Vacuum arc cathode spot motion was investigated on a "roof-shaped" aluminum cathode under an external magnetic field. The cathode included a flat roof-top and four sloped sides, inclined by an angle α. A high speed camera was used to observe cathode spot motion. The spot velocity on the slopes and the distribution of cathode spots on the roof and slopes was determined. It was obtained that under a magnetic field, the spot motion on the roof was slow (<1m/s) and mainly random, while on the slopes fast retrograde motion was observed. This velocity increased linearly with the magnetic field and decreased slightly with α.