Hi some advice needed please, I need to mount a 20kg Optoma screen to the wooden joists in the ceiling. Fortunately the joists are perpendicular to the screen, and the supplied pair of L-shaped brackets can be positioned anywhere along the length of the casing. Original plan was to screw straight into the wooden joists. The Optoma instructions suggest it will stay up by using four upwards-facing screws per L-shaped bracket (40mm number 5.0). There's two brackets in total. My best-friend builder mate is nervous. He's suggesting running some ply between the joists to spread the load, and then screw the brackets into the ply. I'm more than happy to follow his advice but is it necessary?