Finally, after doing much research and waiting for good weather I will be installing a roof vent. What made me hesitate was the roof ribs, didn't know how to compensate for them. My research indicates that I should build up between the ribs with butyl tape then apply a continuous band of the tape on top of that. Once the vent is screwed down it should all push together for a good seal. After that use self leveling lap sealant on all screw heads, along the base of the butyl tape and anywhere else you might suspect could potentially be a problem. I'll let y'all know how it goes!