Monday, February 20, 2012

The Story of the Gazebo (V)

Here we are at the 5th post in this series, the 1st post, if I might be optimistic, of the next 500 posts to come on this blog. It is only fitting, I suppose, with 'fives' coming up everywhere, that the subject of today's post also revolves around 5, literally and figuratively.

I spend a lot of 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 6

8 comments:

  1. Chris,

    One quick question: when do you sleep?

    ReplyDelete
  2. Tico, I have been thinking about that too sometimes, but I guess not having a TV helps.

    Chris,
    The question of design and purpose is very relevant today I think. Designers, not only of houses, often seem to forget the purpose of the thing they are designing. This has been so for many years and not really getting any better in general. I agree with you in the 2 "simple" examples you found, not very practical nor aesthetic. The same with eaves, I personally love large graceful eaves, but a lot of architects seem to have forgot that the purpose of eaves is to keep water of the walls and foundations of the house. Where I live, eaves on most houses has been reduced to a minimum for many years. This makes the houses look boxy and the houses being of wood get completely soaked every time it rains. In combination with equally bad designed flashing and roof connections, this has led to a lot of serious problems with mold in newer buildings. This is beginning to cost a lot of money and worse, giving a lot of people otherwise completely unnecessary health problems.
    On the other hand, the drawing you come up with is a very good example of purposeful design. It does everything it is supposed to do in a graceful manner and provides the carpenter with work for a long time too!

    ReplyDelete
  3. Per,

    yes, not having a TV does help, though I do confess to watching the odd video on my computer.

    I think the reason that "a lot of architects seem to have forgot that the purpose of eaves is to keep water of the walls and foundations of the house" is two-fold. They are not builders, as such, but developers of design and specifications, and are often more concerned with adhering to a philosophical or stylistic ideal rather than a practical ideal, and two, architects rarely go back and see how their old designs are standing up over time, learning some lessons, and applying those lessons to their newer work. If they did, they would have abandoned modernism, with its liking for flat roofs and no eaves, long ago.

    Glad you like the glimpse I have so far provided of my design. Yes, it would be a sweet carpentry project and hopefully I will find a client who wants me to build it some day!

    ~C

    ReplyDelete
  4. I really enjoy the read of this share. It motivates me about gazebo design. Actually, i want a perfect gazebo ion my patio and this post gives some innovation to me for designing my own gazebo. I love this blog and subscribe to it for sure..

    Rich Cuts

    ReplyDelete
  5. Rich,

    thanks for your comment and glad you enjoyed the read.

    ~C

    ReplyDelete
  6. Don't know if you still visit this page but I hope so. I am a bricklayer by trade and have built a Gazebo in my backyard. I was wondering what I was going to do with the roof because I didn't want it to look plane, I came across Reciprocal roof truss design and this post. It will go good with what I have because I have twisted brick piers. If I could? I can send you some pics so you can see what I have done so far. Thxs for posting.

    Bill

    ReplyDelete
    Replies
    1. Bill,

      it's never too late to comment on a post - the comments always reach me and I thank you for chiming in. And sure, send me pictures of your work anytime.

      ~C

      Delete

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