What to Do If Your Chimney Cap Blew Off

Right after the storm

A cap that came down in high wind can land on the roof, in the gutter, in the yard, on a neighbour's flat roof or on the sidewalk. If it is on the ground, move it somewhere out of the way and keep it. If it is on the roof or hanging from the chimney by one screw, leave it alone. A wet, windy roof is dangerous, and a dangling cap can drop at any moment, so keep people and cars clear of the area below.

While you are outside, look for other damage. A cap that tore off sometimes takes a corner of the crown or the top of a flue tile with it, dents the gutter on the way down, or cracks slate or shingles where it hit.

• Stay off the roof and away from anything hanging from the chimney
• Close the fireplace damper and skip fires for now
• Test the CO alarms if a boiler or water heater uses the chimney
• Note any broken brick, crown pieces or tile shards on the ground
• Keep the cap if you can reach it safely, and note where it landed

Can you still use the fireplace and the boiler?

The fireplace should wait. Without a cap there is nothing to stop sparks or embers from landing on the roof, and an open flue that has taken in rain or debris should be looked at before a fire goes in. Keep the damper closed so rain and cold air stay out of the room.

A gas or oil boiler or water heater is different, because the flue is still open for exhaust and the house needs heat. It can usually keep running in the short term. The concern is what falls in next: leaves, a bird's nest, pieces of crumbling tile. Any of those can settle where the flue meets the boiler connection and narrow the exhaust path. Carbon monoxide is dangerous, so keep working alarms on every level. Should an alarm go off or a person in the house become ill, step outside first, then call 911 or the gas company.

What an open flue lets in

An uncapped flue takes rain straight down. Water lands on the damper and the smoke shelf, rusts the metal, soaks the masonry around the firebox and can show up as a damp patch on the chimney breast a day or two later. In freezing weather, that water stays in the masonry and expands. On a heating flue, rain mixes with the acidic deposits on the flue walls and runs down to the base, where it can rust the cleanout door and the flue pipe connection.

Animals notice an open flue quickly. Birds and squirrels move into uncovered chimneys in spring, and raccoons look for dens from late winter. Leaves and twigs pile up in fall. The longer the cap stays off, the more the job grows from fitting a cap to cleaning out what got in.

Shared chimneys and neighbouring roofs

On attached rowhouses and twins, a chimney often straddles the party wall, with flues from both houses in one stack. A single full-width cap may have covered all of them, so when it blows off, both households have open flues. The cap may also have landed on the neighbour's roof. Let them know, both so it can be recovered safely and because their heating flue may be just as exposed as yours.

Getting a shared cap replaced goes faster when both owners agree on who is coordinating access. The crew may need to reach the chimney from one roof or the other, and the replacement has to be sized for every flue it covers.

Why some caps keep blowing off

If this is the second or third time, the cap itself is probably not the whole problem. Caps come off because of how they were attached, or because of what they were attached to.

• Wrong size: a cap too big for the flue tile has nothing solid to clamp onto
• Friction fit: some caps only press onto the tile with nothing holding them
• Crumbling tile or crown: screws and anchors pull out of soft, cracked masonry
• Rusted fasteners or a galvanized cap that corroded through at the base
• A tall, exposed stack above a flat roof with no extra fastening for wind
• A cap knocked loose earlier by ice, a branch or a careless roof repair

Wind exposure in the region

Nor'easters bring strong, gusty wind for hours at a time, often from the same direction, and that steady push works a loose cap back and forth until the fasteners let go. Chimneys that stand well above a flat rowhouse roof, chimneys on corner houses, and chimneys on homes near open water along Long Island, the Jersey Shore or the Delaware River take the brunt of it.

Summer thunderstorms add short, violent gusts. A cap that has survived for years can fail in a single storm if its base has been rusting quietly, which is why a loose or leaning cap spotted during an inspection is worth fixing before the windy season rather than after.

Choosing the replacement

The replacement depends on what the top of the chimney looks like and what each flue serves. The point is a cap that matches the flue and is held by something that will still be solid after the next few storms.

Mesh matters too. Wood fireplace flues take a standard mesh that stops animals and larger embers. An oil boiler flue sheds soot that can clog fine mesh, so the opening size has to suit the fuel. Gas appliances that vent through a metal liner often require the termination cap the liner or appliance maker specifies, and their instructions come first.

• Single-flue cap with a clamp band: for a flue tile that stands proud of the crown and is sound all the way around
• Outside-mount cap: legs that screw into the crown around the tile, for tiles too short or too irregular to clamp
• Full-width cap: one hood over the whole chimney top, for stacks with several flues close together
• Liner termination cap: fitted to the top of a stainless liner that vents a boiler or water heater

Getting it fixed for good

A lasting fix starts at the top of the flue. If the clay tile is cracked or the crown is breaking up, it is repaired first, or the new cap will fail the same way. Then the cap is sized to the tile, or to the whole chimney top for a multi-flue stack, and fastened to sound material: screws or masonry anchors into a solid crown, or a clamp band on an intact tile. Stainless steel holds up far longer than galvanized steel in wind and rain.

Keep the old cap if you found it. It shows the size and style that was there, and the bent or rusted base usually shows exactly how it failed. The crew checks the flue for anything that fell in while it was open and confirms the heating flue draws properly before leaving.