A new roof, a lifted water-heater flue and a sick family: why vents need checking after a reroof

The owner of a 1950s house in California described the case this week on Reddit's r/Roofing forum. After the new roof went on, the family was sick for about a week while the carbon monoxide alarm stayed silent. A technician who came to service the gas water heater found its flue raised 12 to 14 inches and the furnace vent partly disconnected, with combustion gases getting into the house. The owner asked whether the roofers had simply made a mistake.

Building official Glenn Mathewson described a similar case in the Journal of Light Construction. After a hail storm, a roofer slid a universal cap too far down a furnace's Type B vent and blocked it. The family's headaches and nausea stopped once the cap was raised and clamped at the right height. He lists other frequent errors. Turning the whole vent instead of just the cap can unlock joints inside the house, and those often cannot be reconnected from the roof. A B-vent that moves freely on the roof probably has a separated joint.

Alarms built to UL 2034 must sound within 60 to 240 minutes at 70 ppm and within 4 to 15 minutes at 400 ppm, and alarm maker X-Sense notes that a unit can register lower levels without sounding. The Consumer Product Safety Commission recommends a CO alarm on each floor and outside the bedrooms.

The International Fuel Gas Code requires a gas vent to pass through the roof flashing and end in a listed cap or roof assembly, and a Type B vent must terminate at least 5 feet above the draft hood or flue collar it serves. After a reroof on a house with gas appliances, go into the attic and follow each vent from the appliance to the roof. Joints should be locked, the pipe rigid where it exits, and the cap at its listed height. Mathewson's advice to roofers: “If you touch it, understand it.”