Masonry Chimney Height, Hearth Extension and Flashing on a 6:12 Roof
A masonry fireplace and chimney are being built on the rear slope of a one-story house with a 6:12 gable roof covered in asphalt shingles. The chimney is 40 inches wide at the roof and passes through the sheathing 6 feet horizontally from the ridge. Measured at the roofline, the top of the flue is 30 inches above the sheathing, and the masons plan a concrete cap. The ridge surface is 3 feet above the roof surface where the chimney penetrates. The fireplace opening is 36 inches wide by 30 inches high. The hearth extension is brick over a wood-framed floor, laid 16 inches in front of the opening and 8 inches beyond each side. The owner wants oak flooring run up to the hearth edge. No cricket is planned.
1. Is the chimney height acceptable? State both height rules, apply them to this geometry, and give the required dimension.
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Two rules apply, and the taller result controls.
Rule 1: the chimney must extend at least 3 feet above the highest point where it passes through the roof. Here the flue top is 30 inches, or 2.5 feet, above the sheathing, so this rule alone is already failed by 6 inches.
Rule 2: the chimney must be at least 2 feet higher than any portion of the building within 10 feet horizontally. The ridge is only 6 feet away, so it counts, and the roof rises 3 feet over that 6 feet at a 6:12 slope. To clear the ridge by 2 feet, the flue top must be 3 + 2 = 5 feet above the roof sheathing where the chimney penetrates.
Rule 2 governs: the flue needs to be 5 feet above the sheathing at the penetration, which is 2.5 feet more than what is proposed. Extending the masonry is the usual fix; adding a listed chimney extension or a factory-built section is only acceptable where the system is listed and installed per its listing. Height matters for draft and for spark clearance, so it is not a finish item to be resolved later.
2. Are the hearth extension dimensions correct, and can the oak flooring be run to the hearth edge?
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Size the hearth by the area of the fireplace opening. Here 36 inches x 30 inches = 1,080 square inches = 7.5 square feet, which is more than 6 square feet. That places the design in the larger-opening category, which requires a hearth extension of at least 20 inches in front of the opening and at least 12 inches beyond each side.
The proposed 16 inches in front and 8 inches at each side is short on both dimensions, by 4 inches each. The extension must also be noncombustible and supported, which matters here because it is brick over a wood-framed floor.
The oak flooring cannot run to the hearth edge. The required noncombustible extension has to occupy that full 20-inch by side-clearance footprint, so wood flooring and any wood subfloor protection stop at the outer edge of a compliant extension, not at the face of the hearth. Approval should be withheld until the hearth is enlarged and the floor layout is revised.
3. What roof flashing requirement applies to a chimney of this width, and does it change the design?
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Yes, it changes the design. A chimney that is 30 inches wide or less can be flashed on the high side with standard backer flashing. A chimney wider than 30 inches requires a cricket, or an equivalent backer detail, on the uphill side to divert water around it. This chimney is 40 inches wide, so a cricket is required and none is planned.
Beyond the cricket, the flashing itself must be corrosion-resistant and installed in the sequence the roof covering requires: base flashing at the roof-to-masonry joint, step flashing woven into each shingle course along the sides, and counterflashing let into the masonry above the roof line so water cannot enter behind the flashing. Sealant alone is not flashing.
The judgment point is that a wide chimney is a water-diversion problem, not just a sealing problem. Without the cricket, water and ice collect against the uphill face, and the resulting leak will not show up until the ceiling below is stained, long after the final inspection.