Chimneys & the 3-2-10 Rule
~12 min read · Evaluate liners, crown, cap and flashing, and apply chimney height above the roof.
The chimney is a masonry exhaust stack living outdoors: its height follows the 3-2-10 rule, its liner must be whole, its crown sheds water or the freeze-thaw cycle dismantles it course by course — and the flashing where it pierces the roof is a leak franchise.
Height and clearance
The stable geometry: the chimney terminates at least 3 feet above the roof at its penetration point AND at least 2 feet above anything (roof, structure) within 10 feet horizontally — the 3-2-10 rule; short chimneys draft poorly and swirl smoke into the house. The masonry keeps 2 inches of clearance to combustibles inside the structure (visible in attics — framing tight to the chase or packed with insulation is a finding). Exterior lean, separated corbels, and step cracks in the stack are structural findings.
- 3 ft at the roof, 2 ft above anything within 10 ft
- 2-in combustible clearance where masonry passes framing
- Lean and stack cracking: structural referrals
Crown, cap, and masonry weathering
The crown (the sloped concrete top) sheds water away from the flue; cracked/flat/mortar-parged crowns admit water that freeze-thaw converts into spalling brick and open joints down the stack — the deteriorating-from-the-top-down pattern. A cap (screened) keeps rain, animals, and sparks in their places; missing caps fill smoke shelves with water and raccoons. Efflorescence banding on the stack marks chronic saturation; repointing needs get noted by extent.
- Crown sloped and sound — the stack's umbrella
- Cap present, screened
- Spalling and open joints: freeze-thaw's account ledger
Liner and interior passage
Flues need intact liners (clay tile historically): from below with a light — offset/cracked/missing tiles, mortar debris on the smoke shelf (tiles shedding), and heavy creosote glazing (a sweep/Level-II referral — the general inspector's view is honestly limited, and saying so is standard: 'full flue evaluation requires Level II [camera] inspection'). Shared-flue arrangements and abandoned openings sealed poorly complete the passage findings; the orphaned-appliance flue issue cross-references HVAC venting.
Worked example
A chimney rises 2 feet above its roof penetration, with the ridge 8 feet away standing a foot taller than the termination; the crown is cracked through with a chunk missing; no cap; the top four courses spall and shed brick faces; and flashlight-from-the-firebox shows tile shards on the smoke shelf. Assemble the chimney report.
Geometry first: 2 feet at the penetration fails the 3-foot rule, and a taller ridge within 10 feet violates the 2-foot clause — the stack is short twice over, explaining any drafting complaints; extension is the fix. The waterworks: cracked crown + no cap = the freeze-thaw engine running for years — the spalling top courses are its output, and the missing cap also explains water/animal exposure; crown rebuild, cap installation, masonry repair by extent. The shards on the shelf: the liner is shedding tiles — combustion gases and heat may be reaching places they must not; Level II camera evaluation referral before further use, stated as the standard limitation of the visual inspection. Height, water, liner — the chimney's three stories, each with its own remedy.
Common exam pitfalls
Judging height by looks.
Apply 3-2-10 with actual distances — the within-10-feet clause catches ridges and dormers that eyeballs forgive.
Calling crown cracks cosmetic.
The crown is the stack's roof — its failure funds every freeze-thaw repair below; report it as the cause it is.
Blessing a flue seen by flashlight.
Visible-portion language plus Level II referral is the standard — tile debris makes the referral urgent.
Three at the roof, two over ten; crown and cap keep the water out; and the liner's truth needs a camera.
Recap
- 3-2-10 termination rule; 2-in interior clearance
- Crown sound and sloped; cap present and screened
- Spalling/open joints = freeze-thaw from failed crown/cap
- Liner: visible checks + tile-debris evidence + Level II referral
- Creosote glazing → sweep referral
- Lean and structural cracking → specialist evaluation
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