Internal vs External Floating Roof Tanks

The Difference on the Top of Your Tank

Large aboveground storage tanks are a critical part of the global energy infrastructure. From crude oil terminals to chemical storage, these tanks safely store massive volumes of volatile liquids like gasoline, ethanol, and refined fuels. One of the most important design choices in these tanks is what sits on top of the stored liquid. Understanding the differences between internal vs external floating roof tanks can help operators choose the most effective solution for emissions control and storage efficiency.

Floating roofs are engineered to reduce vapor losses, improve community and environmental safety, and help control emissions. Instead of leaving an open vapor space above the liquid, the roof floats directly on the product surface and moves up or down as the tank fills or empties. By minimizing the space where vapors can accumulate, floating roofs significantly reduce evaporative losses and volatile organic compound (VOC) emissions.

However, not all floating roofs are the same. In modern storage tanks, operators typically choose between External Floating Roof Tanks (EFRTs) and Internal Floating Roof Tanks (IFRTs). Understanding the difference between these designs is essential for selecting the right emissions control strategy and optimizing tank performance.

What Is a Floating Roof Tank?

A floating roof tank uses a buoyant roof that rests directly on the liquid surface, rising and falling as the product level changes. This design greatly minimizes the vapor space inside the tank and dramatically reduces evaporation and product loss compared to traditional fixed-roof tanks.

A seal around the perimeter of the floating roof – known as the rim seal – creates a tight barrier between the roof edge and the tank shell, further limiting vapor escape.

These systems are widely used in petroleum and petrochemical storage because they help facilities:

Reduce VOC emissions
Improve environmental compliance
Protect product quality
Minimize economic losses from evaporation

The two primary floating roof configurations differ mainly in where the floating roof sits in relation to the structure of the tank.

External Floating Roof Tanks (EFRTs)

An External Floating Roof Tank is an open-top storage tank where the floating roof sits directly on the liquid and is exposed to the outside environment.

How EFRTs Work

In an EFRT, the tank has no permanent fixed roof. Instead, the floating roof forms the top surface of the tank. As the stored liquid level changes, the roof moves with it, maintaining contact with the product surface and limiting vapor space.

Because the roof is exposed to the elements, it typically requires:

Roof drains to remove rainwater
Rim seals to limit emissions
Support legs that allow the roof to rest when the tank is nearly empty

Advantages of External Floating Roof Tanks

EFRTs are often selected because they offer:

Lower construction cost due to a simpler design
Effective vapor loss reduction compared to fixed-roof tanks
Ease of inspection and maintenance in some operations

These tanks are commonly used for large-scale crude oil or diesel storage, particularly in regions with relatively mild weather conditions.

Limitations of EFRTs

The primary drawback of EFRTs is environmental exposure. Because the roof is open to weather, it can experience:

Rainwater accumulation
Wind movement
Sunlight and corrosion
Seal wear from environmental stress

These conditions can accelerate maintenance requirements and potentially increase emissions if seals degrade.

Internal Floating Roof Tanks (IFRT)

An Internal Floating Roof Tank includes two roofs:

A fixed roof on the tank structure
A floating roof inside the tank resting on the liquid surface

The internal floating roof performs the same function as an external one, but the fixed roof above it protects the system from the outside environment.

How IFRTs Work

The internal floating roof moves with the product level, minimizing vapor space and controlling evaporation. Because the system is enclosed within a fixed roof, the floating roof and seals are shielded from wind, rain, and sunlight.

Advantages of Internal Floating Roof Tanks

IFRTs are often preferred when:

Strict emissions control is required
The stored product has high vapor pressure (such as gasoline or jet fuel)
Facilities must meet tight environmental regulations

The enclosure improves operational stability by preventing environmental forces from affecting the floating deck and seals. This can result in more consistent emissions performance and reduced risk of fire or ignition sources compared to open systems.

Limitations of IFRTs

Despite their benefits, internal floating roof tanks typically involve:

Higher construction costs
More complex installation
Additional structural components

For some applications – particularly lower volatility products – operators may choose the simpler external floating roof design instead.

Key Differences Between Internal vs External Floating Roof Tanks

Feature	External Floating Roof Tank	Internal Floating Roof Tank
Tank structure	Open-top tank	Fixed roof with internal floating deck
Environmental exposure	Roof exposed to weather	Floating roof protected by fixed roof
Typical applications	Crude oil, diesel, large terminals	Gasoline, volatile fuels, stricter emissions environments
Emissions control	Effective but weather-dependent	Generally more stable emissions control
Construction cost	Lower	Higher

Both designs share the same goal: reducing emissions, product loss, and improving safety by eliminating the vapor space above stored liquids.

Where Tank Components Make the Difference

While the floating roof design is critical, performance often comes down to the components installed on the tank.

Key systems that influence emissions and operational reliability include:

This is where custom engineered tank products play a major role. For example, advanced materials and installation systems can dramatically improve seal performance and durability over the lifetime of the tank.Mesa Engineered Tank Products develops solutions designed to improve reliability and emissions control in both internal and external floating roof tanks. Innovations such as Armor Fabric™ vapor barrier materials, ClipLock^® fastener-free fabric attachment systems, and Resist-All-Clad^® flexible, multi-coil roof drains are engineered to address common challenges like seal wear, emissions leakage, and maintenance complexity.

Choosing the Right Floating Roof Design

Selecting between an internal and external floating roof tank depends on several operational factors:

Stored products
Environmental regulations
Climate conditions
Emissions control targets
Budget and lifecycle costs

In many modern facilities, engineers evaluate both tank configuration and component performance to ensure the storage system meets environmental, safety, and operational goals.

The Bottom Line for Internal vs External Floating Roof Tanks

external floating roof tank diagram EFRT

Floating roof tanks remain one of the most effective technologies for controlling emissions and product loss in large hydrocarbon storage facilities.

External floating roofs offer a simpler and lower-cost design.
Internal floating roofs provide greater protection and emissions stability in demanding environments.

Regardless of configuration, the success of these systems often depends on the quality of the seals, drain systems, and vapor barriers installed on the tank – making engineered tank components a critical part of modern emissions control strategies.

References and Further Reading

Center Enamel. “Internal Floating Roof Tank vs External Floating Roof Tank.” Center Enamel, www.cectank.com/Internal_Floating_Roof_Tank_vs_External_Floating_Roof_Tank

EngineerFix. “How Floating Roof Tanks Work: Internal vs. External.” EngineerFix, www.engineerfix.com/how-floating-roof-tanks-work-internal-vs-external

Inspenet. “Internal Floating Roof Tank Function and Emission Control.” Inspenet, www.inspenet.com/en/articles/internal-floating-roof-tank-function

Kaiser, Mark J., and E. W. McAllister. Pipeline Rules of Thumb Handbook. Gulf Professional Publishing, 2023.

ScienceDirect. “Floating Roof Tanks.” ScienceDirect Topics, www.sciencedirect.com/topics/engineering/floating-roof-tanks