According to the Occupational Safety & Health Administration (OSHA), diesel fuel has a lower flashpoint than gasoline, meaning it does not ignite as readily. However, diesel can still be a fire hazard if stored improperly.
OSHA regulations permit a maximum of 60 gallons of diesel fuel to be stored inside a single storage room. Additionally, transfer of the fuel must occur in a ventilated environment by approved methods, such as from the top of the indoor tank, through closed pipes or via a self-closing valve.
Storing diesel tanks indoors helps protect the fuel from environmental degradation due to conditions like high temperatures and moisture. Diesel absorbs moisture from the air, and a tank that is too large will result in condensate mixing with the fuel. Regular removal of dirt and water will extend diesel life, according to BP (British Petroleum).