Purchase the exhaust fan (if you don't already have one) and a roof vent. Roof vents can be dome-shaped, square (like the one shown here.), rectangular or chimney-shaped. They all have a cap over ...

this Roof vent is also ideal for installation in other areas such as kitchens, bathrooms, and attics. It can be used in conjunction with plastic ducts or exhaust fans for easy ventilation.

From the fan, a single exhaust duct exits through the roof. Each bathroom has its own switch and operates independently.