Contents
About this tool
The Roof Evaluation calculator records and checks the roof of an existing tank to API 653 4.2: the roof plate rule of 4.2.1.2 (an average under 2.2 mm in any 250 mm × 250 mm area, or any hole, is repaired or replaced), cracks, punctures and through-pitting of floating roofs before the next internal inspection (4.2.3.1, 4.2.3.2), the supports, frangible joint and appurtenances (4.2.2, 4.2.3.3, 4.2.3.4), and what a change of service brings in (4.2.4).
Scope
An indicative evaluation, not a regulatory submission. It follows API 653, 5th edition (2014) with Addendum 4 (July 2025), 4.2. The structural integrity of the roof and its supports (4.2.1.1) is verified by the inspector and the storage tank engineer; floating roof buoyancy and the frangible joint have their own tools.
Quick start
Choose a sample and press Load sample. The external floating roof has a 4.8 mm deck, 20 years in service and the next internal inspection in 10 years; the deck near the centre drain is pitted through before then. The supported cone roof has one plate under 2.2 mm and a pipe column to repair.
- 1Identification: Tank ID, record type and units, which choose the printed limit (2.2 mm over 250 mm × 250 mm, or 0.09 in. over 100 in.²).
- 2Section 1: roof type, nominal plate thickness, years in service and years to the next internal inspection. For a fixed roof with a frangible joint, tick the box.
- 3Section 2: one row per area examined, with its average (or the readings over it), the least thickness at pits, a rate if known, and any hole, crack or puncture.
- 4Section 3: the condition of each support and appurtenance; for a frangible joint, what affects it.
- 5Section 4: tick any change of service; section 5: tick the record items, then Calculate & Save and Print / PDF report.
Roof plates and pontoons (section 2)
| Check | Rule | Sample (external floating roof) |
|---|---|---|
| Average, 4.2.1.2 | Average over the area under 2.2 mm (SI) or 0.09 in. = 2.286 mm (US): repair or replace. Type the average, or the readings over the area to have them averaged. | deck centre: five readings average 4.1 mm ✓ |
| Holes, 4.2.1.2 / 4.2.3.1 | Any hole through a roof plate, or a crack or puncture in a floating roof plate or pontoon: repair or replace. | none |
| Through-pitting, 4.2.3.2 | Floating roofs: least thickness at pits minus rate × years to the next internal inspection. Zero or less: the pits go through first — repair or replace. On a fixed roof the same projection is shown as a Note. | drain area: 1.4 − 0.17 × 10 = -0.3 mm, through in 8.2 yr ✗; deck centre 2.4 mm left; pontoon 7 bottom 3.45 mm |
| Average at the next inspection | The average less its rate × years: a Note when it will be under the limit before the next internal inspection. | drain area: 3.0 − 0.09 × 10 = 2.1 mm — plan the repair |
- Rates. A blank rate is linear from the nominal thickness and the years in service: (nominal − measured) ÷ years, taken separately for the average and for the pits. An entered rate is used for both.
- Areas. List the thinnest areas found. API 653 asks about any 250 mm × 250 mm area, so a passing table only covers the areas entered.
- Locations: roof plate for a fixed roof; deck, pontoon top, bottom or rim for a floating roof. The chart shows each average as a bar against the limit and each pit as a diamond with an arrow to its thickness at the next inspection.
Supports, joints and appurtenances (section 3)
| Roof | Items | Clause |
|---|---|---|
| Fixed | Rafters, girders, columns (pipe columns checked for internal corrosion), column bases, roof-to-shell joint, vents and screens | 4.2.2.1, 4.2.1.1, 4.2.4.5.2 |
| Floating | Roof supports, perimeter seal, rolling ladder, anti-rotation devices, roof drain, vents | 4.2.3.3, 4.2.4.5.2 |
| Frangible joint | Bottom-to-shell joint corrosion; reinforcement, handrail or other change to the roof-to-shell joint | 4.2.2.2, API 650 5.10.2.6 |
- Each item is acceptable, repair or replace (a failing check) or not inspected / not applicable (a Note while some remain).
- A frangible joint with corrosion at the bottom-to-shell joint, or a changed roof-to-shell joint, fails until it is re-evaluated against API 650 5.10.2.6 (Roof Design tool).
- Floating roofs are evaluated with API 650 Annex C (external) or Annex H (internal) as guidance; upgrading to meet API 650 is not mandatory (4.2.3.4). The External and Internal Floating Roof tools and the Rim Seal Survey cover those checks.
Change of service (section 4)
| Change | Clause | Requirement | Tool |
|---|---|---|---|
| Internal pressure | 4.2.4.1 | Current standard (e.g. API 650 Annex F) for the roof and roof-to-shell junction | Roof Design |
| External pressure | 4.2.4.2 | Roof supports and roof-to-shell junction for the design vacuum, API 650 Annex V | External Pressure (Annex V) |
| Above 93 °C (200 °F) | 4.2.4.3 | API 650 Annex M | Elevated Temperature (Annex M) |
| Lower temperature | 4.2.4.4 | Current standard for the lower temperature | Brittle Fracture |
| Venting | 4.2.4.5.1 | Effects on normal and emergency venting | Tank Venting (API 2000) |
Each ticked change adds a Note to the summary and a record item.
The checks
| Check | Clause | Fails when |
|---|---|---|
| Roof area | 4.2.1.2 (4.2.3.1, 4.2.3.2 floating) | average under the limit, a hole, crack or puncture, or (floating) pits through before the next internal inspection |
| Supports / appurtenances | 4.2.2.1, 4.2.3.3 | an item marked repair or replace |
| Frangible roof-to-shell joint | 4.2.2.2 | bottom-to-shell corrosion or a changed joint |
| Change of service | 4.2.4 | a Note for each change |
| Evaluation record | 4.2 | a Note until every visible item is ticked |
Evaluation record (section 5)
| Clause | Item | Shown for |
|---|---|---|
| 4.2.1.1 | Structural integrity of the roof and supports verified. | All |
| 4.2.1.2 | Thicknesses over the most corroded areas, averaged over 250 mm × 250 mm; holes recorded. | All |
| 4.2.2.1 | Supports inspected; pipe columns checked inside; out-of-plumb columns evaluated. | Fixed roofs |
| 4.2.3.2, 4.2.3.4 | Pitting judged to the next inspection; Annex C or H as guidance. | Floating roofs |
| 4.2.4.5.2 | Vents operate; screens clear. | All |
| 4.2.4 | Change of service evaluated. | A change ticked |
| 9.12 – 9.14 | Roof repairs to Section 9, inspected and tested. | A failing area |
How the numbers were checked
- 4.2 was read from the 5th edition with Addendum 4 (July 2025); the SI and US limits are the printed values (2.2 mm / 250 mm × 250 mm; 0.09 in. / 100 in.²).
- Both samples are recomputed in the test suite and in this manual’s build: readings averaged, pit and average projections, the 8.2-year through-pitting time, the 2.25 mm area that passes SI and fails US.
- Each rule was made to fail in turn: average, hole, floating-roof pits, a support to repair, a changed frangible joint; and the fixed-roof pits Note.
Saving, reopening and reports
- Kept on this device as you type, with Restore or Discard; readings are kept exactly in mm.
- Show saved roof evaluations lists the records for the tank; Open reloads the roof, areas and supports. Records are kept in SI in the
roofEvaluationCalcscollection. - Print / PDF report: summary, roof, areas table and chart, supports, change of service and the record list.
Limits and interpretations
- Rates are linear; a blank rate assumes uniform loss since the plates went into service.
- 4.2.3.2 asks for the likelihood of through-pitting; the tool takes it as the least thickness reaching zero at the projected rate. A pit closer than that may still need judgement.
- Structural adequacy of rafters, columns and floating roof members, buoyancy and the frangible joint are not calculated here.
- Bottom evaluation (4.4) is in the Inspection Interval and Annular Plate tools; repairs in Repair Design Check.
Glossary
| Average | Mean thickness over a 250 mm × 250 mm (100 in.²) area. |
| Least at pits | Thinnest remaining thickness at pitting in the area. |
| Through-pitting | A pit reaching through the plate. |
| Frangible joint | Roof-to-shell joint designed to fail before the shell-to-bottom joint (API 650 5.10.2.6). |