Question:
How do I know if the structure of a one-story home is adequate to support an additional story?
Answer:
In general, you will need to consult a structural engineer or local building official to determine the design loads in your area and whether or not the structural members in your house will support those loads.
For a general idea of loads, you can consult the Structural Design Guide, which you can download at www.huduser.org/publications/destech/residential.html. After determining the load that the structure will need to support, you could check manufacturer's data or general wood property data to determine the size member you need (although this determination is best left to an engineer). For example, view chapter four of The handbook,Wood as an Engineering Material.