Contain 100% of the tank’s volume, plus a rainfall allowance (at least 10% of the tank volume or use the rainfall volume that could result from a 25-year storm event lasting for 24-hours, whichever is greater). For containment areas with multiple tanks, use the volume of the largest tank within the area. NOTE: Tanks that are indoors or covered (with a roof) do not need to include an additional rainfall allowance.