Generally, economics and environmental rules dictate when a floating roof is needed. Most areas let you choose internal or external. But watch for special local rules that may apply. For instance, New Jersey USA requires all tanks to be fixed roof tanks with floating roofs if they store volatile organic compounds. The decision of internal vs external is otherwise more a matter of lifetime cost. External roofs are subject to environmental loads and degradation, internal roofs require a fixed roof on the tank. If you are in an area where wind, sun, salty air, rain, odd weather conditions and/or snow will affect an external floating roof's efficiency, operation or longevity go internal. Try to evaluate the lifetime costs of maintenance, painting, inspections and repairs for both types as well as capital costs.
