Your Furnace Flue Cap Should Be Checked Before the Crew Leaves

A roof replacement touches more than shingles. The furnace flue, the water heater vent, the bathroom fan duct and sometimes a dryer vent all pass through the roof, and each one has a collar, a cap or a flashing that the crew has to lift, reset or replace. A flue that ends a few inches short of where it used to, or a cap that was pulled off and set back with the wrong fastener, can run for years without anyone noticing. The furnace keeps running, the house stays warm, and nothing looks wrong from the driveway. That is why the flue belongs on the checklist before the crew leaves, not after the first winter.

Why the flue matters more in a St. Louis winter

A gas furnace in St. Louis may run for four or five months a year, through long stretches of cold, damp air and the freeze-thaw swings that come in late winter and early spring. Furnace exhaust is warm and moist, and when it cools inside a flue or cap, some of it condenses. In cold weather that condensation can freeze at the cap opening, narrow the passage, or drip onto the metal and rust it from the inside. A cap that keeps out rain in July can still be partly blocked by ice in January.

The flue also has to clear the roof by a set height and sit a set distance from the roof surface and from anything it could heat. Those numbers come from the appliance manufacturer's venting instructions and from the building code your municipality enforces. Ask the crew which instructions they checked when they reset the pipe. If the answer is that they put it back the way it was, ask them to show you the measurement.

Where a reroof disturbs the flue

Four places tend to get disturbed during a tear-off:

• The flashing collar around a metal flue, which is often sealed to the old shingles and pulled loose when the old layers come up.
• The cap, which may be unbolted to make room for the new underlayment and then reinstalled with a different screw, or not reinstalled at all until the crew returns.
• The decking around the pipe, which can be cut or damaged when the crew removes old sheathing to repair rot.
• The pipe itself, which a ladder, a dropped tool or a stack of shingle bundles can bend or crush.

None of these is a sign of bad work by itself. They are the normal hazards of taking a roof apart around pipes that were installed decades ago. The point is to inspect them while the roof is open, because once the new shingles are on, the flue and its collar sit under finished roofing and any problem is hidden.

Questions to ask before the crew leaves

1. Did you measure the flue height above the roof surface, and against which instruction?
2. Is the cap the same size and type as before, and what fastener did you use to reset it?
3. Is the flashing collar sealed to the new underlayment and the new shingles, and can I see that in a photo?
4. Did you find any rust, soot, cracked joints or loose sections in the pipe?
5. Who checks the flue after the first cold snap, and how do I reach them?

Ask for a written answer on each point. A roofer who has photographed the flue, the cap and the collar on the day the roof was finished has given you something you can compare against later. Keys Roofing puts the written scope, with photos and measurements, in the homeowner's hands. If a vent or flue is part of the job, the scope should name it and show where it sits on the roof.

Signs the flue needs attention now

You do not have to wait for a roofer to notice a problem. Check these in the first cold stretch after the new roof goes on:

• Dark soot or staining on the cap, the collar or the shingles around the flue.
• Rust-colored streaks down the outside of the pipe, especially at the joints.
• A cap that looks tilted, loose or missing a screw.
• A furnace that cycles off early, runs longer than usual, or makes a different sound when the wind picks up.
• Condensation or water stains on the ceiling near the furnace or water heater.

If the appliance shows signs of poor venting, shut it off and call your gas utility or a licensed heating contractor before you run it again. Missouri does not license roofers, so the person who can tell you whether the appliance is venting correctly is the heating technician, not the roofer. A carbon monoxide alarm near the sleeping areas costs little and is worth testing once the roof is finished.

Permits and the flue

A roof permit is written around the roof work itself. A change to a gas vent or appliance flue may need its own mechanical permit, depending on the municipality where the house sits and on exactly what was altered. Ask the permit desk for your address before the work starts, not after, and ask whether they want the flue inspected. The desk's answer is the one that counts, not a guess from the crew.

Keys Roofing documents the flue, its cap and its collar with photos and measurements on the job, and the written scope shows what was reset and what was replaced. Keys attends as the contractor on the work. Questions about the furnace itself belong with your heating technician, and questions about your insurer belong with your insurer. If you want to ask about a specific pipe before the crew arrives, call the office at (314) 220-2333.