Masonry (C-29) · Lesson 1 of 8

Masonry Fireplace: Hearth, Firebox Walls and Clearances

"Masonry Fireplace: Hearth, Firebox Walls and Clearances" is lesson 1 of 8 in the Masonry (C-29) Learning Hub. It opens with "What the Code Controls on a Masonry Fireplace" and works through 5 short steps, then a worked example you can check yourself against.

Open the free trialSee pricing

One-time purchase, no subscription. Law & Business included.

Steps in this lesson

  1. What the Code Controls on a Masonry Fireplace
  2. Hearth and Hearth Extension: Thickness and Size
  3. Firebox Walls, Depth, Throat and Lintel
  4. Clearance to Combustibles and Mantels
  5. Footing, Cleanout and the Order of Checks

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Find the opening area" through "Check the framing".

What you'll be able to do

  • Explain the main idea of "Masonry Fireplace: Hearth, Firebox Walls and Clearances" in plain language you can use on the exam.
  • Apply this step with confidence: "Firebox Walls, Depth, Throat and Lintel".
  • Apply this step with confidence: "Clearance to Combustibles and Mantels".
  • Apply this step with confidence: "Footing, Cleanout and the Order of Checks".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Masonry Fireplace: Hearth Extension SizingPLAN VIEW - HEARTH EXTENSION36 in openingHEARTH EXTENSION60 in wide x 20 in deep12 in36 in12 in60 in overall20 inFIREPLACE CHECK (CRC R1001)Opening 36 x 30 = 1,080 sq in1,080 / 144 = 7.5 sq ft (≥ 6)Hearth extension (R1001.10):front 20 in, each side 12 inwidth 12 + 36 + 12 = 60 inFirebox walls (R1001.5):2 in firebrick, 8 in solid totalMantel 10 in above the opening:10 x 1/8 = 1.25 in max projectionnever within 6 in of the openingHearth extension 60 in wide x 20 in deep, 2 in thick minimum7.5 sq ft opening is over 6 sq ft, so the larger extension appliesWood framing: 2 in from front and sides, 4 in from the back (R1001.11)Source: 2025 CRC R1001.5, R1001.9-R1001.11 (CBC 2111.6, 2111.10-2111.12 print the same numbers)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: What the Code Controls on a Masonry Fireplace

A site-built masonry fireplace in a house is governed by 2025 CRC Section R1001 (CBC Section 2111 carries the same rules for other buildings). Table R1001.1 summarizes the key numbers, but the table itself says it does not cover all requirements, so the text sections control. CBC 2111.2 also requires the construction documents to show the location, size and construction of the fireplace, the thickness and characteristics of materials, and the clearances to walls, partitions and ceilings.

Step 2: Hearth and Hearth Extension: Thickness and Size

The hearth is at least 4 inches thick (R1001.9.1) and the hearth extension at least 2 inches thick (R1001.9.2). Exception: if the bottom of the firebox opening is raised at least 8 inches above the extension, a 3/8-inch-thick brick, concrete, stone, tile or other approved noncombustible extension is allowed. Both are concrete or masonry, supported by noncombustible material, reinforced to carry their own weight and imposed loads, and no combustible material may be left against the underside (R1001.9). Size comes from the opening area, width times height divided by 144 to get square feet: under 6 sq ft the extension runs 16 inches in front and 8 inches beyond each side; 6 sq ft or more it runs 20 inches in front and 12 inches beyond each side (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.

More lessons for this trade

Browse all lesson previews

Ready to practice for real?

Try the real classroom free, no sign-up, or get the full course for your trade.

Open the free trialSee pricing

Independent exam preparation. Contractors Classroom is not affiliated with the Contractors State License Board, and no course can guarantee a passing result.