Made some progress today on framing out the curved front soffit. Lot's of pictures to follow.
The soffits I'm building will have a light tray attached to them. The curved soffit is being built with that feature in mind. So there are actually two curves. One is full height behind the soffit tray. The other is 5" high and is front of the soffit tray.
I started by making a template of the soffit. This was accomplished by rolling out a 36" roll of kraft paper (you can get this at HD or Lowes) the full width of the room. I then transferred measurements taken from the ceiling to the paper, taking care to layout the location of the framing members I had previously installed prior to installing the ceiling drywall.
I found it a bit tricky to layout the curve. At first I attempted the string method, but it was too inaccurate. The string I used was too strectchy, so the arc traced very erratically.
I then tried using a full length wooden curve, which worked great. The curve was created from leftover 16' baseboard that I planed down to 3/8" thick.
The process I used essentially uses an arc from an elongated ellipse. Go here for more info on how to draw an ellipse - http://www.mathopenref.com/constellipse1.html
Here's the procedure I used.
1. Draw layout lines, perpendicular to the major axis of the curve. These were drawn every 12" starting from each end until reaching the center.
2. Place the wooden curve on the template paper and bend it until reaching desired curvature. Mark the curve's offset from the major axis.
3. Then hold the curve by tacking in place. In my case, I used small finish nails tacked into the plywood subflloor.
3a. Start by tacking a finish nail on the minor axis at the offset marked in step 2. This nail should be pretty firmly installed.
3b. Tack finish nails - one each - on the ends. These should be exactly the same offset from the major axis.
3c. Place the board in front of the center finish nail and bend it back behind the end nails. This should hold the curve, assuming the wood extends beyond the end nails.
3d. Measure the offsets at each layout line and note it on the template. Do this on one half of the curve. Transfer each measurement to the respective layout lines on the second half of the curve.
3e. At each layout line, tack the curve in place for the offset measurement noted on the template. This may require a nail on front and back of the board.
4. Trace the curve with a marker.
This picture shows one end of the template using string method.
This is showing the other end.
Here you see the arc drawn using the string method. Unfortunately, it was not satisfactory.
A better method - this one shows the curve drawn for the soffit tray.
This one shows the full height curve behind the soffit tray.