We looked at lots of options for how to go about creating them, including cutting them out of solid stock, ordering prefab beams, or curving the trim over an angled structure. None of the prefab companies seemed to guarantee the materials they were using, so given that and the lead times we didn't pursue that any further.
The easiest option turned out being building laminated beams on site. They're constructed from thin marine plywood cut into strips, screwed, clamped, and laminated together with marine epoxy. The dimensions were all determined by our engineer. If you attempt to do this yourself, do not underestimate how many clamps you'll need!
The curve was made by attaching the strips directly to the angled floor joists. Quite to our surprise, the header beam was built first, using the floor joists to shape it. Once it had cured they attached it to 2x6's (on edge) to maintain the shape and help lift it, as well as a ratchet strap on the open edge.
Once it was finished it was moved to the side, and the curved joist was built in it's place.
Lots of work and planning, but straightforward enough. Now, if only curved handrail were as easy!
