Long ago, I was asked to build a curved portico by adding a roof structure and columns to an existing semicircular front porch on my client's Greek Revival-style home. The critical dimension for the new roof structure was the outside arc of the existing masonry porch below, which the new roof would have to match. And since the curved crown molding had to be special-ordered to size, accurate layout of the structure's outside arc was vital for a good fit.

Working on a circular rather than a rectangular shape requires a different mode of thinking. All of our layout would need to start at the outside of the crown and work inward toward the center. So instead of a lot of complex math and "air framing," I decided to take the same simple approach I use to build gable end walls: Lay out and...

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