Blocking a Ceiling Joist

Ceiling joists span from wall to wall, providing the base of the roof structure, in addition to being used to support ceiling lights and to attach drywall. In specific instances, such as the installation of a return air duct, a recessed can light or stovepipe, you may have to insert joist blocking to control airflow or to keep insulation away from a hot can or pipe. Blocking a ceiling joist is a relatively simple project.

Step 1

Measure the space between the two ceiling joists.  If the joists sit on 16-inch centers, meaning it’s exactly 16 inches from the center of one joist to the center of the next joist, the space between the joists will be 14 1/2 inches. For joists on 24-inch centers, the space between the joists is 22 1/2 inches. 

Step 2

Cut a board to match the measurement.  To create a full block between the two joists, the board should be the same dimension as the joists. For example, if the joists are constructed from 2-by-10s, use a 2-by-10 for the block.  In some instances, a 1-by-10, or a piece of drywall, cut to the same dimension, suffices. If possible, though, opt for same-size dimensional lumber. 

Step 3

Position the cut board sideways, between the two joists, and secure it with two 12d nails, inserted from the outsides of the supporting joists, into each end of the board.  Space the nails evenly. Because you're driving the nails in a restricted space, a nail gun works better than a hammer. 

Things You Will Need

  • Circular saw
  • Tape measure
  • Dimensional lumber
  • Nail gun with 12d nails


  • Joist blocking prevents something from occurring. It might prevent air in the joist space from being drawn into a return air vent or an attic fan, or it might block insulation from filtering into the edge of an attic where it could block fresh air from a soffit vent.
  • Joist blocking is different from joist bridging, which entails adding staggered boards between all the ceiling joists for additional structural support. Bridging installs in the same manner as blocking, but on a larger scale and for a different purpose. The words are often used interchangeably, though.
  • If you’re cutting out a ceiling joist to install a skylight or an opening for attic stairs, you will need to install additional support between the remaining joists, in the form of a header, and you’ll have to double the adjacent joist with the addition of trimmer joists.


  • If the ceiling contains trusses and not joists, do not install blocking without first consulting an engineer or the truss manufacturer.

About the Author

Glenda Taylor is a contractor and a full-time writer specializing in construction writing. She also enjoys writing business and finance, food and drink and pet-related articles. Her education includes marketing and a bachelor's degree in journalism from the University of Kansas.