When rain reaches the firebox only on the windiest days

Why some storms wet the firebox and others do not

Many owners notice the same puzzle. The fireplace stays dry through ordinary afternoon showers, then a squall line or tropical storm blows through and there is water in the firebox the next morning. The difference is the angle of the rain. Rain falling straight down lands on the cap hood and the crown, which are shaped to shed it. Rain driven sideways by strong wind reaches surfaces that vertical rain never touches.

In the Brick Industry Association's design note, the Eastern Seaboard and the Gulf Coast carry the severe rating for wind-driven rain, because rain there can last for hours alongside high winds. The National Weather Service office in Miami cautions that its wet-season rainfall outlooks can be strongly influenced by tropical systems. For a chimney, how hard the rain is pushed matters as much as how much of it falls. When rain coming down the chimney shows up only in certain storms, the wind is part of the diagnosis.

The open sides of a chimney cap

A cap is a hood held above the flue on a screened frame. The hood keeps vertical rain out, and the screened sides let smoke leave. Those open sides are the first place sideways rain gets in. Wind blowing across the top of the chimney can carry rain under the hood, through the screen and straight into the flue, especially if the hood is small for the flue, bent upward or no longer centered.

Closing the sides is not the answer. Section R1003.9.3 of the 2023 Florida residential code sets the open area beneath a masonry chimney's rain cap at no less than four times what the flue outlet itself provides, so a cap has to breathe. The Brick Industry Association's chimney guidance also notes that a manufactured rain cap affects the flow of flue gases and that the maker should be asked about it. What helps is a cap sized to the flue with enough hood overhang for the exposure, checked on the chimney rather than chosen from a picture.

A crown with no drip edge

The crown is the slab over the top of a masonry chimney, and Florida's code wants it pitched so water runs off, with a drip edge at its outer rim (R1003.9.1). The drip matters in wind. Without an overhang and a drip, water running off the crown can cling to the underside of the edge, run back onto the brick face and soak into the top courses. Wind pushing rain across a flat or cracked crown sends water toward the flue instead of away from it.

Chimney detailing guidance from the Brick Industry Association makes the same point about shape, noting that thickened sides and overhangs reduce the potential for water penetration. A crown finished flush with the brick, or a thin mortar wash with no overhang, gives driven rain an easy route into the masonry. That water can travel down inside the chimney walls and surface in the smoke chamber or high on the firebox walls hours after the storm.

Chase cover seams and the storm collar

The top of a factory-built fireplace is different. Its metal chimney pipe rises through a framed chase, and the chase is closed by a sheet-metal cover or flashing. DuraVent's parts list for its DuraChimney II, a metal chimney built for masonry fireplaces, shows the same arrangement, with a storm collar fitted to the pipe just above that top to help keep rain from entering the chase, and the collar is required wherever a flashing is used. When the sealant at the collar splits or the collar slips, wind-driven rain runs down the outside of the pipe and into the chase.

The cover itself has seams, corners and a skirt. Rockford Chimney Supply's chase cover guidance describes diagonal creases that keep water from pooling on top and drip edges that throw water clear of the siding. A flat cover with no creases, a skirt corner lifted by wind or a split seam gives sideways rain a way in. In a factory-built unit, that water can end up on the firebox floor or the refractory panels, sometimes with rust flakes from the metal above.

The gap around the flue

Where a clay flue liner passes through the crown, the joint between the two is a weak point. Florida's residential code requires a caulked bond break around each flue liner, and the Brick Industry Association's chimney guidance, which calls the crown a chimney cap, says it should be primed, backed and sealed where it meets the liner to reduce water penetration. When that joint fails or was never made, a thin gap opens around the liner. Rain pushed across the crown finds the gap, runs down the outside of the liner inside the chimney and reaches the damper area or the back of the firebox. When the firebox is wet and the cap looks sound, this joint is one of the first places to check.

Write down the wind as well as the water

Because wind is part of the cause, a record of the wind is part of the evidence. You can gather it without going near the roof. Weather apps and the National Weather Service both show recent wind direction for your area, and the side of the house that took the rain is often obvious from wet walls and windows.

• The date and approximate time you found the water, and whether the storm was still going or had passed hours earlier.
• The wind direction during the storm, and which side of the house faced it.
• Whether it happens in every heavy rain or only when storms come from one direction.
• Where the water lands, such as the back wall, the front of the hearth, the damper area or the floor of a metal firebox.
• Anything that came with it, such as rust flakes, black streaks, white deposits or a wet smell.
• Whether you have seen water dripping in the fireplace before, and whether any work was done at the top since then.

What to do until the chimney is checked

Mop up the water and let the firebox dry with the damper closed, which limits how much damp air and rain reach the room. If the fireplace has gas logs, leave its damper as the log set's manual says. Move rugs, baskets and firewood away from the hearth. Resist lighting a fire to dry it. Heat will not close the opening the rain used, and a wet flue should be checked before it is asked to carry smoke.

Leave the roof alone during and after the storm. A wet roof is no place to inspect a cap, and a hose on a ladder is no way to reproduce a leak. The record you keep from the ground is enough to start with.

Reading the repair proposal for sideways rain

Because more than one opening can let sideways rain in, a good proposal starts with the opening that was found, and only then with the part or material that will close it. Look for a statement on whether the cap is sized and seated correctly for the flue, whether the crown has an overhang, a drip edge and a working joint at the liner, and whether a chase cover and storm collar are sealed. When looking is not enough to tell two paths apart, we may run water from a hose over one part at a time, starting on the side the wind came from. That test is used only when the job calls for it.

If the water shows up on a ceiling or a wall rather than in the firebox, it is taking a different path. For that, start with our guide to the question why does my chimney leak.

Matching your storm notes to the openings

Bring your notes on the wind and the timing to our free inspection. We compare them with the cap, the crown and the seal around the flue liner, and the chase top on factory-built units, then write up the openings we found and what it will cost to close them, free of charge and before any work. The fix may be a chimney cap replacement, chimney crown repair or both, and the written record of the work tells you exactly which openings were closed.

Sources

• Brick Industry Association Technical Note 7, Water Penetration Resistance, Design and Detailing: The Eastern Seaboard and Gulf Coast rated severe for wind-driven rain exposure.
• Florida Building Code, Residential, 8th edition (2023), chapter 10: Crown slope, drip edge and bond break under section R1003.9.1, and the open area beneath a rain cap under section R1003.9.3.
• Brick Industry Association Technical Note 19B, Residential Chimneys: Crown overhangs reducing water penetration, sealing the joint between the crown and the flue liner (the note calls the crown a chimney cap), and asking the maker about a rain cap's effect on flue flow.
• DuraVent DuraChimney II component literature: The storm collar's role in keeping rain out of a chase, from one maker's metal chimney for masonry fireplaces.
• Rockford Chimney Supply chase cover guidance: Diagonal creases and drip edges on chase covers.
• National Weather Service Miami, Rainy Season 2024 Outlook: The South Florida wet season, and tropical systems influencing wet-season rainfall outlooks.