How to Install a Floor Joist Over a Girder


A girder is a beam made up of solid lumber, built-up lumber or engineered products. These are used in floor construction to hold the live load and dead load of the subfloor. A girder beam's most common use is when floor joists do not span the entire width of the structure. That means a pair of floor joists will meet and overlap on top of the girder beam. Next, solid blocking is added between each pair of floor joists to stiffen and strengthen the floor. Floor joist material commonly consists of two-by-six to two-by-10 lumber.

Things You'll Need

  • Tape measure
  • Speed square
  • Joist lumber
  • Hammer
  • 16d sinkers
  • 8d sinkers
  • Circular saw
  • Safety glasses
  • Lay out the locations of the floor joists on the header joists, or sill plates if no header joists are present. Working left to right, hook the butt end of the tape measure to the outside corner of the header joist/sill plate. Look at your floor plans to determine what type of spacing you need between each floor joist; the typical layout is the first mark at 15 ¼ inches followed by every 16 inches. Follow these marks with a speed square, scribing straight lines across the top and side if possible. On one side of the building, place an X to the right side of each line. Place an X on the left side of each line on the opposite wall.

  • Hook the butt end of the tape measure to the end of the building and place marks on the girder beam matching those along the header joist/sill plates. Scribe a line across the top using the speed square followed with an X on the right side.

  • Set the first joist with one end on the wall and the other resting on the girder beam. Line up the butted end of the joist to your first scribed line with the X along the header joist. Hammer three evenly spaced 16d sinkers through the face of the header joist into the end of the floor joist to secure it into place.

  • Adjust the floor joist over the girder beam until it is lined up with the first mark with the X. Make sure the floor joist is parallel to the end joist and check that it overlaps the girder beam by at least three inches. Toenail it into place using two 8d sinkers on one side and one on the opposite side. Continue down one side of the structure until one half of the building floor is complete. Double-check the measurements to make sure the floor joists are all parallel to each other.

  • Start the other half of the building in the same manner but with the X on the left side of the scribed lines. Secure the floor joist to the header joist/sill plate. Check that the floor joist overlaps the beam by three inches and press it tight against the installed floor joist on the other half of the building. Make sure it is parallel to the end joist and toenail. Install each joist until the floor is complete.

  • Measure the space between the installed floor joists directly above the girder beam. Transfer this measurement to scrap joist material and cut it to size with a circular saw. Slide the block between the floor joists so one end rests on the girder beam. The opposite end of the blocking should be flush with the top of the floor joist. Toenail it into place for solid blocking. Continue with each space between the joists along the girder beam.

Tips & Warnings

  • When using hand and power tools, wear safety glasses to protect your eyes from flying debris.

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