In construction, blocking refers to short pieces of lumber or wood used as bracing between frame members. The intervals between blocks are specified in the building code, or calculated by a structural engineer.
Blocking reduces bounce and twisting of floor joists by distributing vertical load evenly across all the joists in a row. It also helps maintain spacing between joists and can increase lateral strength of the floor structure.
The International Residential Code (IRC) requires blocking or rim joists to be installed every 8 feet between 2x10 and taller floor joists. The American Wood Council’s 2015 edition of the National Design Specifications (NDS) also requires rim joists or band joists to be installed at both ends of a joist system.
Cross-bracing, or bridging, is another system that reinforces a floor structure similar to blocking. Instead of using a solid block between joists, this method uses a pair of strips that create an X shape.
Fire-blocking is a construction technique that seals off concealed spaces like stud cavities and soffits to slow or even stop the spread of flames, smoke and gases in a wall. It’s especially important in situations where the top plates of a wall or ceiling are not tight to their framing members, such as balloon-frame walls or dropped soffit ceilings.
When installing drywall, back-blocking is a useful reinforcing technique. It helps minimize ridges between butted joints and provides panel edge support, which makes drywall more resistant to deformation or cracking. It can also help secure cabinets and other heavy items that hang off the wall.
