Floating Roof Drains Explained: Design, Types, and Why Flexible Drains Are Becoming the Industry Standard

Aboveground storage tanks (ASTs) play a critical role in the oil, petrochemical, and chemical industries. Many of these tanks use floating roofs to reduce evaporative losses and control emissions from volatile liquids like crude oil, gasoline, and chemicals.

But floating roofs introduce a practical challenge: rainwater accumulation. Without a reliable way to remove water, even a well-designed tank can face structural risks, corrosion, operational issues, and potential environmental hazards.

That’s where floating roof drain systems come in. In this guide, we’ll explain how floating roof drains work, why they’re essential to emissions control and tank integrity, and why flexible floating roof drain systems are rapidly replacing older pivot style designs.

What Is a Floating Roof Tank?

A floating roof tank is an aboveground storage tank where the roof rests directly on the liquid surface, rising and falling as the tank fills or empties. This design dramatically reduces the vapor space above the liquid.

Reducing vapor space helps control emissions of volatile organic compounds (VOCs) – one of the main goals of environmental regulations governing storage tanks.

Federal rules such as the EPA’s 40 CFR 60 Subpart Kc require tanks storing volatile liquids to use emission control technologies such as floating roofs and sealing systems to limit vapor releases.

These systems are a key component of AST emission control and storage tank vapor control strategies.

Why Floating Roof Drain Systems Are Necessary

Even though floating roofs reduce vapor emissions, they are exposed to the environment. Rain, snow, and condensation accumulate on the roof surface.

If this water is not removed:

The roof weight increases, potentially causing structural stress
The roof may lose buoyancy or stability (potentially causing a spillover event)
Corrosion and contamination risks increase
Water may enter the stored product

To prevent these problems, floating roofs include roof sumps and drain systems that collect and remove the liquid that settles on top of the AST floating roof.

Most external floating roofs include one or more roof drains that carry water from the sump to a nozzle at the bottom of the tank shell, allowing the water to exit safely outside the tank.

How a Floating Roof Drain System Works

A typical floating roof drain system includes several components:

Roof sump: A low point on the roof where rainwater collects
Drain inlet: Allows water to enter the drain pipe while preventing product contamination
Drain pipe: Transfers water from the roof to the tank floor outlet
Tank shell nozzle: Where water and debris exit the tank safely

Because the roof constantly moves as the liquid level changes, the drain system must adapt to vertical movement, often over tens of feet.

This requirement is what differentiates modern flexible drains from older mechanical systems.

Types of Floating Roof Drain Systems

1. Pivot Style Drains

Historically, many tanks used articulated rigid pipe systems made from steel sections connected by joints.

These systems function like a mechanical arm, where two rigid pipes are connected by a joint (or elbow), allowing the drain to pivot as the roof moves up and down.

However, these systems have several limitations:

Numerous mechanical joints
Increased leak points
Higher maintenance requirements
Risk of joints binding or failing
Higher chances of sediment or debris slogging joints or connections

Over time, operators began seeking alternatives with fewer failures.

2. Flexible Floating Roof Drain Systems

Flexible drain systems replace rigid pipes with continuous flexible hoses designed to move smoothly with the roof.

Modern flexible drains are engineered as a single or multi-coil continuous assembly that lays in a predictable pattern on the tank floor.

For example, Mesa Engineered Tank Products manufactures flexible floating roof drain systems such as Armor-Clad^® and Resist-All-Clad^®, which are engineered to drain rainwater and debris while accommodating constant roof movement.

These systems typically include:

Flexible armored pipe
Full-flow end fittings
Superior chemical compatibility to reduce degradation
Smooth internal fluidways
Custom engineering for tank dimensions and rainfall data

Flexible drains eliminate the complex mechanical joints required by pivot-style drains, thereby removing the risk of a clog, failure, or slow draining.

Why Flexible Floating Roof Drains Outperform Pivoting Systems

Over the past two decades, flexible drain systems have become the preferred option for many tank operators. Think of your drain as a garden hose or straw, if there’s a kink or clog it won’t flow.

1. Fewer Leak Points

Flexible drains are often built as one continuous length, eliminating multiple couplings or pivot joints that can leak or fail.

This reduces:

Product contamination risk
Maintenance requirements
Potential seal failures

2. Predictable Movement

Flexible drains are engineered to form a controlled coil or lay pattern on the tank floor.

Some systems use a single coil design that rises and lowers vertically with the roof, others use multiple coils and tethers to maintain a consistent position within the tank.

This predictable motion helps prevent pipe dragging, tank shell and floor abrasion, and interference with other tank components.

3. Higher Flow Efficiency

Flexible drains often use a smooth internal fluidway rather than corrugated piping. This design improves flow rates and prevents sediment buildup that could restrict drainage.

Better flow capacity ensures that large rainfall events can be handled without water pooling on the roof.

4. Longer Service Life

Modern flexible floating roof drains are engineered for harsh tank environments – including exposure to crude oil, gasoline, chemicals, and temperature fluctuations.

High-quality systems can remain in service for decades when properly installed and maintained.

The Role of Floating Roof Drains in Emission Control

Floating roof drains themselves do not control emissions directly, but they are essential to maintaining proper floating roof operation.

Regulations under U.S. environmental law require floating roofs and associated components to remain in proper operating condition to minimize emissions.

For example:

Tanks must maintain continuous seals between the floating roof and tank wall to limit vapor escape.
Roof openings and penetrations must be properly sealed or equipped with protective covers.

If excessive water accumulates on a roof, it can damage seals or alter roof performance – ultimately increasing VOC emissions.

Because of this, reliable roof drainage is an important part of AST emission control and regulatory compliance.

Engineering Considerations for Floating Roof Drain Systems

Designing an effective floating roof drain system requires careful engineering.

Key factors include:

Tank diameter and roof height
Roof travel distance
Stored product compatibility
Expected rainfall
Internal tank components

Manufacturers like Mesa ETP custom engineer flexible pipe systems to match each tank’s dimensions and operating conditions, ensuring proper fit and long service life.

The Future of Floating Roof Drain Technology

As emissions regulations tighten and storage operators prioritize reliability, the industry is shifting toward:

Flexible floating roof drains
Integrated AST emission control technology
Advanced seal materials and vapor barriers

Companies like Mesa Engineered Tank Products develop custom engineered tank products – including flexible roof drains, supporting roof leg boots, and gauge pole covers – to help operators meet evolving environmental standards while improving long-term tank performance.

The Breakdown

Floating roof drain systems may not be the most visible component of a storage tank, but they play a critical role in safe and efficient operation.

By removing rainwater and maintaining proper roof performance, these systems help protect:

Tank structures
Stored products
Emission control systems
AST regulatory compliance

As a result, flexible floating roof drains are increasingly becoming the industry standard, offering improved reliability, fewer leak points, and longer service life compared to older articulated drain designs.

References & Further Reading

Cornell Law School. 40 CFR §63.685 — Standards: Floating Roofs. Legal Information Institute, Cornell University

Environmental Protection Agency. Volatile Organic Liquid Storage Vessels (Including Petroleum Liquid Storage Vessels).

Environmental Protection Agency. Standards of Performance for Volatile Organic Liquid Storage Vessels (40 CFR Part 60 Subpart Kc).