Tuesday, February 5, 2013

Details, details...

For a while now I've been mulling over how to construct the vaulted roof section of the garage, where there are no trusses. Peter's drawings specified 140x45 F17 rafters, terminating at two 120x45 F17 top plates bolted together around the Hebel panels above the rammed earth wall.


I've decided I'm not going to use Hebel panels there any more, for a couple of reasons:

  1. They're heavy, and even with the do-it-all bobcat I'm not sure I could safely lift them up above the top of the rammed earth wall, then secure them to the frame above.
  2. This wall is inside the building envelope (being mostly beneath the vaulted roof line), and so doesn't need the fire protection that the Hebel provides to the external walls. The gable section which will be exposed above the roof line will be Hebel, but that's a pretty trivial exercise given that I'll have the roof as a working platform.
Instead, I'm going to keep an eye out for some second hand corrugated iron roofing sheets and use them over the regular 25mm battens instead. This should be a nice rustic complement to the rammed earth and bush poles of the garage.

Now that I've made that decision, I know how I'm going to attach the rafters - rather than use two top plates bolted either side of the Hebel panels, I'm going to check one plate 20mm into the stud wall, then use standard joist hangers to attach the rafters. These will give me protection against lift, and with the use of coach screws or bolts to attach them to the plate, they will also tie the rafters (and therefore the half trusses, bush pole frame and outer wall frame) together nicely.

So that's one little detail resolved.

The other which has been plaguing me for some time now is how to construct the little triangular wall sections which occur at either end of the vaulted roof, between it and the trussed roof. Not getting it? Here, these ones:

The one at the north end (the one pictured above) has a window in it, but the one at the other does not. The problem I have is, how to construct the framing? They're really a cross between roof trusses and wall frames; their role is both supporting a (small, albeit) roof load plus the weight of the Hebel panels. Now, wall frames I'm good with, and trusses aren't rocket surgery either. I've seen enough and drawn enough in Sketchup to know how they work structurally, and I've got plenty on site I can copy in any case. But those have horizontal bottom chords; these won't, necessarily.

So how to build them?

I've had a go at one design in Sketchup, but I'm not completely satisfied with it.

I suppose I'm struggling a bit with the dual roles. I actually think this will be more than strong enough in all the right places to do the job - I can't imagine how it could be built to be stronger, but there's an element of doubt. Do I really need the bottom chord? Would the single vertical spar at the log end be strong enough to support the roof load as well as that of the Hebel? Instinctively I don't think so - providing a bottom chord and using it to support some of the load of the structure above will transfer some of the weight off the single vertical, which I think will be a good thing, and also stiffen the whole thing longitudinally.

So maybe I'll do it this way after all. Either way I think I'll run it past Peter for his opinion...

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