To provide a building ruler which enables a tile to be accurately and easily cut along a corner ridge or a valley and which is suitable for the cutting of the tile in a place except the top of a roof, and a usage method for the building ruler.

This building ruler 1 is used for cutting a roof tile along the corner ridge or the valley. The corner ridge is formed by crossing the two roofs in which a pitch of the roof, expressed by the ratio of the vertical height of a roof surface to the horizontal length of the roof surface, is 10:n. In this case, a first linear scale portion 10, which has scales provided on both sides, and a second linear scale portion 20 are integrally formed; and a specific inner angle is provided.