Hello,
API 650 does not mention the criteria for the selection of Roof types.
Generally, Roof types depends on the Size (dia) of the Tank, Liquid stored, Hazardous etc,.
for example, If the vapour pressure is more and if there is a possibility of forming a Vapour cloud above the liquid surface (due to hydrocarbon service), then you need to choose Floating roof. Again, your Process Engg needs to calculate the escape velocity of the vapour and that decides whether to select Single deck or Double deck or Internal floating roof type etc.,
You can refer SHELL standard, where it has been clearly mentioned about the criteria for selecting the roof type based on Dia and the liquid stored.
On an overview (without any details), for your 100m Tank, you could choose a Geodesic Aluminium roof type (but, check for the temperature criteria, liquid stored).
Thanks,
JAS