Fireplaces and Chimneys for Roofers and HVAC Techs

Why this matters

A masonry chimney is three systems in one stack: a roof penetration the roofer flashes, a fire-safety structure with clearances the insulation crew has to respect, and often a vent for the furnace or water heater that the HVAC tech has to size. Each trade sees only its own slice. The roofer chips the crown and caulks the counterflashing; the insulator packs the gap around the chimney; the HVAC tech swaps a furnace and leaves a water heater alone in a flue built for two. None of those moves looks dangerous on the day. The results are leaks, a chimney fire path, or a water heater spilling carbon monoxide into the basement.

Safety first

• Carbon monoxide: if a CO alarm sounds or anyone has headache, nausea or dizziness, everyone goes outside, call 911, and nobody goes back in to check the appliance. A water heater or furnace with debris in its flue, or one that has just lost its flue partner, is not left firing until an HVAC tech has checked its draft.
• Never cover or cap the top of a flue that serves an appliance, even for an hour to keep roofing debris out. If debris is a concern, the homeowner shuts the appliance off first and the HVAC company is told before it is relit.
• Do not lean over an active flue on the roof. Flue gas at the top is hot and carries CO.
• Loose masonry: a chimney top with loose bricks or a broken crown can shed pieces onto anyone below. Keep the ground under it clear and do not use the chimney as a handhold or a ladder rest. Fall protection on the roof follows the work type; see related: The Roof Assembly for Trades Who Penetrate It.

The chimney top, piece by piece

 MASONRY CHIMNEY TOP, SIDE VIEW

      [ RAIN CAP / SPARK SCREEN ]  metal, on liner
          |  flue liner  |         clay tile or
     _____|______________|_____    stainless steel
    /   CROWN: sloped concrete \   sheds water off
   /____ drip edge overhangs ___\  the top of brick
   |  brick                     |
   |  counterflashing let into  |
   |  a cut mortar joint, bent  |
   |  down over the step flash  |
   |======= roof line ==========|
   CRICKET (saddle) on the upslope
   side, sheds water around a wide chimney

Crown. The concrete or mortar cap on top of the brick, sloped away from the flue and overhanging the brick so water drips clear. A cracked crown lets water into the masonry, where freeze-thaw breaks bricks and mortar from the inside.

Flue liner. The clay tile or stainless steel pipe inside the masonry that the smoke or flue gas actually travels through. The liner, not the brick, is what contains heat and corrosive condensate.

Rain cap. A metal hood over the flue that keeps rain and animals out and, on wood-burning flues, carries a spark screen.

Flashing. Step flashing along the sides under each shingle course, a base or apron flashing at the downslope face, and counterflashing let into a cut mortar joint and bent down over the step flashing so the chimney and the roof can move separately.

Cricket. A small ridged roof section on the upslope side that diverts water around the chimney. The residential code requires one when the chimney's dimension parallel to the ridge is more than 30 inches and it does not meet the ridge (IRC R1003.20 in the 2021 edition, as adopted by your jurisdiction).

Height. A masonry chimney must extend at least 3 feet above where it passes through the roof and at least 2 feet higher than any part of the building within 10 feet (IRC R1003.9 in the 2021 edition, as adopted). A roofer adding a dormer or a second-story addition near a chimney can make an existing chimney too short.

See related: Fireplace + Chimney Reference.

What a roofer can damage

The crown. Standing on it, resting a ladder on it or chipping it while cutting counterflashing cracks it. Roofing cement smeared over a crack is not a crown repair; it traps water in the masonry. Report a cracked or crumbling crown to the homeowner and a chimney sweep or mason.

The counterflashing. The usual reroof mistake is to pull the old counterflashing out of its joint, lay new step flashing, then glue the counterflashing back over it with sealant instead of resetting it into the mortar joint. Sealant on masonry fails within a few seasons. Properly, counterflashing is reset into the joint (re-cut if needed) and pointed with mortar or a sealant made for that joint.

The flue. Roofing debris, nails or broken mortar dropped down a flue land at its base, where the furnace or water heater connector enters. A partial blockage there is enough to make an appliance spill. If anything falls in, tell the homeowner and the HVAC company the same day.

The cap. Removing a rain cap to flash around it and not putting it back lets rain run straight down the liner, which on a gas flue wets the appliance and rusts the connector.

Factory-built chimneys. Many fireplaces use a metal chimney inside a framed chase. The top has a listed termination cap and a chase cover. The listing sets its clearances to combustibles and its termination parts; do not shorten it, swap its cap for another or frame tight to it. See related: Penetrations Near Flues and Chimneys.

What packing the clearance does

A masonry chimney inside the house needs an air space of at least 2 inches to combustible framing, and 1 inch where it sits entirely outside the exterior walls (IRC R1003.18 in the 2021 edition, as adopted, with the exceptions it lists). The code says that air space is not to be filled except for the noncombustible fireblocking it requires at floors and ceilings (IRC R1003.19).

The clearance exists because a chimney fire, or years of hot flue gas, heats the outside of the masonry. With air around it, the heat moves off. Packed with cellulose or fiberglass, the heat stays against the framing, and wood that is heated repeatedly can char and ignite at temperatures far below what it takes to light fresh lumber.

So in an attic, the insulation stops at a dam: a strip of sheet metal or other noncombustible material set around the chimney at the clearance line, tall enough to hold back the insulation depth, with the gap at the ceiling sealed with noncombustible material. A chimney buried to the shoulders in blown insulation is a finding to report, not a finished attic.

What leaving a shared flue does

Many older homes vent a gas furnace and a gas water heater into one masonry flue, through a shared connector or two connectors entering the same liner. The flue was sized for both, and the two appliances together keep it warm enough to draft.

When the furnace is replaced with a high-efficiency condensing unit that vents through its own plastic pipe out the side or roof, the water heater is left alone in that flue. This is called an orphaned water heater. Three things follow:

• The flue is now too big for what is in it. The fuel gas codes' venting tables (NFPA 54, the National Fuel Gas Code, or the International Fuel Gas Code, whichever your jurisdiction has adopted) give both a minimum and a maximum flue size for a given appliance. A flue that was within range for two appliances can be over the maximum for one.
• The flue stays cold. Less heat goes up it, so it takes longer to establish draft and the flue gas cools as it rises.
• The water vapor in the flue gas condenses inside it. Gas combustion makes water. In a cold, oversized flue it condenses on the liner and the masonry, it is mildly acidic, and over a few years it eats mortar, spalls bricks and soaks the chimney from the inside. Meanwhile, at every cold start, the water heater spills flue gas at its draft hood until the flue warms.

The fix is the HVAC company's: resize the venting for the water heater alone, usually with a liner sized from the venting tables, or vent the water heater another way. Swapping the furnace without checking the water heater's vent is how the problem gets made.

Worked example: a reroof finds an orphaned water heater

A roofer is tearing off a house with a center masonry chimney. On the chimney's north face, the top three feet of brick are spalled, with white crusty deposits (efflorescence, salts carried out by water moving through the masonry), and the mortar is soft enough to scrape with a fingernail. The crown has a crack across it. In the attic, the brick is stained dark. The homeowner mentions the furnace was replaced two years ago.

What the signs point to. Spalling and efflorescence concentrated high on a chimney, on the cold side, two years after a furnace swap, fit water coming from inside the flue, not only rain from outside. A cracked crown adds outside water too. The roofer cannot tell the two apart and does not need to; both are findings for others.

Putting a number on it. In the basement, the water heater label reads 40,000 Btu per hour input. The old furnace, per the homeowner's paperwork, was 80,000 Btu per hour. Together they put 120,000 Btu per hour into the flue; now the water heater alone puts in 40,000. The flue is receiving 40,000 / 120,000 = one third of the heat input it was sized around. Whether that leaves the flue outside the venting tables' range is the HVAC tech's calculation, but the direction is not in doubt: much less heat, same size flue, colder walls, more condensate.

What the roofer does. Reflashes the chimney with new step flashing and resets the counterflashing into its mortar joints. Does not patch the crown with roofing cement, does not cap the flue, keeps debris out of it, and checks with the homeowner that nothing fell in. Leaves the cracked crown, the spalled brick and the vent question alone and writes them up.

What gets said. "Your chimney is spalling on the upper north face with white deposits, and the crown is cracked. Your water heater now vents alone into a flue that used to serve the old furnace too. That combination often means condensation inside the chimney. Have the HVAC company check the water heater's venting, and a chimney sweep or mason look at the crown and the brick. Make sure you have working CO alarms."

What changes the answer

A wood-burning fireplace in the same chimney. A gas appliance may not vent into a flue that serves a wood-burning fireplace or stove under the fuel gas codes, in substance. A chimney with two flues side by side, one for the fireplace and one for appliances, is common; tell them apart before assuming anything.

A chimney that was already relined. A stainless liner sized for the water heater, with a cap on top, means someone already solved it. Spalling may then be rain alone.

An oil-fired appliance. Oil flues follow their own venting rules and their own soot behavior. The same rule holds: do not cover the flue, and report what you see to the heating company.

References

• International Residential Code, 2021 edition, Sections R1003.9, R1003.18, R1003.19 and R1003.20, as adopted by your jurisdiction.
• NFPA 54, National Fuel Gas Code, or the International Fuel Gas Code, as adopted, for appliance venting.
• See related: Fireplace + Chimney Reference
• See related: Penetrations Near Flues and Chimneys
• See related: The Roof Assembly for Trades Who Penetrate It