A tool for installing a length of backer rod, the backer rod being sized for insertion into a contraction joint within a concrete slab, the tool including a first carriage; a plurality of wheels mounted upon the carriage, the wheels being arranged for facilitating rolling motion of the first carriage along the contraction joint; a second carriage sized for suspension beneath the first carriage; a plurality of backer rod depressing disks including a frontmost disk, each disk being mounted rotatably upon the second carriage; jack screw disk positioners interconnecting the carriages and being adapted for, upon the rolling motion along the contraction joint, downwardly extending the disks into the contraction joint; a backer rod guide tube connected to the first carriage and being adapted for continuously positioning the length of backer rod beneath the disks; and a handle connected operatively to the first carriage.