Concrete (or masonry) steps are commonly built between a garage that is on grade and a home which is on a basement or crawl space type foundation. This is fine unless you decide to convert your garage into living space and you are confronted with how to cover the masonry stairs with wood so that they no longer look like exterior stairs.
Measure and cut treated wood fir strips with a table saw, the same width as the concrete tread. Position one fir strip against the concrete riser and one at the front edge of the step. Apply a serpentine bead of construction adhesive to the back of each fir strip and press it into position.
Divide the width of the tread into even sections from twelve to sixteen inches apart and mark the tread. Measure and cut fir strips to fit between the two fir strips already installed, positioned at your marks. Apply a serpentine bead of construction adhesive to the back of each fir strip and press it into position. Repeat for each tread.
Measure and cut treated wood fir strips the same width as the concrete riser. Position one fir strip against the concrete tread and one at the front edge of the tread fir strip. Apply a serpentine bead of construction adhesive to the back of each fir strip and press it into position.
Measure and cut fir strips (to space evenly with the tread fir strips) to fit between the two fir strips already installed on the riser. Apply a serpentine bead of construction adhesive to the back of each fir strip and press it into position. Repeat for each riser. Your risers and treads should now be faced with treated fir strips. Add finish nails joining the tread and riser edge strips after the adhesive cures (about 24 hours).
Fit the treated wood stringer so that it aligns with the top of the fir strips for both the treads and the risers. One end of the stringer should rest on the floor. The other end should finish at the top of the stairs. For stairs that are flush against a wall, you will only install one stringer. For stairs open on both sides, you will fit a second treated wood stringer on the remaining side.
Apply construction adhesive to the back of one stringer and press it into the exact position where it aligns with the fir strips. (You may need a friend to hold the stringer in place while drilling.)
Mark the stringer at two inches from the top and bottom edges and every twelve to sixteen inches in between, roughly centered on the stringer. Drill the top mark (through the wood and into the concrete) using a rotohammer installed with a one quarter inch by four inch long masonry bit. Wear eye protection as you drill. Pull the bit out frequently to remove debris.
Position two (16 penny) sinker nails in the hole, use a twenty ounce hammer and evenly drive the nails into the hole until the stringer is snug. Make sure your alignment with the fir strips has remained perfect. Repeat this process for the hole closest to the bottom of the stair. Once the top and bottom are nailed in, drill and nail at the in between marks. Repeat for both stringers. Your concrete stairs are now ready for finish wood.
Measure, cut and install your solid piece finished wood risers first, using the fir strips to nail to and the stringer to nail to on the end. Measure, cut and install solid wood treads allowing the stair nose to overhang the riser. Allow for sheetrock or other types of facing to be applied over the stringer when you fit your treads.