What is the maximum span for double 2x12?

Hi Chica. What species do you plan to use? Any with a rating of select structural will work for that span. Be sure to nail 4 rows every 12" o.c. on the beam itself. From what I am reading into your question, you're just supporting roof trusses, so your live load is less than with a floor system. You can use Doug Fir, but it's not necessary. Good luck.

EDIT: The end of the beam on one side is on top of a wall, and the other end is cantilevered out 3' past the wall. Is that right? As long as you have the proper support under the beam inside the wall (I'd do 3 or 4 studs under each end), you'll be fine with a 13' clear span. Good luck.

Go with Douglas fir, usually offered at a true lumber yard in 20' lengths or more. It has greater tensile strength. The extra cost is well worth the price. If you can pick it out yourself, try to get as few knots as possible, look down from the end of the board to see how straight it is. The load it is bearing will determine if you need to increase to 3 2x12s, i.e. if you plan on putting a 4 person hot tub or grand piano upstairs. Good luck!

Macy-Skye Lily-Skye Hayleigh-Skye Some suggestions.

yes,if the finished opening is13'.ply.should be no less than1/2''thick.good luck.