With sprung crown molding, only the top and bottom edges touch the ceiling and the wall. For a proper installation, those edges have to be nailed into the ceiling joists and wall studs, respectively. While there are always studs available in the walls, there may not be a joist where you need it in the ceiling. When that happens, This Old House general contractor Tom Silva fills the void with an angled, solid-wood backing strip that he fastens to the studs. Then he can nail his crown every 16 inches without worrying about missing a joist. Here's how to make nailing strips using a circular saw.
1. Draw the angle. Label the adjacent edges of a square-cornered piece of scrap lumber “ceiling” and "wall." Stand the butt end of your molding on the face of the scrap and line up the molding's top and bottom edges flush with the "ceiling" and "wall" edges, respectively, as shown in the photo. Draw a line along the back of the molding onto the face of the scrap. Cut the scrap along this line. You now have a triangular model for the backing strip's profile.
2. Set the saw angle. Remove the saw's battery or unplug it, and loosen the bevel screw. Place the model against the saw blade, as shown, with the “ceiling” edge against the saw's shoe. Now tilt the blade until it's flush with the model. Tighten the bevel screw to hold the blade at that angle.
3. Cut the strips. Place the model's ceiling-side edge on the face of a 2x4. Line up one end flush with the 2x4's edge; make a mark on the 2x4 at the model's opposite end. Extend this mark into a cutline along the length of the 2x4, then rip-cut the 2x4 at this cutline. Mark and cut as many 2x4s as you need, then nail them to the studs. You now have a secure attachment point for nailing your crown.
TOH Tip: When you rip-cut a backer strip, run the saw blade along the wall side of the cutline. That way, the backer strip will be slightly narrower than the gap behind the crown, and the molding will pull up tight against the wall and ceiling as it's being installed.