Cut the stringers. Cut three 2" x 12" stringers to accommodate a 3-1/2" ladder height. Cut a notch in the bottom of the stringers to fit over a 2" x 4" piece, making sure to cut notch. in the same place on each stringer.
For each stair, cut 10" rungs from 2" x 10" lumber and 7" posts from OSB sheets. Also, cut a 2" x 4" piece of treated lumber to 3' long.
Note: These stairs will rest on a concrete floor, but the untreated wood will absorb moisture and rot out of the concrete. To combat this, install the treated 2" x 4" squarely against the floor.
Place a stringer against the wall and have a helper hold it in place. Fit the treated 2" x 4" into the notch and press it flush to the wall. Use a masonry drill bit to drill through the 2" x 4" and into the concrete floor, then fasten the 2" x 4" to the floor with concrete nails.
Step the first stringer down to the 2" x 4" and attach it to the top stair bracket with metal brackets. Attach a second stringer in the same manner at the other 2" x 4" end. Attach the end stringer midway between the other two.
Install the 2" x 10" treads by nailing through the treads into the stringers. Nail the OSB risers to the stringers (Image 1).
The stairs are ready to be finished as desired (Image 2).
