What angle do I need to incline my solar collectors to insure that they drain properly in a drainback system?

Different installers have advocated different answers to this question for years. In Tom Lane’s book he advocates a pitch of 1/16” per foot. While this can work I would advocate a more nuanced approach. A collector can drain if it is mounted with the headers parallel to the ground although I would never advocate this approach. The general principle that we are trying to accomplish is having just enough angle to enable the water to run out of the collectors. The steeper the roof pitch the more confidence we have that the headers will drain even with a slight tilt back towards the inlet pipe. When you are working on a shallow pitch roof (4/12 or less) more pitch is recommended. I would recommend doubling the pitch of the collectors when dealing with shallow pitch roofs of 3/12 or 4/12. I would not recommend using a drainback system with a pitch of 2/12 or less. Another concern to watch out for is on lower pitch roofs pay special attention to any waviness in the roof. It can be catastrophic as in the case of this picture where the collectors were ruined from freeze damage because they didn’t drain.