What's wrong with API 650 Annex H? Aluminum and steel internal floating roofs are extremely common for gasoline tanks. If you are not in the USA, consult the standard that parallels API 650 in your jurisdiction. If you are looking for design help to meet the requirements of API 650 you will not find that here and most manufacturers won't share their methods or 'secrets'. Designing your own aluminum IFR suggests you want to begin a new company - if not, learn the differences between the extremely wide variety of cheap and barely acceptable through expensive and high performance AIFRs and then buy the one that suits your situation and budget. If you want to design your own steel IFR, that can be relatively straightforward for a pan type or more involved for a bulkhead or double deck version. At some point though, you may want to just buy the version or style that meets your needs and budget.