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Secondary containment is the single most consequential design decision in an AST project — it drives tank selection, site footprint, and permitting scope all at once. Here's what it means, how it's typically satisfied, and where buyers most often get the sizing wrong.
Secondary containment is a barrier designed to hold the full volume of a tank's contents (typically 100–110% of capacity) if the primary tank leaks or ruptures, preventing fuel from reaching soil, groundwater, or storm drains. Both SPCC (as an environmental requirement) and NFPA 30 (as part of a broader fire and life-safety framework) require it in some form for most aboveground fuel storage — but as with the frameworks themselves, the two evaluate containment for different reasons and don't automatically satisfy each other. A containment design that clears your SPCC plan review isn't guaranteed to clear your local AHJ's NFPA 30 review, and vice versa.
Built-in double-wall containment. A UL 142 double-wall tank has containment engineered into the tank shell itself — no separate structure needed. This is the fastest path to compliance for most projects and is the default construction across our double-wall product line. See our UL 142 guide for how the interstitial space and monitoring work.
Berms and dikes. A site-built earthen or concrete containment structure surrounding a single-wall tank, sized to hold the tank's full capacity plus precipitation allowance. Requires civil design, additional construction, and ongoing maintenance (drainage, liner integrity, periodic inspection for cracking or erosion).
Remote impoundment. Contents are directed via piping and grading to a containment area away from the tank itself — typically reserved for large or unusually sited installations where on-site containment isn't practical, such as tightly constrained industrial lots or facilities with existing stormwater infrastructure that can be adapted.
Containment capacity is calculated against the tank's full rated volume, not typical operating volume — undersizing to reflect "normal" fill levels is a common and avoidable compliance gap. For multi-tank systems, containment is generally sized to the largest single tank in the manifolded group, not the combined total, though your specific AHJ and SPCC plan should confirm this for your configuration. Precipitation freeboard also needs to be factored in for outdoor berm/dike systems — a containment structure sized exactly to tank capacity with no margin for rainfall accumulation is a common finding in SPCC plan reviews.
Whichever method you choose, containment needs periodic inspection as part of ongoing operation, not just at installation. Berms and dikes need drainage checked and liner integrity confirmed; double-wall interstitial monitoring needs the sight gauge or sensor checked on whatever cadence your SPCC plan or local AHJ specifies. Accumulated rainwater in an uncovered berm system is a particularly common finding — standing water reduces the available containment volume exactly when you'd need it most, and it's an easy thing to overlook between inspections.
Sizing a berm to typical fill level instead of the tank's full rated capacity — a gap that only becomes visible during an SPCC plan review or after an actual overfill event.
Not accounting for precipitation freeboard on an outdoor, uncovered berm or dike system.
Assuming a double-wall tank's built-in containment satisfies both SPCC and your local AHJ's NFPA 30 review without confirming both separately.
Sizing multi-tank system containment to combined capacity instead of the largest single tank, which can lead to over-building (and overpaying for) containment.
Choosing double-wall construction upfront generally simplifies both SPCC and fire code review compared with specifying a single-wall tank and adding containment separately later — it also avoids the ongoing berm/dike maintenance burden and the site footprint a separate structure consumes. See our tank comparison for how containment approach scales across capacity tiers.
This overview is general information, not legal or engineering advice. Confirm current requirements with your local AHJ and a licensed Professional Engineer.
FAQ
Typically 100–110% of the tank's full rated capacity, calculated against the maximum volume, not typical operating fill level. Outdoor berm/dike systems also need precipitation freeboard added.
Usually not — the interstitial space between shells generally satisfies containment requirements on its own, but confirm against both SPCC and your local AHJ's specific NFPA 30 interpretation.
Generally to the largest single tank in the manifolded group, not the combined total — but confirm this with your specific AHJ and SPCC plan for your configuration.
Yes — berms and dikes need drainage and liner integrity checked periodically, and double-wall interstitial monitoring equipment needs regular verification, on whatever cadence your SPCC plan or AHJ specifies.
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