Dormer Dilemma

Hello all, my partner and I are building a timber frame home up here in Northern BC and want to include a dormer. For the challenge we want to design, engineer and build a timber dormer. In the Guilds Timber Frame Joinery and Design Workbook, Ed Levins drawing of a similar style dormer has the jackrafters joining the side of the valley rafter instead of the top (plane of the roof). This seems to make the joinery more complicated and yet I am unable to see the design advantage? Is it stronger, more aesthetically pleasing, or just a technical challenge. The associated article was good in explaining the Hawkindale angles and how to approach the drawing of compound angles but it fails to answer the why's of joinery. As these are Ed's drawings I am sure there is a 'why' and I am just missing it. What am I missing?
Thanks

Thanks for your interest in the metaphysics of timberframing. What you find on the web page is a way of doing all wooden joinery dormers, not the official Guild-sanctioned way (there is none). Dormer framing styles vary by size, region, species, designer, and of course, client. Most timberframers are prepared to go to some lengths to achieve all-wood joinery in their complex roof work. Clearly a dormer would be much simpler to fabricate and install if it were, say, made on the ground of structural foam-core panels, and hoisted into place after trim and painting. If simplicity were the only criteria, we'd all be building pole barns.

Clients, and owner-builders, and timberframers need to find the balence point between the competing interests of time, money and quality. Let me know how I can help.