Floating Roof Tank Inspection Under API 653

Short answer: API 653 treats the floating roof as part of the tank's mechanical integrity. The inspector checks deck and pontoon plates for pitting and leaks, the rim seals, roof supports, drains, vents, and anti-rotation devices. Existing roofs are judged against the floating-roof annexes of API 650. Separately, US air rules (40 CFR 60 Subpart Kb, 40 CFR 63 Subpart WW and others) set seal-gap limits and inspection frequencies. A sound programme has to meet both regimes.

This guide is for terminal managers, tank inspectors, mechanical-integrity engineers and contractors planning out-of-service and in-service work on external floating roof (EFR) and internal floating roof (IFR) tanks in the US and Canada. It explains what the codes require and how each roof component is inspected. It also covers the NDE methods that produce the data, where the environmental rules overlap with API 653, and the mistakes that show up again and again in tank files. All interval and acceptance decisions belong to the owner and the owner's API 653 authorized inspector. Treat this page as a guide to the code, not as a substitute for the current edition.

What API 653 actually requires for floating roofs

API 653 covers the roof as one of several tank components that the authorized inspector has to evaluate. It does not set up a separate floating-roof code. For a floating roof the code's expectations fall into four groups.

• Structural soundness of the roof. Roof plates and pontoons that are cracked or punctured have to be repaired or the affected sections replaced. Pitted areas are evaluated for the likelihood of through-pitting before the next scheduled internal inspection. A pit that will not survive the interval is a repair item now, not a monitoring item.
• Evaluation basis. API 653 points to API 650 for the criteria used to evaluate existing floating roofs. The external floating roof annex covers EFRs and the internal floating roof annex covers IFRs. Upgrades or replacements are designed to the current API 650 annex. The inspector compares what is installed with that design intent: buoyancy compartments, drains, supports, venting and seal design.
• Repair methods. API 653's repair section covers leaks in pontoons or in the compartments of double-deck roofs, usually repaired by rewelding the leaking joints or adding patch plates. It also covers replacement of roof seal systems.
• Seal work while the tank is in service. API 653 limits how much of the primary seal may be removed at one time on an in-service tank, so the roof stays centred and vapour exposure stays controlled. The 2009 edition limited this to one-quarter of the seal system with temporary spacers in place. Confirm the wording in the current edition before you plan seal work.

The current API 653 is the fifth edition with subsequent addenda. Addendum 4 (2025) revised the text on tank-bottom thickness measurement, which shows that the document still changes between editions. Check your licensed copy and the official API standards catalogue. Many free copies circulating online are superseded editions, and the clause numbers in them may no longer line up.

Inspection intervals for the tank as a whole (routine in-service checks by owner personnel, formal external inspections by an authorized inspector, and internal inspections based on corrosion rate or risk) are set by API 653 and the owner's programme. Those intervals cover the roof as well. For how they are calculated, see our API 653 inspection intervals explainer.

External vs internal floating roofs: what changes for the inspector

EFRs and IFRs face different damage drivers and different access limits, and their inspection plans differ to match.

External floating roofs are exposed to weather. Rainwater has to leave the deck through the primary roof drain, which is a hinged or flexible pipe system running through the tank to an external valve. A blocked or failed drain is a classic cause of a sunk roof. Snow and ponding load the deck. Pontoons give the roof its buoyancy, so a leaking pontoon compartment matters directly to structural integrity. The rolling ladder, gauge pole and well, anti-rotation devices, rim vents, emergency overflow drains and the weather shield over the secondary seal all need regular checks.

Internal floating roofs sit under a fixed roof. They are often aluminium pontoon-and-skin designs, though some are steel pan or steel pontoon designs. Weather loading is gone, but venting of the fixed roof, the condition of floats and skins, deck fittings (legs, column wells, ladder wells, sample hatches) and the seal become the focus. IFR access for in-service checks usually happens through roof hatches, and the US air rules build their annual visual inspection around that access.

The two types also behave differently when there is trouble. A tilted or partly sunk EFR is an emergency that exposes product to atmosphere and to lightning. A damaged IFR may stay hidden under the fixed roof until a routine hatch inspection finds it. Both need an inspection plan written for the actual roof design. A generic "roof OK" box on a checklist is not enough.

Component-by-component inspection and the NDE that supports it

The table below maps each floating-roof component to what the inspector looks for and the NDE that usually supplies the data. Method choice is a site-specific decision made by the owner's inspector and the NDE Level III. The table describes typical practice and does not replace a procedure.

Two practical points help. First, deck plate pitting is often worst on the underside, which you cannot see from the roof. UT taken from the topside, or scanning during an out-of-service inspection, is how underside loss gets measured. Second, a roof can look fine from the gauging platform and still hold a pontoon that is half full of product. Open every compartment manway on the walkdown, not a sample of them.

For more on tank NDE choices, see MFL testing, ultrasonic testing and our API 653 tank inspection checklist.

Seal-gap rules: where the EPA overlays API 653

API 653 asks whether the seal is sound and does its job. US air regulations ask whether the seal holds vapour within numerical limits, and they set the frequency of measurement. The regulation that applies depends on when the tank was built, what it stores and what kind of facility it sits in. Two common examples are summarised below from the published eCFR text:

• 40 CFR 60 Subpart Kb (NSPS for volatile organic liquid storage vessels). For EFRs, the primary seal gap is measured during hydrostatic testing or within 60 days of initial fill, and then at least once every 5 years. The secondary seal gap is measured within 60 days of initial fill and then at least once per year. The accumulated gap area may not exceed 212 cm² per metre of tank diameter for the primary seal (maximum gap width 3.81 cm) and 21.2 cm² per metre for the secondary seal (maximum width 1.27 cm). For IFRs, the roof and seals are visually inspected before filling, at least once every 12 months, and each time the vessel is emptied and degassed. Defects are repaired, or the vessel emptied, within 45 days, and an extension may be requested.
• 40 CFR 63 Subpart WW (NESHAP storage vessel control standards). The gap limits are the same: 212 cm²/m with 3.81 cm (1.5 in) maximum for primary seals and 21.2 cm²/m with 1.27 cm (0.5 in) maximum for secondary seals. EFR seals get their first measurement within 90 days of initial filling. After that, secondary seals are measured annually and primary seals at least every 5 years. IFRs get an annual inspection plus a full inspection each time the vessel is emptied and degassed or every 10 years, whichever comes first. Repairs are due within 45 days, with up to two 30-day extensions allowed.

Other tanks may fall under 40 CFR 63 Subpart CC (refinery MACT), older NSPS subparts K or Ka, a state implementation plan, or local air district rules. Your Title V or state air permit names the rule that applies. Confirm it for each tank before you build the inspection calendar. Canadian facilities follow provincial environmental approvals and, where they apply, CCME guidance. Confirm with the provincial regulator.

The practical consequence is that seal-gap measurement is an environmental compliance record. It is often done by the operator's environmental staff or a specialist contractor, and it has its own notification and reporting duties. API 653 asks the inspector to record seal condition, rim spaces and seal-to-shell gaps around the circumference as part of the in-service check. Many owners therefore coordinate the two so that one roof visit produces both records. Gap measurements alone do not show whether the roof is structurally sound, and API 653 checks alone do not prove emissions compliance.

How a floating-roof inspection runs in practice

The work splits into in-service activities and out-of-service activities.

In service. Owner personnel do routine checks from the gauging platform. Is the roof level? Is there water or product on the deck? Are the drains clear, the seals in visible contact, and the ladder tracking? The formal external inspection under API 653 is done by or under the authorized inspector and includes the roof as far as it can be reached safely. Going down onto an EFR deck is a confined or hazardous-atmosphere activity. API RP 2026 (Safe Access/Egress Involving Floating Roofs of Storage Tanks in Petroleum Service, 4th edition 2022) is the industry reference for the precautions: atmospheric testing, roof level relative to the shell top, attendants and rescue planning. Your site entry permit rules apply on top of it.

Out of service. When the tank is emptied, cleaned and gas-freed for its internal inspection, the roof is landed on its legs and can be examined from both sides. This is the chance to do the following:

• Grid UT or scan the deck plates, concentrating on low spots, areas around drains, and lap welds where water collects.
• Open every pontoon and measure compartment plates.
• Inspect the underside of the deck and the support-leg sleeves.
• Test the roof drain.
• Examine the shell in the seal travel zone.
• Replace or repair seals while full access is available.

The roof data then goes into the same remaining-life calculation the inspector uses for the next internal interval. It should not be filed as a separate report that nobody trends.

Worked example (qualitative). An EFR in gasoline service is opened for its scheduled internal inspection. The topside walkdown finds light pitting near the centre deck drain. UT pit-depth readings show that the deepest pits would go through the plate before the next planned internal date, based on the measured pitting rate. Under API 653's through-pitting logic these areas are repaired now with patch plates, and the welds are leak tested. Two pontoon compartments hold product. They are drained, their seams are tested, and the leaking joints are rewelded. The primary seal shoes show corrosion at the hangers, so the seal is replaced to the current design basis. The inspector records all of this, updates the roof corrosion rate, and the owner sets the next interval. Separately, the environmental team schedules the post-fill seal-gap measurement required by the tank's air rule.

Common failure modes and mistakes in tank files

• Sunk or tilted roofs. These usually trace back to blocked or failed drains, heavy rain or snow loading, pontoons that had been taking on product for a long time, or a roof binding on an out-of-round shell. Each of these is visible on inspection if someone looks.
• Pontoon checks done by sampling. If you open three compartments out of twenty, you get three compartments of data. Many owners require all compartments to be opened at every external inspection.
• Underside deck corrosion missed. A topside visual inspection cannot show it. You need UT or scanning, and the internal inspection is the only time it is fully accessible.
• Seal gaps measured but seal hardware not examined. A seal can pass the gap test while its shoes, hangers or fabric are close to failing.
• Old-edition criteria in the checklist. Inspection forms copied from superseded editions of API 653 or API 650 cite clause numbers that have moved. Check the current editions.
• Roof data not trended. Deck and pontoon thicknesses should carry corrosion rates the same way shell and bottom data do. Our guide to long-term and short-term corrosion rates explains the calculation logic.
• Lightning and rim-seal fire controls left out. Shunts and bypass conductors between the roof and the shell are part of the roof hardware on many EFRs. Lightning guidance for tanks was historically published as API RP 545, which API has since withdrawn. Confirm your company's current design basis and include these items in the visual checks.

Documentation the authorized inspector expects

For each floating-roof tank, a complete file usually contains the following:

• Roof type and design basis: EFR or IFR, single-deck or double-deck, steel or aluminium, and the API 650 annex edition it was built to.
• Drawings or sketches showing pontoon numbering, leg locations, drains and fittings.
• Thickness data for deck and pontoon plates with locations, each tied to a grid or compartment number.
• Pit-depth measurements and the through-pitting assessment for the next interval.
• Pontoon inspection log: every compartment, its contents and its condition.
• Seal inspection record and, where an air rule applies, the official seal-gap measurement record, kept on the regulatory schedule.
• Drain test results.
• Repair records: weld procedures, welder qualifications, NDE and leak-test results.
• NDE reports carrying the technician's certification level, instrument calibration and the procedure used.

The inspector of record uses this file to sign the inspection report and to recommend the next inspection dates. Good NDE data shortens that review. Missing compartment numbers or unlocated thickness readings slow it down and weaken the next remaining-life calculation.

Regulatory overlay in the US and Canada

Besides the air rules, floating-roof tanks may also fall under the following:

• EPA SPCC (40 CFR 112). Bulk storage containers need regular integrity testing under an industry standard. API 653 and STI SP001 are the usual references, so the API 653 programme often doubles as SPCC compliance.
• OSHA PSM (29 CFR 1910.119). Where the tank is part of a covered process, mechanical integrity applies to it, and API 653 is the recognised and generally accepted good engineering practice (RAGAGEP).
• PHMSA 49 CFR 195. Breakout tanks on hazardous liquid pipelines must meet the API 653 requirements that the regulation incorporates by reference.
• Fire codes (NFPA 30) and local fire marshal requirements.

In Canada, provincial fire codes, environmental approvals and, for pipeline-connected terminals, the Canada Energy Regulator or provincial pipeline regulators may apply. Check which authority governs each tank.

For the full tank context, see the API 653 tank inspection guide, the API 650 vs API 653 comparison and our page on tank floor scanning, MFL vs UT.

How Atlantis supports floating-roof tank inspections

Atlantis NDT supplies the NDE data the owner's API 653 authorized inspector needs. Our ASNT-certified technicians work under ASNT Level III oversight. The scope covers UT thickness and pit-depth grids on deck and pontoon plates, corrosion mapping, tank-floor MFL, MT or PT on suspect welds, vacuum-box leak testing and visual examination during out-of-service work. Results are reported by location so they can go straight into the inspector's remaining-life calculation. We are not the authorized inspector and we do not set intervals or approve repairs. The owner's API-certified inspector remains inspector of record, and seal-gap compliance stays with the operator's environmental programme. See aboveground storage tank inspection and API 653 tank inspection support. Quotes are returned within 24 hours. Request an NDE quote for your floating-roof tank.

Frequently asked questions

What does an API 653 floating roof inspection include?

It covers the roof deck and pontoons (pitting, cracks, leaks, product in compartments), roof supports, primary drains, rim seals, vents, gauge pole and anti-rotation devices, and the shell surface the seal runs on. Existing roofs are evaluated against the floating-roof annexes of API 650.

How often are floating roof seal gaps measured?

That depends on the air rule that applies. Under 40 CFR 60 Subpart Kb and 40 CFR 63 Subpart WW, EFR secondary seals are measured annually and primary seals at least every 5 years, after an initial measurement soon after first fill. Confirm the rule named in your permit.

What are the EPA seal gap limits for floating roof tanks?

Under Kb and WW, the accumulated gap area may not exceed 212 cm² per metre of tank diameter for primary seals (no single gap wider than 3.81 cm) and 21.2 cm² per metre for secondary seals (no gap wider than 1.27 cm).

How are pontoons inspected on an external floating roof?

Every compartment manway is opened. The inspector looks for product or water and for internal corrosion, and checks bulkheads and covers. Leaking compartments are drained and the leaking joints are rewelded or patched, then leak tested.

How is floating roof deck pitting evaluated?

Pit depths are measured, usually with UT and pit gauges. API 653 asks whether pitting is likely to go through the plate before the next internal inspection. If it is, the area is repaired now.

Who can inspect an internal floating roof?

Owner personnel do the routine and air-rule visual checks through roof hatches. The formal API 653 inspection is done by or under an API 653 authorized inspector. NDE technicians provide supporting measurements under their own certifications.

Is it safe to walk on a floating roof for inspection?

Only under a site permit with atmospheric testing and rescue planning. API RP 2026 is the industry reference for safe access and egress on floating roofs in petroleum service.

What causes a floating roof to sink?

Common causes are blocked or failed roof drains, rain or snow loading, pontoons that have filled with product, and a roof binding on a damaged or out-of-round shell. Routine checks of drains, deck water and pontoons catch most of these early.

Does API 653 require seal replacement at every internal inspection?

No. Seals are replaced when the inspection finds them damaged or ineffective, or when an emissions rule requires it. API 653 also limits how much of the seal can be removed at one time while the tank is in service.

Which API 650 annex applies to floating roofs?

API 650 has separate annexes for external floating roofs and internal floating roofs. API 653 uses them as the criteria for evaluating existing roofs and for designing replacements.

Talk to an NDE Level III about your next tank outage or ask for a scope review of deck and pontoon NDE.