UPDATE 2015-07-03: I tightened the tolerances slightly for a better fit. Make sure your printer is well calibrated before printing.
Update 2014-11-11: For a more elegant solution that uses less plastic, try out my new adjustable angle 3-way elbow: http://www.thingiverse.com/thing:537497
Here are reconfigurable 22.5 & 45 degree 3-Way Elbows designed to fit 1/2" Schedule 40 PVC pipe. They are part of a series of 1/2" PVC fittings I'm designing to supplement conventional plumbing fittings. These elbows can be used to make angled roofs for structures and other forms. My original purpose was a roof for a small, plastic-wrapped greenhouse to hold plants over the winter.
If you're making a roof, you might notice that each corner requires a different fitting. In order to make one design instead of four, I made the elbows reconfigurable. The 22.5 and 45 degree fittings each allow you to make 4 different elbows and 1 Y fitting. Print 1 of either the 22.5 or the 45 degree fitting, then print 2 sleeves and 2 caps. Slide the sleeves into the holes of your choice and cap the other holes. Glue for a secure fit.
I put the triangular holes in the pipes to help the glue dry. On the first iteration, I noticed that being air tight prohibited drying. The ledges on the top of the horizontal holes make it printable without support structures.
I will continue to add new fittings to the series and share them as a derivative of my original 3-Way Elbow. Please let me know if there is a fitting you need that I haven't created yet.
You are highly encouraged to make new derivatives. Please tag your new creations with #HalfInchPVCFittings.