The rule: which clock is your tank on?
NFPA 25 Chapter 9 covers water storage tanks that supply a water-based fire protection system. Its summary table, Table 9.1.1.2 (“Summary of Water Storage Tank Inspection, Testing, and Maintenance”), has two interior rows. One reads “Interior — steel tanks without corrosion protection” at 3 years. The other reads “Interior — all other tanks” at 5 years. In both the 2014 and 2017 editions, the requirement text behind those rows reads the same way: “The interior of steel tanks without corrosion protection shall be inspected every 3 years,” and “The interior of all other types of tanks shall be inspected every 5 years.”
So the first question is not how big the tank is or how old it is. It is whether the tank is steel, and whether that steel has corrosion protection. The table below sorts common tank types onto the two clocks. Treat it as the shape of the rule, and confirm each row against the edition your AHJ enforces.
| Tank | Interior interval | Table 9.1.1.2 row | Note |
|---|---|---|---|
| Steel tank, no interior corrosion protection | Every 3 years | Interior — steel tanks without corrosion protection | The short clock. Bare steel plate in contact with water and air. |
| Steel tank with an interior coating or lining | Every 5 years | Interior — all other tanks | Ask the AHJ how it treats a lining that has visibly failed. |
| Concrete tank | Every 5 years | Interior — all other tanks | Look for spalling, cracking, and exposed reinforcing. |
| Wood tank | Every 5 years | Interior — all other tanks | Look for rot. Hoops and grillage are also on an annual exterior check. |
| Embankment-supported coated fabric tank | Every 5 years | Interior — all other tanks | Not steel, so it falls under “all other tanks.” |
| Pressure tank | Set by material | Whichever interior row matches the tank | The rule sorts tanks by material and protection. Confirm the row with the AHJ. |
Intervals: confirm the adopted edition with your AHJ.
Which edition of NFPA 25 applies in Virginia
Virginia does not adopt NFPA 25 on its own. It gets there through the Virginia Statewide Fire Prevention Code (SFPC), 13VAC5-52. The 2021 SFPC took effect on January 18, 2024 and incorporates the 2021 International Fire Code (IFC) by reference. SFPC Section 102.4 says the codes and standards referenced in the IFC are the ones listed in IFC Chapter 80. The IFC’s maintenance table, Table 901.6.1, sends water-based fire protection systems to NFPA 25, and the Chapter 80 list for the 2021 IFC names NFPA 25-20. Virginia’s own Chapter 80 amendment adds a few standards but does not change the NFPA 25 edition. The result: the 2020 edition of NFPA 25 is the one the 2021 SFPC enforces.
The next cycle is underway. The Department of Housing and Community Development’s 2024 SFPC base document, last updated October 28, 2025, updates the reference from NFPA 25-20 to NFPA 25-23. When this page was written in September 2026, the Virginia Regulatory Town Hall still showed the 2021 SFPC as the code in effect. Check the Town Hall and your local fire marshal before you assume the 2023 edition applies.
Local fire marshals enforce the SFPC, and some publish the edition they expect to see on ITM records. Fairfax County’s fire marshal guidance, for example, lists NFPA 25 / 2020 Edition, with water storage tanks in Chapter 9. A locality can also adopt a local fire prevention code on top of the SFPC. That is why every frequency on this page carries the same instruction: confirm the adopted edition with your AHJ.
Why the section number you were quoted may be wrong
The 3-year and 5-year intervals have held steady, but the section numbers have not. In the 2014 edition the interior rule is at 9.2.6.1.1 (3 years) and 9.2.6.1.2 (5 years). In the 2017 edition it is at 9.2.5.1.1 and 9.2.5.1.2. Checklists, contractor websites, and old inspection forms still quote whichever edition their author had on the desk.
We could not confirm the section number in the 2020 or 2023 edition from a source we can quote, so we do not print one for those editions. Open the edition your AHJ enforces, find Table 9.1.1.2 in Chapter 9, and use the reference column in that table. If an inspection report cites a section number, check that it matches the edition on the report’s cover page.
| Edition | Status in Virginia | 3-year rule | 5-year rule |
|---|---|---|---|
| NFPA 25-2014 | Superseded | 9.2.6.1.1 | 9.2.6.1.2 |
| NFPA 25-2017 | Superseded | 9.2.5.1.1 | 9.2.5.1.2 |
| NFPA 25-2020 | Enforced (2021 SFPC) | Confirm in your copy | Confirm in your copy |
| NFPA 25-2023 | In the draft 2024 SFPC, not yet in effect | Confirm in your copy | Confirm in your copy |
What “without corrosion protection” means for scheduling
Steel without corrosion protection is on the short clock because water and oxygen attack bare plate. A steel tank with an interior coating or lining is on the 5-year clock. Concrete, wood, and coated fabric tanks are not steel, so they fall under “all other tanks” at 5 years.
The 5-year clock does not mean the tank is safe for 5 years. A coating that has failed in patches leaves bare steel exposed, which is the condition the 3-year rule was written for. If the last interior report showed coating failure, ask the AHJ whether it wants the tank treated as unprotected until the coating is repaired. Coating repair is qualified coatings work. FLOW does not perform it.
What the interior inspection looks for
The standard’s interior checklist looks for pitting, corrosion, spalling, rot, other forms of deterioration, waste materials and debris, aquatic growth, and local or general failure of the interior coating. The 2017 text also calls for looking at tanks on ring-type foundations for voids beneath the floor, at the heating system and its piping, and at the anti-vortex plate for deterioration or blockage.
Sediment matters twice. It hides the floor from the inspector, and it is the material a fire pump or the tank outlet ingests during a fire. Where the interior inspection is made by underwater evaluation, the 2017 text calls for silt to be removed from the tank floor first. That removal is done by others before the visit; FLOW does not remove silt. The rover inspection documents the silt and sediment conditions it finds.
In service or drained: two ways to meet the interval
In service — the tank stays full and the interior is looked at under water. FLOW does this with an underwater rover (a remotely operated vehicle) flown by a pilot, which records the walls, floor, and fittings while the tank keeps protecting the building. It avoids taking the fire protection water supply out of service.
Out of service — the tank is drained and the interior is looked at empty. This is the right method when sediment must come out, when the floor cannot be seen through the water, or when a coating has failed and the steel needs closer testing. FLOW does not drain, empty, or refill tanks and does no out-of-service visits. We refer drained inspections to a licensed partner firm.
When a steel tank shows pitting, corrosion, or coating failure, NFPA 25 sends it to a further set of tests on a drained tank. Industry practice for that step includes coating adhesion testing, dry-film thickness, and ultrasonic thickness readings where there is pitting or corrosion. That is qualified tank inspector work. FLOW does not do it. We route it to a licensed fire protection partner firm.
Keeping the clock: dates, records, and the AHJ
The interval runs from the last documented interior inspection. If there is no record, assume the tank is overdue and schedule one. Keep the report with the rest of the NFPA 25 records for the property, because the AHJ will ask for the date and the finding, not just the invoice.
The property owner is responsible for inspection, testing, and maintenance under NFPA 25. The AHJ enforces the adopted edition and decides what it accepts, including whether an in-service rover inspection meets the requirement for a particular tank. FLOW performs the inspection visit. We are not the AHJ, and we do not sign a tank back into service.