For 18 feet of run you would have a rise of 72 inches.
Furnace vent pipe height above roof.
Single wall metal pipe shall terminate at least 5 feet 1524 mm in vertical height above the highest connected equipment draft hood outlet or flue collar.
Vent caps 12 and smaller may terminate a distance above the roof if 8 feet or more away from a vertical surface as follows.
If the pipe is farther than 10 feet from the roof.
The top of a chimney pipe must extend at least 2 feet above the peak of the roof if the pipe is within 10 feet of the peak.
General rule the chimney should not be outside a window or other house opening.
Put the roof opening directly above the exhaust opening on the furnace.
If the previous calculation results in the required height of 64 inches we can see that the top of the pipe will not rise above the roof peak but would still meet the requirements of the rule.
Table of rooftop chimney clearances for metal b vent chimneys flues.
Roof pitch rise run in inchesminimum height flat to 7 12 1 0 foot above the roof surface 7 12 to 8 12 1 5 feet above the roof surface 8 12 to 9 12 2 0 feet above the roof surface.
These galvanized steel vents were all installed at the same time but only the combustion vent is staining the roof.
Any moisture entering this pipe will be drained off before it reaches the furnace by components designed to divert condensation so it doesn t need to be protected.
5 above the flue collar e g.
Chimney chimney flue must terminate min.
2 0 feet above the roof surface.
However i have searched archives on this and another site and have come across various interpretations of the height requirement.
Minimum height above the roof surface 1 flat to 7 12.
1 0 foot above the roof surface 1 7 12 to 8 12.
Water heater installed close to the roof framing would need at least 5 long vertical vent pipe single wall vent pipe requires minimum 6 of clearance to combustible materials.
For the next example you have a shorter chimney height.
1 5 feet above the roof surface.
2 run the vent pipe up through the roof and install a metal or pvc flashing collar over it.
The photo shows a b vent from a fan assisted furnace flue on a steep sloped hip roof.
2 5 feet above the roof surface.
I know about the 2 feet taller than the roof when a vertical wall is within 8 feet as in illustrated in my code check books.
