Transition from flat to sloped truss ceiling
I am working on finalizing my roof truss order. My building has flat ceilings at both ends, with a sloped ceiling over the center portion. In other words, there will be two transitions between flat and sloped ceilings. My truss designer has asked if I want a supplemental-flat-bottom-chord-truss to make these transitions. I had assumed this would be how it was done – a scissor truss with a flat bottom chord truss right next to it – but now that he’s asked, I am wondering what the alternatives are.
How do you guys prefer to handle the transition from a flat ceiling to a “cathedral” style ceiling under a trussed roof?
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The two end trusses for the scissor truss section should be a gable end truss. Hopefully all trusses are heeled to allow full insulation. The gable end trusses should be next to a flat roof truss on the outside.
The transition is to run sheathing outside the gable end truss all the way down to the top plates of the walls, creating structural stability and a way to insulate the end of the wall at the gable end. On the flat roof side it allows to install roof sheathing on the outside, and either insulate a conditioned attic or blown insulation over netting. My proffered way to insulate flat roofs is install rigid foam on top of the roof decking and open cell under the roof decking. For the Scissor truss insulation, it all depends on the roof/ceiling pitch.
I don't know any other way to do it. Unless the transition falls on a 24" oc dimension, you need the extra trusses.
In many situations the vaulted portion of the ceiling is over a discrete space or room, and in that case you could just extend the walls up to the underside of the last scissor truss, but that comes with the attendant air-sealing and fire stopping problems of a wall extending up into the attic.
Alright - extra truss at the transitions makes sense to me. Thanks for the help guys!