The purpose of fire blocking is to prevent fire from spreading through the concealed spaces of a building. It works by dividing framing cavities into separate compartments, slowing the passage of flames and combustion air.

In an unblocked balloon-framed building, for example, a fire that starts in the basement can rapidly travel up the stud bays and spread into the joist bays and attic. In a platform-framed structure, though, the top plates separate stud bays from joist bays. This means a fire that starts in a wall cavity would have to burn through the plate to...

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