An apparatus for installing siding boards comprises a body member, a siding-engagement member; and a cam member rotatably engaged with the body member. The cam member includes an eccentrically curved cam segment. The body member, siding-engagement member, and cam segment enable adjustment of vertical overlap of siding boards during installation with one of the boards engaged with the siding-engagement member and another board engaged against the eccentrically curved cam segment. A method for installing siding comprises engaging a pair of siding installation tools with an installed first siding board; positioning a second siding board against the eccentrically curved cam segments; adjusting vertical overlap of the boards by rotation of the cam members; securing the second board to an installation surface; rotating the cam members to disengage the cam segments from the second board; and disengaging and removing the installation tools from the first board.