Roof Struture vs. Ceiling Structure

[Scotbow]

The last house I built had an unvented roof with the following layers, top to bottom:

1. Standing seam metal roof surface, 1:12 pitch

2. Ice & water shield membrane

3. 3/4" plywood 

4. 24" mono-trusses, with 6" closed cell foam on the underside of the plywood. Enough to prevent condensation, in my climate.18" spray in dense pack fiberglass, under the foam, to fill the cavity.

5. Netting to hold the dense pack fiberglass

6. 5/8" drywall

7. vapor barrier primer & paint

I had to frame the overhang, to keep the fascia to 16".   This has performed very well.

 

My question is, "How should the ceiling structure and surface", be defined?

[GeneDavis]

Ceiling structure can be under floor structure and include as many layers as you want.  You cannot do insulation such as sprayfoam or dense-pac as a layer, the way you are describing it as full fill or partial fill of a structural member layer such as joists or trusses.  It is not uncommon for ceiling structure to have an air gap layer.

 

Roof structure can be specified in as many layers as you want, and builds up from structure (lumber, truss chords, I-joist members) with "cover," i.e. sheathing, then maybe rigid insulation, film, and finishes like standing seam steel or shingles.