The Story of the Gazebo (IV)

I spend more than a little time designing things, many of which I might never build. The process is incredibly valuable however as each time i work through a design I learn new things and re-confirm previous conclusions, often with fresh insights. I’ve come up with a timber framing system which makes use of Japanese joined timber framing, an adapted form of German wall framing method and natural infill. It’s different than anything else out there, at least which I’ve come across, and I look forward to the chance of prototyping the design with a real building someday. Maybe I’ll do some posts on that at a later date.

I’ve also been working upon designs for various pavilions, which are simpler structures in certain respects, and if you’ve perused the previous posts in this series, you will see that there is a fairly wide range of structural approaches out there. By and large, the Western approaches I’ve seen don’t do much for me, either in structural ingenuity or aesthetics. The Chinese forms are a bit overly flamboyant for my tastes, though i have found considerable inspiration in some of the structures I have come across. In the end, the Japanese forms of pavilion are of the greatest interest to me and I find them the most satisfying in visual terms.

One of the forms of framing I looked at in a previous post in this thread was reciprocal framing, as is well exemplified in the Bunraku Puppet Theatre. I’m not new to reciprocal framing. Nearly 10 years ago I built a reciprocal roof model while working in California:

You can see the developed drawing dimly on the piece of plywood in the background.

I didn’t come up with the drawing method on my own – I found a few pages on it in the German text, Bassiswissen Shiften. Like a lot of German timber work that I’ve seen, the joinery tends to be on the simpler side, from my perspective at least – for the above model, the connections between the reciprocal rafters were basic lapped notches on the underside of each stick. This makes for reasonably straightforward assembly of the parts, however the joint allows for a slight amount of creep in one direction as the parts sink onto one another – multiply this by 8 sticks and the structure will compress down below the design height. Further, if the structure is loaded from above, then the end grain face of the half lap presses against the side grain face of the timber below, and thus a bit of crushing ensues, which causes the whole thing to slide down a little further. And any sliding means that the rafters effectively push outward at their base, which is less than ideal. Of course, one can use timber screws to keep things more or less in the correct place but, obvious to me at least, the joinery solution could use improvement.

I’ve seen reciprocal structures in which the reciprocal rafters themselves are the roof rafters instead of supporting elements. Olga Larsen’s book Reciprocal Frame Architecture has a section which details the work of Englishman Graham Brown, framed structures which are entirely of this type – here’s his ‘Whisky Barrel House’:

Another example which I came across on an Australian site:

This arrangement, in which the external roof form lies directly atop the reciprocal rafters, has several problems, in my view. The aesthetics are a matter of taste, and to me the roof form does not look all that attractive from the exterior. More important by far though is that the roof membrane is chopped up into sections, with short triangular walls introduced at each rafter point. These transitions make a greater percentage of the roof area vulnerable to leakage.

It is a general rule of roof design, if keeping the water out is going to take center stage (and isn’t that the main point?), that the roof have a minimum of such transitions, like valleys, dormers, chimney penetrations, etc. The more places in which the roof covering is replaced with flashing, the more potential there is for leaks down the line. The above picture shows a framing system in which the top of the wall has been angled, along with the window and its frame, to fit against the underside of the roof, and this adds extra work and complication for dubious gain. It most likely means fixed windows.

Finally, many of the reciprocal frames end at the wall and thus the resulting structure has no eaves, which means that the wall envelop takes a lot more punishment from the weather. And fitting gutters to such a roof shape would seem awkward, with separate gutters and downpipes needed for each roof facet.

It makes a lot more sense to me to use the reciprocal frame as a support structure for the roof rafters above, and not make the reciprocal frame the roof itself. The idea of creating a double roof so that the interior and exterior structural and aesthetic modes can be somewhat separated from one another happens to be a Japanese invention – and one which I admire. Indeed, the Bunraku Puppet Theatre is a fine example in which the external form of the roof makes good sense in terms of keeping the weather out, and has deep eaves, while the reciprocal support structure inside remains hidden from exterior view:

There are many different structural framing systems used on gazebos, and I am choosing to employ reciprocal framing because it readily provides a clear opening in the enter of the roof for a top-mounted lantern, which will allow light in to the structure – that light makes for a much more pleasant space, and shows the framing off to good effect.

Many reciprocal frame designs that I have come across place the lower ends of the reciprocal beams directly atop the corner posts. Since the reciprocal rafters are at a different orientation, plan-wise, than the posts, the connection between the post and the reciprocal beams is often a bit awkward.

I decided to place the reciprocal beams atop the wall plate beams which span from post to post. Further, I chose to lengthen the reciprocal beams so that their lower ends project out beyond the wall to help support the eave:

The reciprocal beam is colored brown in the above drawing, and does double duty as a support for both the inner and outer roof structure. It acts party as a cantilever.

Now, coming up with an interior arrangement of reciprocal beams which would cleanly tie in with a polygonal plan and the raised lantern and a double roof structure was not all that easy. In fact, as it turned out, the most ideal plan for this structure is pentagonal. In the next post, I’ll explain why pentagons ‘r us, and look at this framing solution in more detail. Thanks for dropping by the Carpentry Way. Comments most welcome.  ➜ on to post 5