How to Build a Floor Joist


Floor joists support the floor of the building. The joists run across the short length of the building and may be supported in the middle of the building for wider structures. Standard home type construction places the ends of the joists on the sill at the top of the basement wall. The building process of creating floor joists requires some planning before the construction begins.

Things You'll Need

  • Hammers
  • Tape measure
  • Power saw
  • 16d nails
  • Locate the position of any toilets. The drain for a toilet requires a space of at least 13 inches between floor joists. Mark the two joists on each side of the planned drain.

  • Mark the joist positions 16 inches on center starting at the two initial joists. Mark the joists on the sill on both sides of the home.

  • Place a board on edge along the outside of the sill. This board is toe nailed, nails driven at an angle into the sill, with 16d nails.

  • Place a joist at the marked location. Toe nail through the bottom of the joist into the sill. Drive nails through the upright board above the sill into the joists further securing the joists. Use 16d nails in both situations.

  • Continue to add the joists in the marked positions across the floor of the house.

Tips & Warnings

  • Check the lumber for straightness before using as a floor joist. If the board is twisted discard it. If it has a minor bend place the high part of the board on the top side of the joist.
  • The building plan indicates the lumber size for the floor joists. The lumber ranges from 2-by-6-inches to 2-by-12-inches or manufactured joists built of plywood and lumber components can be used. The size of the joists is determined by the span of the joists and the anticipated weight load in the room.
  • Check the plans for additional joists. Some plans call for extra floor joists under heavy loads such as bathtubs or large fireplaces or wood stoves.

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