Greenhouses, Sheds and Sun Rooms - Longest span for joist...???

I have a patio that is 15' wide and was wondering if it was possible to use one piece of wood to span the entire 15' without having to put another post in the middle to support it. Could I use like a 2x12x15' or something?

Depends on the snow load you may have to meet, what you are placing on the joists, the species of wood you use and how far apart you place the joists.

First, you also need to know how long your joists are to figure the size of beam needed. A deck that is 15' wide but only extends 4' from the house has a very different requirement than a deck that extends 10' from the house because the beam is supporting much more floor.

I don't have the books in front of me, but I don't recall seing spans greater than 12' in the standard tables for regular 2x lumber. You will probably need an engineered lumber like a glue-lam or LVL (laminated veneer lumber). Beware: most lumber yards stock engineered lumber but most of it is NOT treated so you will have to protect it from the elements or special order a treated beam.

I've got a 22' span under my covered deck/screened porch. It can be done, and it looks really cool, but it is a big A$$ beam (LVL).

Some nice info here, if it helps.

http://www.edeck.net/copeland/sketches/framing_detail.htm