The curving of the purlins is also relatively mundane, as they derive their incremental rises at various spots based on the same rises that develop the perimeter fascia, but of course given that the purlins in this roof are not rolled over (like the perimeter fascia) to sit flush with the rafters, the amount of rise that lifts the kaya-oi in a given spot needs to be converted for use at the same spot involving the purlin. That may not make sense to some folk, so you’ll have to take my word for it at this point as I am not going to expand upon that topic any further. Even if I explained in detail, it wouldn’t make sense unless you were familiar with hip rafter layout in general and more particularly, how Japanese roof systems are configured. It’s a long story.
Here’s a view mid-way through the development of one of the curved purlins:
You can see a straight line defining the arris of the piece and then a curved line which has sprung up from it. The section in view is only half, lengthwise, of the total piece – I develop one half, then copy and mirror image the copy then stitch them back together to form the piece.
Here’s a few shots then of the bell tower with the moya and ko-mai added to the decorative roof:
And a close up of one of the hips with its ‘cricket neck’ nose treatment, or kera-kubi:
One more bit, also developed from the kaya-oi, is the next layer in the eave build-up, a piece of fascia termed the ura-gō. Ura-gō (裏甲) translates as ‘rear carapace’ as it’s thought of as somewhat akin to a shell, as on a tortoise:
Like the kaya-oi below it, the upper and lower surfaces of the uragō are not parallel to one another – both pieces swell up in thickness as they move up the curve towards the hip. This thickening of the sections is done to counteract the effect of optical foreshortening when looking along a pair of lines (similar to the way we perceive the rails of the railway tracks meeting off in the distance):
There’s another layer yet to add to the eave build-up, but next up I will be returning to the fan rafter drawing work, and developing their individual curved and twisted shapes. When that is done it will mean I only need to place the ceiling boards and a few other minor pieces to complete the decorative roof portion of the layout. Once that milestone is passed, I will be turning to the hidden roof structure which will have many challenges and delights. I’m looking forward to that stage – it’s all good!
With a return of attention to the fan raftering, I will resume my blog series on that topic, so that is likely to be the next posting that you will see.
Thanks for dropping by today on your travels. –> on to post 5
3 thoughts on “This One Rings a Bell (4)”
Hi there, chris – I especially love the thickening of the urago pieces as they approach the hip- so elegent. Is there anything that the japanese craftsman have not considered in their perfecting of their craft? To account for optical distortions, care in proportioning, spacing, etc., etc.,is aproaching the divine. It is exciting to see your depth of coverage, and the finished piece will be sublime.
Mitch Rose, British Columbia, Canada.
I'm not sure why the roof is divided into 'decorative' and 'hidden'. Is it because the detail would be lost in the shadows, if the functional (and steeper pitched) roof were decorated? Or some structural necessity?
And Mitch is right. Your depth of coverage is exciting.
nice to read the words of someone as enchanted with this architecture as I am. and I don't believe there is much that to be found in these constructions that hasn't been considered by Japanese carpenters in the last 500 years.
the decorative and hidden roof system is one of the most brilliant aspects of Japanese roof structures in my opinion, though I have found it is not so easy to convince people of this. As I detail the build, the system will become more apparent. At this point, I have only been working on the exposed decorative roof. The actual roof which will carry a final pitch of 7.5/10 or so and keep the rain off the rest of the wood isn't even in the picture yet. All will be revealed in time, so please stay tuned.