For a masonry chimney coming through the high ridge, how high must the flue top be above the roof ridge. If you have an answer, state which model code is being applied (UBC, CABO, IBC, . . . ).
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NFPA 211 says a chimney (masonry or prefab) must be 3 feet above any portion of the roof within 10 feet measured on a horizontal plane. So, 3' above the ridge would solve safety issues. There are factors that could affect draft that might dictate a taller chimney as well as cosmetics.
We build stone-clad chimneys here with big flat stone or concrete "roofs" over the chimney flue top. The cap adjacent the flue top has stone or concrete "columns" supporting the "roof."
Would the 3' rule apply to the flue top, or "roof" top? "Roof" is typically a foot or more above the flue top end.
I don't have code to cite but have installed a couple hundred or so of metal flues and had a couple dozen masonry ones done. Thje standard is two feet above the ridge or three feet above any point within ten feet of the chimney. That is to top of flue, not rain cap. The minimum height of the flue from firebox should be fifteen feet.
Things that can cause need for a higher chiney are elbows below, which decrease draft by about 15% for each one, a hill or tall trees upwind that can cause downslope conditions, or a large firebox openning such as a two sided fireplace. generally a taller flue creates a higher stack velocity or draft.
There are specific formulas that my masons sometimes refer to, but I go with rules of thumb and we tend to agree. It generally never hurts to go too high but it can be a source of failure to have too low a top. I also like the appearance of a chimney that tops out three or four feet above the ridge.
You obviously have a reason for asking. Care to share? already done too short and arguing with your mason? or just planning a new job?
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My mason just finished a job within sight of my project, a big rustic Craftsman style thing that closed last week for a cool million, and the stone chimney coming straight through the center of the highest ridge only pokes through with the top of clay flue looking like about 16 or 18 inches above the ridge. Looks ridiculous. Kinda like a little stone button on top.
He's got my fireplace core structure up and over the end of the gable (it's an outside-the-wall flue), and weathered in at the roof, with the stone finish yet to go. Things are looking a little short to me on this one, and I want it higher.
Per the diagram below, the flue top, for one going right through the ridge like mine (and the button up the street), needs to be 3-feet up from the elevation where the chimney side comes through the roof. So, with that rule, 3-feet over the RIDGE is higher than required by code. If a 36-inch-width stoneclad chimney goes dead center through the ridge of a 7/12 roof, the flue top should be (36" minus ((36/2)/12)*7), or 25.5 inches above the ridge. Still too short for yours and my taste.
My mason quoted this job from drawings that show the chimney flue clearly dimensioned out as ending 3'-0" above the ridge. I'll not cite code, but simply haul out the drawings.
Here is what I have found out re code:
The minimum chimney height for fire safety is the greater of 3 ft. above the highest point where the chimney penetrates the roofline, or 2 ft. higher than any portion of the structure or adjoining structures within 10 ft. of the chinmey.
It is interesting that the note with the drawing you posted says that min "for fire safety" should be three feet. I had always understood it to be connected to draft needs rather than fire safety, though I can see that too, with a wood roof.
If he quoted based on your drawings, just be sure to remind him that's waht the drawiongs say.
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