Wednesday, October 6, 2010

Fan of the Fan (VIII)

Today marks the 300th post in the history of this blog!! Thank you to all the readers, followers, supporters and contributors to this blog, and I plan to continue moving forward with the Carpentry Way. Blogging has been fun so far, and rewarding in various respects. In the next month or two I will be releasing the next volume in the Art of Japanese Carpentry Drawing Series, so please keep an eye out for that.

I expect that there may be readers out there, who, after digesting the information contained in the first 7 posts in this thread, might be wondering why anyone would want to employ fan rafters on a building. After all, it is more than a little complex to determine an elegant spacing and position for them geometrically, and, if the eave edge is curved, each fan rafter must be a uniquely shaped piece, both in its parallelogram-shaped cross section, and differential curvature. Fan rafters are tough pieces to fabricate, and to achieve a nice clean fit among other curved roof parts requires a bit of skill, care, and patience.

So why bother with them? For many carpenters, regular hips with parallel-oriented jack rafters are challenging enough to deal with. In fact the number of carpenters who can lay out regular hips and jacks, especially when joinery is involved, is a fairly small number, and getting smaller by the day, sad to say. We're in the golden age of fly-in truss packages after all.

Well, I think fan rafters are worth the trouble. I have a few reasons for making that assertion, and in today's post I would like to explain my reasoning in regards to this matter.

Take a look first at this model of a regular hip, Japanese style, with parallel rafters:

Bear in mind that the model above is only of the decorative hip and rafters. It's only half of the roof, but once you can do that half, figuring out the structural roof above is no great stretch.

If you take a look at the jack rafters attached to the hip, you might notice that the four rafters, counting from the left side as '1', are all doing very little, structurally speaking. That this is a decorative roof and structural concerns are less critical I'll set aside for the moment as the model illustrates a condition common to any hip rafter with parallel jack rafters.

The lowest jack rafters attach at the hip with stub tenons, a weak connection typically reinforced with a screw or spike, and lower down are affixed to the kaya-oi at the eave edge with a couple of screws. At best, these first four rafters simply hang off the hip and the perimeter fascia, and about all they would support would be the lightweight ceiling boards.

If you left those rafters off because of their limited structural usefulness, there would be a major gap in the rafter spacing rhythm, and that wouldn't look right. So, for aesthetic seamlessness, those rafters have to be there. Their main function is for continuity of appearance, period.

It gets worse though. Take a look at the next three rafters in line, numbers 5, 6 and 7. You can see that while the upper end of the rafter at last rests on the plate, they remain structurally limited in function. As cantilevers go, the fulcrum (the wall plate) is at the wrong end of the stick, isn't it? That means that these three rafters continue to do very little to support the structural parts of the roof; instead they carry ceiling boards and maintain the continuity of rafter spacing along the eave.

Only when we reach the last rafter, number 8, do we achieve a rafter of some structural usefulness, as it roughly balances on the wall plate, 50% inboard, and 50% hanging out in the eave. Still not ideal as a cantilever, but it would work. If the hip were extended further uphill, you can see that the structural support or the jack would get better and better as we move along to the right from rafter 8.

Now consider an eave with fan rafters:

Taking note of the above discussion about the jack rafters in relation to their structural usefulness, perhaps the reader can immediately apprehend the advantage to fan rafters. For nearly any given position of a rafter tip along the eave edge, the fan rafter is longer and better supported. That means stronger. While the first rafter over is not quite to a 50/50 balance point on the wall plate, its immediate neighbor, rafter number 2, is at at 60-on/40-off position. All in all, the rafters have much greater structural usefulness. Every other subsequent rafter is in that 60/40 relation, or better, and if i were to slice the in-between rafters so they could nestle in among the longer one their position would improve even more.

A further advantage to fan rafters is that they do not, in Japanese work at least, connect to the hip rafters. This eliminates the need to mortise the hip rafter, or make housings for the jacks (or both), ineffective wood-joint connections at best, and thus the hip is going to remain intact and therefore will be stronger. Considerable labor is thereby saved in the removal of any need to cut mortises or tenons to attach rafters to the hips.

In curvilinear hip work, regardless of whether parallel rafters or fan rafters are employed, the rafters need to be of parallelogram cross section, so the same amount of work is required to form either type of rafter section.

In terms of aesthetics, the experience of looking up at a fan raftered eave corner is quite delightful, in my opinion. While of course such things are a matter of taste, there is something to be said for an elegant sweep of rafters up to the hip, accentuating the curvilinear shape and projection of the hip.

Fan rafters, when you think back to earlier ages of man, to the age of building wooden structures before the advent of metal tools, one can see how fan rafters harken back to the primitive. After all, if you are lashing poles together to build a roof, one does not tend to gravitate toward mortise and tenoning the rafters to a hip corner. It makes much more sense to distribute the rafters radially:

So, in that sense, fan rafters are a connection to the primitive hut, even, I dare say, if done with high elegance.

There are reasons therefore to make use of fan rafters, though I will say the path to understanding how to lay them out is a steepish climb and probably best attempted after conventional hip rafter work with parallel jack rafters is mastered.

Thus concludes my look at fan rafters. I hope this thread was an enjoyable read, and your comments are most welcome should you have anything to add.


  1. Congratulations on #300 Chris.....

  2. Chris

    Not too long ago I didn't even know what a fan rafter was, now I'm a fan, too. That was a GREAT series of posts and very informative. And, yes, for most of us regular hips and jacks are difficult enough.



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