A furnace flue leaking water when it rains usually means rain is getting past the parts that seal the flue pipe where it passes through your roof: the rain cap, the storm collar or the flashing boot. Water then runs down the outside or inside of the pipe and drips near the furnace, the water heater closet or a ceiling.

If water is dripping around a gas appliance right now, book your roof diagnosis online or call so we can stop the roof-side leak quickly. Have a qualified HVAC technician check the appliance itself as well, since water inside a flue can affect equipment.

How a furnace flue passes through the roof

Most standard gas furnaces and water heaters in Orange County homes vent through a metal flue pipe that runs up through the attic and out the roof. On the roof, three parts work together to keep rain out:

  • The rain cap at the top of the pipe, which keeps rain from falling straight down the flue.
  • The storm collar, a metal band clamped around the pipe just above the flashing that sheds water running down the pipe outward.
  • The flashing, a metal base or cone that sits under the roofing upslope and over it downslope, sealing the hole cut for the pipe.

When any of these fails, the leak tends to show up only in rain, which helps narrow down the source. Water heaters often share the same type of flue, so the same checks apply if the drip is in a garage or water heater closet rather than near the furnace itself.

Why your furnace flue is leaking water when it rains

Missing or damaged rain cap

Wind can bend or knock off a rain cap, especially during Santa Ana events. Without it, rain drops straight down the flue and can reach the appliance. This kind of leak often shows up as water or rust inside the furnace cabinet or at a water heater's draft hood.

Loose or unsealed storm collar

The storm collar has to be snug against the pipe and sealed at the top. If it slides down, separates or its sealant cracks, water running down the pipe slips into the gap between the pipe and the flashing cone and drips into the attic.

Worn or poorly installed flashing

Flashing can rust in coastal air, crack at the seams, or be installed incorrectly, for example with its upslope edge sitting on top of the shingles instead of tucked under them, so water runs straight beneath the flashing. On tile roofs, a flashing base that is not shaped to the tile profile leaves gaps that wind-driven rain can enter. We cover salt-air wear in our guide to salt air roof corrosion.

It may not be rain at all

Some water around a flue is condensation from the exhaust gases rather than a roof leak. This is more likely if the drip appears on cold, dry days, not only during storms. High-efficiency furnaces that vent through plastic pipe are designed to produce condensation and drain it, so water there may be a drain issue rather than a roofing one. An HVAC technician is the right person to check that side.

Signs the leak is coming from the roof penetration

  • Stains on the ceiling or wall around where the flue passes through.
  • Water lines or rust on the outside of the pipe in the attic.
  • Wet insulation or damp decking around the pipe opening.
  • Drips that start during rain and stop soon after.
  • From the ground, a tilted rain cap, a storm collar that has slipped or visibly rusted flashing.

Flue leaks are one type of penetration leak. If the source is not obvious, our article on roof penetration leaks explains how vents, pipes and HVAC curbs are traced and repaired.

How we repair a furnace flue roof leak

Our roof repair and restoration technicians start by confirming the water path from both the roof and the attic. The repair then targets the failed part:

Resetting or replacing the storm collar and cap

When the flashing is sound, a new or reset storm collar, properly sealed, and a secure rain cap often solve the problem.

Replacing the flashing

If the flashing is rusted, cracked or installed wrong, the surrounding roofing is lifted, any damaged decking and underlayment are repaired, and a new flashing is installed with the correct laps. On clay or concrete tile, tiles are removed and reset around a flashing suited to the tile profile.

Keeping the required clearances

Metal gas flues require clearance from wood framing and roofing materials, so the repair maintains the space the pipe manufacturer specifies rather than packing material tight around the pipe.

Coordinating with your HVAC contractor

We handle the roof penetration. If we find signs that the pipe itself needs replacing or the appliance has been affected, we will tell you so your HVAC contractor can address it.

Getting a flue leak fixed with Rescue Roofer

Rescue Roofer is a licensed roofing contractor, St. Lic #1137524, serving Orange County since 1993. Our T.E.A.M. Certified technicians are trained in-house, and our "Warehouse on Wheels" trucks arrive stocked with flashing and roofing materials. For active leaks during storms, our emergency roof repair service comes with a 1-hour arrival guarantee.

Frequently asked questions

Is a leaking furnace flue dangerous?

Water reaching a gas appliance or corroding a flue can affect how it operates and vents, so have an HVAC professional inspect the equipment, and get the roof leak fixed promptly.

Can I fix a flue leak by adding roofing cement?

Roof cement may slow a leak briefly, but it cracks in sun and heat. A slipped storm collar or rusted flashing needs to be properly replaced or reset.

Why does my flue only leak during windy storms?

Wind pushes rain sideways under a bent cap, a gapped storm collar or a flashing edge that handles straight-down rain fine.

A flue leak is small at first, but it sits right above equipment you rely on. Let us find where the rain is getting in and seal the penetration properly. Book your roof diagnosis online or call 1-888-346-7663 to speak with a Roofing Advisor.