Drywall (C-9) · Lesson 4 of 5

Rated Walls: Box Openings and Concealed-Space Signs

"Rated Walls: Box Openings and Concealed-Space Signs" is lesson 4 of 5 in the Drywall (C-9) Learning Hub. It opens with "Boxes in a Rated Wall Are Membrane Penetrations" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Boxes in a Rated Wall Are Membrane Penetrations
  2. Exception 1: Steel Boxes in Walls Rated 2 Hours or Less
  3. Keep Opposite-Side Boxes Apart
  4. Listed Boxes, Larger Boxes, and Putty Pads
  5. Marking Rated Walls in Concealed Spaces

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Wall area and allowed aggregate area" through "Count the signs".

What you'll be able to do

  • Explain the main idea of "Rated Walls: Box Openings and Concealed-Space Signs" in plain language you can use on the exam.
  • Apply this step with confidence: "Keep Opposite-Side Boxes Apart".
  • Apply this step with confidence: "Listed Boxes, Larger Boxes, and Putty Pads".
  • Apply this step with confidence: "Marking Rated Walls in Concealed Spaces".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Rated Walls: Box Openings and Concealed-Space SignsPLAN: BOXES ON OPPOSITE SIDESstuds 16 in. o.c. (2 px = 1 in.)Side ASide B32 in. apart (24 in. minimum)BOX AREA: 50 ft x 10 ft wallWall: 50 x 10 = 500 sq ftLimit: 100 sq in. per 100 sq ft = 500 sq in.30 single-gang x 6 sq in. = 180 sq in.6 four-square x 16 sq in. = 96 sq in.Total 276 sq in. - under 500Busiest 10 x 10 ft bay: 10 x 6 + 2 x 16= 92 sq in. (OK); 2 more boxes = 104 (over)CONCEALED-SPACE SIGNS (CBC 703.5): 50 ft wall = 2 signsWithin 15 ft of each end; no more than 30 ft apart; letters 3 in. high15 ft20 ft15 ftBoxes: 276 of 500 sq in. allowed, 32 in. apart | Signs: 2, at 15 ft and 35 ftSources: 2025 CBC 714.4.2 Exception 1 (steel boxes, wall rated 2 hours or less) and 2025 CBC 703.5.Box sizes in the example are assumed from a shop drawing; the sign count is arithmetic on the printed rule.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Boxes in a Rated Wall Are Membrane Penetrations

When a cutout is made in the board of a fire-resistance-rated wall for an electrical box, 2025 CBC Section 714.4.2 treats it as a membrane penetration. It must comply with Section 714.4.1 (a firestop system installed as tested in the approved assembly, or an approved penetration firestop system), and recessed fixtures must be installed so the required fire resistance is not reduced. Six exceptions then say when electrical boxes are allowed without a separate firestop system. The cutout size and gap are yours; the limits are printed numbers.

Step 2: Exception 1: Steel Boxes in Walls Rated 2 Hours or Less

Steel electrical boxes are allowed in walls and partitions rated up to 2 hours when each box does not exceed 16 sq in. in area, the aggregate area of the openings through the membrane does not exceed 100 sq in. in any 100 sq ft of wall area, and the annular space between the board and the box does not exceed 1/8 in. So the gap around a box is a pass/fail number, and a loose cutout fails it.

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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