we replace that wood before the new unit goes on so the fasteners bite into solid lumber.

Not every drip at a skylight comes from outside. In winter, warm, humid air from a shower or a kitchen hits the cold glass and condenses, then runs down the frame. It looks just like a leak, but it happens on cold dry days and stops in summer.

We check the dates and conditions of the drips, look at the flashing and frame, and test with water when needed. If it is condensation, the fix may be better bathroom ventilation, an insulated unit in place of single-pane glass, or attic ventilation changes rather than roofing work.

Fogging between the panes of a double-glazed skylight is a different problem again. The seal between the panes has failed, which does not leak into the house but ruins the view and the insulating value. That calls for a glass replacement or a new unit.

We repair when the unit is sound and only the flashing or a seal has failed. A cracked plastic dome on a curb-mount unit can often be replaced by itself. A flashing problem on a newer deck-mounted unit can be fixed by removing and reinstalling it with the correct kit.

We replace when the frame is rotted or rusted, when the glass seal has failed, when the unit is older than about twenty years, or when the roof around it is being redone anyway. Pairing a skylight swap with a new flat roof or a reroof of the pitched slope costs less than doing them separately and gives one tied-together set of flashings.

Some homeowners decide to remove an old leaky skylight and roof over it, especially in rooms where it lets in harsh afternoon heat. We frame the opening, deck it, and roof it to match. The ceiling below needs a drywall patch, which a painter or handyman can finish.

Adding a new skylight to a rowhouse roof is possible as long as the joists allow an opening without cutting more than one. In historic districts, a skylight visible from the street may need historic review by the Historic Preservation Office, so we usually place them toward the rear of the roof.