Masonry (C-29) · Lesson 2 of 8

Sizing the Flue for a Masonry Fireplace

"Sizing the Flue for a Masonry Fireplace" is lesson 2 of 8 in the Masonry (C-29) Learning Hub. It opens with "Why the Flue Is Tied to the Opening" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Why the Flue Is Tied to the Opening
  2. Get the Opening Area
  3. Pick the Ratio for the Flue Shape
  4. Read the Net Area from the Tables
  5. Check the Passageway and Install the Liner

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Opening area" through "Passageway".

What you'll be able to do

  • Explain the main idea of "Sizing the Flue for a Masonry Fireplace" in plain language you can use on the exam.
  • Apply this step with confidence: "Pick the Ratio for the Flue Shape".
  • Apply this step with confidence: "Read the Net Area from the Tables".
  • Apply this step with confidence: "Check the Passageway and Install the Liner".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Fireplace Flue Sizing: Opening x RatioFIREPLACE OPENING36 in x 30 in= 1,080 sq in36 in wide30 inarea = width x heightSHAPERATIONEEDSELECTED (NET AREA)Round1/1290 sq in10 3/4 in = 90 OK10 in = 78 is too smallSquare1/10108 sq in13 x 13 = 127 OK12 x 12 = 102 is too smallRect. under 2:11/10108 sq in12 x 16 = 131 OKaspect 16 / 12 = 1.33Rect. 2:1 or more1/8135 sq in8.5x18 = 101 FAILaspect 18 / 8.5 = 2.1ratio x opening area = minimum net flue areachoose a listed size with net area at or above itMinimum flue area = 1,080 sq in x ratio for the flue shapePass: 10 3/4 in round, 13 x 13, 12 x 16. Fail: 8.5 x 18 (101 under 135)Passageway above the firebox (throat, damper, smoke chamber) at least the flue area (R1001.6)Source: 2025 CRC R1003.15.1, Tables R1003.14(1)-(2), R1001.6; CBC 2113.16CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Why the Flue Is Tied to the Opening

A big fireplace opening pulls a lot of air and smoke, so the code sizes the chimney flue from the opening, not from the chimney you wish to build. 2025 CRC R1003.15 gives two ways: Option 1 (R1003.15.1) uses a ratio of the opening area, and Option 2 (R1003.15.2) uses Figure R1003.15.2, which sizes the flue from the chimney height measured from the firebox floor to the top of the flue. Option 1 is the one you can do with a tape measure and a calculator, so this lesson works it. CBC 2113.16 carries the same rules.

Step 2: Get the Opening Area

Multiply the fireplace opening width by its height in inches to get square inches. A 36-inch by 30-inch opening is 36 x 30 = 1,080 square inches. The flue net area is also in square inches, so there is no need to convert to square feet here (that conversion is only for the 6-sq-ft hearth extension test in R1001.10).

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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