I'm building an outdoor couch and want to put an angle on the back piece. What is a generally acceptable angle for the back piece? In the photos below, I've rotated the back pieced to 15°, but is just a guess.
I am sure there are some "standards" but note that there are individual preferences. Take car seats for instance with their range of adjustability. However rear car seats in a sedan or those in many pickup trucks are not adjustable and you may be able to use that as a guide.
Note that comfort in seating often has the seat bottom sloped down toward the back some too. Case in point would be for you to look at some plans for deck chairs or lawn chairs. The rear sloped seat bottom is especially pronounced in Adirondack style chairs.
Another thing to think about. If the seat base is too long front to back the seat back slope can be extremely awkward for persons with shorter legs. Your pictures give the impression of seat bottoms that may be too deep from front to back for generally comfortable seating.
It may be worth a half day trip to a furniture store and to a home center where deck furniture and lawn chairs are sold. You can learn a lot first hand from what you find comfortable to sit in rather than guessing while drawing away in a CAD program.
Old question I know but since I am also building a sofa and Google kindly sent me here during my research today, I thought I'd finish off the piece....
15 degrees is good for the back angle, BUT you should also slope the seat base down towards the back 5 degrees also. This stops people sliding forwards when they lean back and maintains the ideal 100 degree angle for lounging...
Michaels last point is extremely good advice. I did exactly that at IKEA and learned a lot. Mostly about how low outdoor sofas were and that most seemed to have quite low backs also. Take a tape measure plus a camera and photograph the tape in place across the parts you're interested in. Ignore the funny looks! Why not take advantage of the small fortunes these companies must spend on design.
Incidentally IKEA sell their water resistant outdoor cushion range independent of the furniture, which is perfect if you're making your own hardware...