Abstract: An adjustable roof ventilator has a base, a ventilation assembly and an interlocking arrangement for securing the ventilation assembly to the base at a desired tilt angle. The base has a bottom peripheral flange mountable on a pitched roof, and an elbow member upwardly projecting from the flange. The ventilation assembly has an upper venting arrangement in communication with a lower duct portion having front, rear and side walls defining a cross-sectional configuration tiltably mountable on the elbow member. The side walls have lower rounded front edges and the front wall has a lower edge extending between upper ends of the rounded edges so that the rounded edges are in pivotal engagement against the flange of the base as the rear wall of the duct portion is in sliding engagement against a curving rear wall of the elbow member when the duct portion is tiltably mounted on the elbow member.