PURPOSE: To costlessly form a modular roof structure in a efficiently constructible simple structure by using Y-shaped modular construction members.

CONSTITUTION: A modular roof structure comprises of first and second Y-shaped modular construction members and the first member is composed of first, second, and third branch sections 12a, 12b, and 12c of substantially equal lengths. The second member is also composed of fourth, fifth, and sixth branch sections 14a, 14b. and 14c of substantially equal lengths. The first and fourth branch sections 12a and 14a are coupled together along a common axis to define the apex 22 or ridge of a roof structure and the second, third, fifth, and sixth branch sections 12b, 12c, 14b, an 14c are coupled between the apex 22 and four corners of a rectangular base frame 30. At least one gable 32 is formed in the intermediate section of the first and second structural members. When the length of the intermediate section of the roof structure 10 is adjusted, the length of the structure 10 can be adjusted along the common axis. A foldable or inflatable roof cover is attached to the roof structure.