Chimney flashing repair in Washington, DC

A chimney is a big masonry box punched through the roof, and the roof and the brick move differently. Flashing bridges that gap in two layers. Base or step flashing tucks under the roofing and turns up the side of the chimney. Counterflashing is set into the mortar joints and folds down over the step pieces, so water running down the brick lands on the roof, not behind it.

At the downslope face, an apron piece wraps the front of the chimney and laps onto the roofing below. On the upslope side, a chimney wider than about 30 inches needs a cricket, a small peaked roof that splits water around the chimney instead of letting it pond against the brick and leaves pile up there.

When any of those parts is missing or done with shortcuts, leaks follow. The most common shortcut we see is a single bent strip of aluminum glued to the brick face with roof cement, with no counterflashing at all. It looks fine for a few years, then the cement cracks and water runs behind it.

Rowhouse chimneys usually rise from the party walls, so the flashing has to tie into the low-slope membrane on both sides and often into the neighbor's roof too. Many are no longer used, with a cracked cap or a tin cover over the flue. We look at the cap and the top courses of brick as part of the repair, because water entering at the top runs down the inside of the chimney and shows up in exactly the same places.

On detached colonials and Tudors in neighborhoods like Chevy Chase, Spring Valley and Tenleytown, the chimney usually sits on a steep shingle or slate slope. These need proper step flashing woven into each course, a soldered or folded apron, and on wide chimneys a cricket framed and covered with the same roofing or with metal.

Slate roofs call for copper or tin-coated copper flashing that will last as long as the slate. Putting aluminum against slate means you will be back up there replacing flashing long before the slate wears out. Our slate repair crews handle the slate that comes off and goes back on around the chimney.

The proper way to attach counterflashing is to grind a reglet, a slot about an inch deep, into a horizontal mortar joint. The top edge of the metal is bent into that slot, wedged, and sealed with a sealant made for masonry. On a sloped roof the counterflashing is stepped to follow the courses of brick.

Face-mounted counterflashing, screwed or glued onto the brick, is the main reason we see repeat chimney leaks. Water running down the brick face gets behind the top edge, and no amount of caulk keeps up with it. We remove it and cut proper reglets, even when it takes longer.

If the mortar joints are soft or crumbling, the reglet will not hold. In that case we repoint the joints around the flashing line first, or recommend a mason for heavier chimney work, before installing new metal.

Brick is porous. A chimney with failed mortar, a cracked crown or no cap can soak up rain and pass it down into the house, and the stain looks exactly like a flashing leak. We check the crown, the cap and the condition of the brick above the roof before we sell anyone new flashing.

Condensation inside an unlined flue serving a gas furnace or water heater can also stain the chimney breast. That is a venting and lining issue rather than a roofing one, and we tell you so when we see it. Separating these causes is part of any leak tracing job near a chimney.

The cheapest time to replace chimney flashing is when the roof around it is being replaced, because the roofing is already off and the new step pieces weave straight into new courses. If your roof has a few years left but the flashing is failing now, we fix the flashing and leave the roofing ready to accept the next roof without tearing it out again.

Masonry work should come before flashing. If a mason is rebuilding the crown, repointing the stack or taking the chimney down a few courses, we schedule our flashing after that work so new metal is not disturbed or splattered with mortar. We coordinate the order with you so the chimney is not left half flashed through a week of thunderstorms.