Wednesday, May 19, 2010

Rafters and Purlins

Now that the poles have been concreted in the guys have been very busy putting up the roof members.  The big rafters are spanning gaps of up to 6 metres. These were actually pretty hard to find because they're an unusual spec (i.e.300x50mm, gauged, kiln dried H3.2). I'll try and explain what it all means:
  • 300x50 is required to provide enough rigidity and strength over the span (as per NZS3604)
  • Gauged means smoothed and planed off to an even dimension all around
  • Kiln dried means it's been force dried so that it won't bow or twist once it's gone up
  • H3.2 is treated for outside exposure. If the rafters remained 100% inside the walls then we could have gone with H3.1 which was more readily available but then we lost the integral verandahs. 
The purlins (the ones that go across the roof) are 250 x 50 gauged, kiln dried, H3.2 and they're spanning up to 3.5 metres in places.

This is different from most houses because we're not using trusses to support the roof. Trusses are strong and easy to make offsite from standard 100x50 timber but they didn't give us the high raking ceilings that we wanted.  We needed something that could support the roof, thick insulation and ceiling yet remain stiff enough to span the width of a room.


Changing plans already

One of the nice things about working with Dale is that he's pretty flexible when it comes to the finer details.  When I was looking up at the purlins one morning I had a hard time imagining the finished roof with the thick edges that were shown on the plan.  I thought that if we tapered the purlins off at the overhangs they'd end up making the roof look lighter and less heavyset (a nice juxtaposition with the thickness and weight of the poles). A quick word to Dale and we were cutting test pieces and trying them out on the rafters.  I think you'll agree it ended up looking pretty good.

Stay tuned for photos of the finished roof...

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