How to Attach Stairs to a Floor Joist

Set a piece of 3/4-inch plywood on a stable surface. Pull a tape measure from one end of the wood. Make a pencil mark 16 to 18 inches in from the end of the plywood. Make another mark perpendicular to the first at a length representing the width of the stairs.

Cut the plywood at the 16 to 18 inch mark with a circular saw. Cut along the stairway width mark with the circular saw to complete the joist extension.

Apply wood glue to the back of the joist extension. Hold the glued side of the piece against the floor joist. Align the top of the piece with the top of the joist. Have a helper run one 3-inch wood screw through the plywood and into the floor joist every four inches with a screw gun equipped with a Phillips tip adapter. Ensure all screws are tight.

Hold the assembled stair stringers against the joist extension. Hold an 8d nail against the bottom of one stair stringer. Tilt the nail to a 45-degree angle with the nail point aiming toward the joist extension. Drive the nail into the joist with repetitive hammer blows. Set one 8d nail through each stair stringer.

Hold a joist hanger against one stringer. Secure the hanger to the joist extension and the stringer by running one 3-inch wood screw through each mounting hole stamped in the hanger. Install one joist hanger to each stringer as described.