Confused Roof layers: Structure vs Surface

[BrownTiger]

So I am "building" a modern house: mcm with a gorgeous skillion roof at 2 in 12".

 

Unvented roof assembly with the following layers:

1) Roof Metal

2) 1/2" ... something like 1"x4" diagonal sleepers  

3) 2" foam

4) 2" foam

5) Grace Ice and water shield HT 

6) 1/2 OSB

7) 2 1/2" x 9 1/2" 16 o.c. I-joist  with fiberglass insulation for my 20' span

8) drywall  underneath

 

I want 9 1/2" I-Joists to have soffit under and some sort of facia. 

 

1) Chief presents me with the following dialog. Now I am totally confused ... What the heck is a difference between a surface and a structure?

Is my foam part of the surface or a structure? Or sleepers.

 

What should realistic eave and gable subfascia be?

 

 

   

[Chopsaw]

It is a bit like when defining the framing layer of a wall.  You can do what you want but it may cause other issues later.  I believe the theoretical roof plane is between the Surface and Structure layers so if that is important make that your dividing line.  I think it really should be all one DBX like walls.  You just need to watch how the other structures build around it like sub fascia and fascia to know if you need to make adjustments.

 

You may be able to trim the rafters on the under side if you don't want a big fat fascia, but unfortunately not much @ 2:12.

[BrownTiger]

I was thinking along the same lines why would they not have it like a wall?

 

 

[DzinEye]

I would say the surface is anything above the plywood sheathing...but like Chop says, may depend on how you want it to finish with other roof types, if any other will be present.

For un-vented roof, if it's an available option for you (is this a real project or theoretical?) , I would recommend closed-cell spray foam directly into the rafter space.   You'll avoid having that insulation hump on top of the roof which you'll either have to have a very wide fascia to hide or leave a step down which isn't all that aesthetically pleasing.  There are other benefits to the closed cell too.
 

[Chopsaw]

If you run into issues I would be glad to take a look at your plan file.  There are a lot of factors that go into a custom roof.

[BrownTiger]

1) SonOfBachelor View Has autobuilt ... Nothing build right

 

 

 

 

 

[BrownTiger]

HandBuilt  What I want to get

 

 

 

[robdyck]

2 hours ago, BrownTiger said:

What should realistic eave and gable subfascia be

Do you mean finish material or measurements?

[CAmichael240114]

The roof metal is the surface layer.

The structure layer is all other material layers below the roof surface and above the ceiling structure and ceiling finish (battens and the gypsum board underneath).

Draw separate roof plans with reduced rafter height for the different soffit depth 

[robdyck]

40 minutes ago, BrownTiger said:

HandBuilt  What I want to get

Here's a section view of your roof. 

Surface:

Structure:

Ceiling:

 

@DzinEye definitely had good advice for an unvented roof. If you want the increased depth for aesthetics, I'd suggest a 14" I-joist and a sheathing layer that's appropriate for the metal roof panels.

Obviously the cost of spray foam varies by region, but reducing the # of layers will lower the labor cost dramatically and the simpler the roof can be, the better it will perform.

[Chopsaw]

I like the Idea of the thicker I Joists and the spray foam.  The drawback I see is that spray foam as well as rigid insulation has really poor sound dampening properties.  This could be an issue depending on the location but almost any where it rains metal roofing is noisy.  I think a nice compromise is a couple layers of Roxul insulation with wiring in between and then spray foam on top which solves a few problems all at once.