How to Build a Wooden Base to Support Bookcases

Measure the width of the bookcase to find the base length. Cut two 2-by-4 pieces to the base length, using a miter saw, to use as runners.

Measure the depth of the bookcase and subtract the thickness of the baseboard on the wall, then subtract 3 inches. Cut two 2-by-4 pieces to the depth calculated, one for end each of the base, plus one additional piece of the same length for every 16 inches of bookcase width over 24 inches.

Place the two 2-by-4 runners parallel on edge. Put the other 2-by-4 pieces perpendicular to the runners on edge between them, one at each end, with the remaining pieces spaced evenly between the two end pieces.

Nail the runners to the perpendicular pieces with two 10d nails through each runner into the end of each perpendicular piece.

Place the wooden base on the floor in the desired bookcase location against the baseboard on the wall. Lay the level on top of the base to check for level from side to side. Place shims beneath the base to adjust the height until the level bubble is centered. Repeat the process placing the level front to back, adjusting as necessary, then recheck from side to side.

Drill a pilot hole 2 inches from each corner -- one in each 2-by-4 -- at a 45 degree angles beginning 1 inch above the floor through the base and down into the floor on the inside of the base. Place a screw into each hole and drive the screw through the wooden base and into the floor. Add two holes and screws for each 2-by-4 between the two on the ends.

Set a piece of base molding against the left side of the wooden base. Mark the front of the wooden base on the molding's top back edge. Place the molding on the miter saw with the table set to 45 degrees right. Align the blade with the mark and cut the left-side base molding off. Cut the right-side base molding with the table set to 45 degrees left. Nail the two base moldings to the sides of the wooden base with finish nails.

Cut one end off a piece of the base molding with the table angle set to 45 degrees left. Measure the distance between the two ends of the moldings already nailed to the wooden base. Transfer the measurement to the base molding on the saw and cut it with the table angle set to 45 degrees right. Nail the molding to the front of the wooden base.