A roof-mounted ventilator comprising a plurality of blades is disclosed. Each blade comprises front and back blade faces and leading and trailing edges. When the ventilator is viewed in side profile, the blades are positioned and configured such that a line of sight into the interior of the ventilator is substantially restricted or obstructed. Restricting or obstructing the line of sight into the ventilator can reduce or prevent various particles, such as water droplets, debris, dust and light, from entering the ventilator interior. For example, rain is thereby forced to strike a face of the blades, thus preventing direct access into the ventilator interior. The blade itself may include additional features, such as a lip that extends along the length of the leading edge, to further prevent particles from entering the ventilator interior, or to redirect the particles away from the ventilator interior in a controlled manner.