Visualizing Crown Moulding Miters Around a Chimney

Help with the three-dimensional geometry for crown moulding installed around a chimney penetration in a cathedral ceiling. November 30, 2009

I just put tongue and groove white pine on a ceiling. I am trying to figure out the angles for trim. The ceiling is 12 x 12 pitch (45 degrees). Chimney comes up through the ceiling in center of room. I need the angles for molding that comes down the front of the chimney at a 45 degree angle and then turns 90 degrees across side of chimney and back up to peak of ceiling at 45 again. Where do I set the miter saw to run from bottom of 45 degree to go around the 90 degree side of the chimney and back up to the peak again? Any help would be greatly appreciated.

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor L:
You can't make the transition directly; you need an intermediary piece. The piece that runs up the rake of the wall needs to transition to parallel to the floor before it will easily make the return to wrap around the chimney.

Imagine you were just wrapping the chimney, but at 3' down from the ceiling. Simple, right? Just miter the outside corners of the chimney and die the returns flat into the wall.
Now imagine running a different piece, following the ceiling line but also held 3' down from the ceiling line. You want it to rake 45* up the wall and then transition to run parallel to the floor. Just miter that transition, right?

What you need to do is combine these two ideas... If you held all the pieces 3' off the ceiling, that horizontal transitional piece between the rake and the chimney would be roughly 3' long. But if you put it right at the ceiling line, it will be very short and look like a pie wedge. If this is still confusing, get up on a ladder with a length of trim and mark it out on the wall. It'll be clear quickly enough.

From contributor S:
If you register your crown off the plane of the ceiling only, then you can use the standard settings on your miter saw for running regular crown.

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From contributor S:
After re-reading your post I realized that I assumed you were talking about crown. Is that true, or are you running 1x stock flat on the chimney? If so, you have two options:
1) Run the horizontal piece 90 off the ceiling.
2) Run the horizontal piece flat against the chimney.

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