Roof type is the single most misunderstood line item in a tank spec. Buyers write "fixed roof" and later discover they needed an internal floating roof to pass emission limits. Below is the field version ZN engineers use.
Fixed cone roof
Best for: diesel, fuel oil, heavy oil, water, lubricant, edible oil.
Vapor space: large ullage, breathing loss on temperature swing.
Cost: lowest.
Watch: needs P/V vent, flame arrestor if flammable, and roof manhole for entry.
Use when product Reid vapor pressure is low and local VOC rules are loose.
Self-supporting dome / umbrella roof
Best for: wastewater, chemical service where internal columns block mixer/coil access.
Benefit: no center column, cleaner internal geometry.
Cost: 8–15 % above cone.
Use when roof live load, snow or seismic demand favors curved geometry.
Internal floating roof (IFR)
Best for: gasoline, naphtha, jet fuel, volatile solvents.
Mechanism: floating deck rides liquid surface inside a fixed outer roof.
Benefit: weather protection + 90 %+ vapor-loss reduction.
Cost: moderate premium over cone.
Watch: rim seal, gauge well bonding, rolling ladder, anti-rotation.
External floating roof (EFR)
Best for: crude oil, large terminal tanks 50 m+ diameter.
Mechanism: open-top shell + deck exposed to atmosphere.
Benefit: near-zero vapor space, lowest evaporation for crude.
Cost: highest, plus roof drain, rim seal maintenance, snow-load risk.
Watch: primary + secondary rim seal, flexible roof drains, pontoon buoyancy.
Selection rule of thumb
Stored product RVP low + water/oil service → cone
Need column-free interior → dome
Flammable + volatile + need weather shield → IFR
Crude terminal at 80 m diameter → EFR
FAQ snippet
"Can I put an internal floating roof on a fire water tank?" Technically yes, but rarely justified-fire water is not volatile, and the floating deck adds cleaning and inspection complexity. Fixed cone or dome is standard.
ZN builds cone, dome and IFR tanks; EFR is offered on large crude projects with third-party naval-architect review because rim-seal fire risk is not a shop problem, it is an operating-procedure problem.
