What is the purpose of roof legs on a floating roof system?

• A. To prevent roof from moving all the way to the tank bottom when the tank is empty
• B. To connect the roof to the top manway
• C. To vent excess pressure
• D. To anchor the roof to the ground

Answer: (A)

Roof legs keep the floating roof from resting on the tank bottom when the liquid level becomes very low or the tank is empty. Without buoyancy to support it, the roof would sink toward the bottom, which can damage seals, the roof structure, or the legs themselves and make re-floating difficult. The legs hold a safe clearance, allowing the roof to rise again as the tank is filled and buoyancy returns. This is why the primary purpose is to prevent the roof from moving all the way to the bottom when the tank is empty. The other ideas—connecting to a manway, venting excess pressure, or anchoring to the ground—do not describe how roof legs function.