Deck Joists

[smithsor]

Does the deck designer base my design and final rendering for materials on a default PSI for static load, active load, etc?

How can I change the joist spacing for all joists to say 12", 16", etc.?

[Elovia]

The Home Design (and by extension Landscape and Deck Design) titles do not perform any engineering; most default settings are based on generic designs, for example, stud walls of 2x4 on 16" centers. If your area requires it, you should have an engineer licensed in your state review your design for compliance to local building codes.

I am not familiar enough with the limitations of the Deck Designer title to render an answer to your last question. In Pro we have tools to build framing and designate those parameters, but I'm not knowledgeable in the tools available to users of LDD.