Laying a floor joist in a wood frame house is a simple procedure that can be done in 4 easy steps. Wood joists are structural framing members that are laid upright (on edge) and nailed equally spaced (usually 16 inches on center) onto the wall plates. Joists range in size from 2 x 6 to 2 x 12 based on the distance being spanned. Joists are cut to lay flush with the outer surface of the exterior walls and usually run parallel to the short overall dimension of the floor plan. If the house is a multi-story structure, the joists would function as ceiling joists for the lower floor and floor joists for the upper floor. When used as floor joists, subflooring or decking is installed onto the upright joists in either 4 x 8 sheets of plywood or interlocking tongue and groove boards.
Step 1: Select Appropriate Boards for the Joists
A board can be curved in 3 different directions. A bow refers to the curve in a plane perpendicular to the face (long dimension) of the board. A board that is twisted is curved in a plane that is perpendicular to both the short and long dimension (thickness and width). A board with a pronounced “crown” or side bend has a curve in the plane parallel to the face of the board. Select boards that are neither twisted nor bowed for the joists. A board with a crown is acceptable and even preferable. Joists are installed with the crown pointing up so that over time the applied loads will cause it to straighten. A board installed with the crown pointing downward is sagging and will most likely sag even further.
Step 2: Remove Material to Expose the Rough Framing
Installing joists in new home construction is much easier than replacing joists in an existing house, but the end result is the same. If you are replacing existing joists, you will most likely have to remove the flooring and subflooring to get to the structural framework.
Step 3: Install the Joists
Measure the distance being spanned and cut the joists to length. If the plate is on an interior wall, the joists may extend past the plate. Layout the joists (mark equal spacings) onto the plates; continue the spacing pattern of the existing adjacent joists. Place the joists upright onto the plates on the marks and toenail each joist into the plate with 2 nails on each side. Make sure that the joists are installed parallel to adjacent joists. Measure the spacings between adjacent joists at both ends of each joist.
Step 4: Install Bridging for Spans Over 15 Feet
Many building codes require bridging for joist spans greater than 15 feet. Bridging consists of short blocks that maintain equal spacing and prevent buckling. Using lumber of the same dimensions as the joists, cut blocks to fit the cavity between each pair of joists. Install the blocks staggered, flush with the top edge and perpendicular to the joists at the midpoint of the span.
TIP: Check your area's building codes to find its specific regulations on joint spans and other requirements for installing joists before you start working.