A metal roof can be installed without using a ridge vent, assuming adequate ventilation is achieved by other methods. Metal tends to generate more heat than other roofing materials, so adequate ventilation is necessary and ridge vents are one of the most effective ways to do so.
A ridge vent is essentially a gap, covered by a cap of roofing material, that is cut along the entire length of the peak of the roof. Heated air in the attic, which naturally rises, exits through this vent drawing fresh air in through vents in the soffit, lower on the roof. This method of ventilation is effective since it relies on the natural convection currents of heated air. One of the other methods of ventilation replaces the vent in the ridge for vents in the gables of the house. Gable vents, which tend to be smaller in surface area than a ridge vent, are also not appropriate for every style of house.
The reasons for roof ventilation differ depending on the climate, but can be achieved with similar venting techniques in both cold and hot seasons. In cold climates, the reason for roof ventilation is twofold: firstly to expel moisture in the attic, and secondly to keep the roof cold. A cool roof temperature needs to be maintained to stop melting snow from sliding down the roof and forming ice damns. In hot climates, the primary purpose of ventilation is to remove warm air from the attic that has been heated by the sun.
Doing this reduces the building's requirement for cooling and subsequently lowers the strain on air-conditioning systems. In mixed climates, correct ventilation will provide both functions at different times of the year. A potential drawback to using a ridge vent, as opposed to another style of vent, is rain and snow entering the attic and causing rot. Alternatively, creating a completely unvented roof is an option, but generally only considered where snowfall is unlikely and humidity is very low.