CSLB General Building (B)Mechanical, Electrical, Plumbing CoordinationEasy
A contractor is selecting fire blocking material for concealed stud cavities in a wood-framed home. Which of the following meets the California Residential Code minimum requirement for solid lumber fire blocking?
- A1/4-inch plywood with taped joints
- BNominal 2-inch lumber installed as a single thickness
- CNominal 1-inch lumber installed as a single thickness
- DNominal 1-inch lumber with joints aligned (not staggered)
Show answer & explanationAnswer & explanation
Correct answer: B. Nominal 2-inch lumber installed as a single thickness
CRC R602.8 allows nominal 2-inch (actual 1-1/2 inch) lumber as a single-thickness fire blocking material. Single 1-inch boards are too thin unless doubled with staggered (broken lap) joints, and thin plywood without backing does not qualify.
Why the other options are wrong
- A. 1/4-inch plywood is too thin and not an approved fire blocking material.
- C. A single 1-inch board alone is insufficient thickness for fire blocking.
- D. Two 1-inch boards must have staggered (broken lap) joints, not aligned joints, to qualify.
Fire Blocking Material Thickness
CRC R602.8 lists approved fire blocking materials, including nominal 2-inch lumber, two 1-inch boards with staggered joints, or approved panel products with backed joints.
- Nominal 2-inch lumber (actual 1.5") qualifies as single-thickness fire block.
- Two 1-inch boards need staggered/broken lap joints.
- Gypsum board, mineral wool, and certain panel products are also approved.
Memory trick: 'Two-by lumber blocks the fire, thin boards alone will only conspire (to fail).'