You may be thinking a bit old school, it is used in this example.



Don't look like shed to me

Once again, it comes down to ones tastes, local codes and CC&R's (if you live in an HOA community). Compared to a standing seam metal roof, corrugated panels looks cheap, but that again is my opinion and others will disagree. So is life.

The problem I see with the functionality of corrugated steel roof panels is that the fasteners are exposed. That is NOT good in a roof design and especially if you have a SIP roof. Exposed fasteners will back out over time and especially so in a 7/16" OSB skin which the SIP has. Exposed fasteners leak, that is fact, it's not "if" they will leak, it's when. The leak will get past the OSB sheathing and cause the OSB to swell and the screw will then begin to pull out. This cycle repeats itself until the fastener fails.

Do you plan on walking on the roof every year to check on the fasteners?

A standing seam metal roof does NOT have exposed fasteners. That is a huge advantage.

If you choose the corrugated panels, best run it past the SIP company to make sure they are OK with it. SIP OSB skins are 7/16" thick and there isn't a lot of holding power in 7/16" OSB sheathing. A corrugated steel roof uses exposed fasteners and that is a bad idea when using a SIP OSB roof with 7/16" OSB skins.



OFF-TOPIC DORMERS: The one thing that stands out in that photo you uploaded is the dormers. I personally dislike dormers but from an energy standpoint, they are horrendous. They create energy leaks, roof leaks and moisture problems. An energy efficient home shouldn't have a dormer. What is the R-Value of that roof in that dormer section? Can't fit more than a few inches of insulation. Flashing that area is almost always a PIA and leaks are common. There are better ways to bring in natural daylight. Oh well, that is a topic for another thread...

Good luck getting your roofers to screw down the screw just right each and every time. Even if they did it correctly 100% of the time, the screw WILL work themselves loose from thermal expansion of the metal roof as it sits in the blazing sun. Then in the winter time when the metal roof sees below freezing weather. As the metal roof does the wiggle and dance of thermal expansion, your screws come along for the ride. Sitting in 7/16" OSB skins does not help the situation. I promise you this, they will work themselves loose over time.

Lbear, you're dead set in your mind there's only one way do it.

so not much point in debating with you on this subject.

But thanks for your input

Gosolar,

The fastener holding power is one of the concerns I have as well. But, if your strapping was spaced such that it fell on your metal roof fastener spacing, you would be screwing in to 7/8" of strapping+SIP OSB (if not installing a second layer of sheathing). The weak point in my view is the strapping to SIP OSB. I think that interface could be detailed to satisfy any of the forces in play. Based on inputs from Dana and others I have no concerns about weather tightness (as much as any roof can be) or the ability to dry.

Not sure I would skip the second layer of sheathing though. Wouldn't that leave the metal roof unsupported in some of the field and thus subject to denting and possibly noise issues? Corrugated metal would be less likely to dent, but noise may still be a problem.

GoSolar,
As you pointed out, this thread is supposed to be about vapor barriers. It's safe to say that none of the roof claddings being discussed will act as a vapor barrier. In your climate zone, the proper location of a vapor barrier is on the inside of your wall or roof assembly. The type of roofing you choose is up to you based on budget and aesthetics. I encourage people to consider roof and wall claddings that are inherently "Self-ventilating" such as the corrugated materials. These profiles promote drying and that is always a good thing. If you were in a harsher climate and you wanted to use a completely non-self ventilating cladding such as fiberglass shingles or standing seam roof, I would recommend a strapping system as discussed earlier. The entire discussion should always consider degrees of wetting and drying and this is a function of location and type of cladding. A common misconception of these ventilating details (Rain screens) is that they are SIP-specific. In fact they are applicable in all structures and more so in ones that are high performance (Air-tight) structures. These tight homes have a reduced drying capacity when they don't leak any air. Any moisture in the assembly needs a path to dry. The beauty of a SIP (which is vapor semi-impermeable), is that it can dry in either direction if you design your assembly accordingly. It's also important to understand the difference between vapor drying from within your assembly and bulk water draining and drying that is a result of wind-driven rain. The bulk water issue is handled primarily by the cladding and secondarily by the underlayment. The decision to add a rain screen or back venting should be based on location and cladding type.

I am not using strapping,
having said that,
I found a few membranes that look great among them, this one, in fact I spoke to the manufacturer and he is first one that actually under stood sips.

he said "the real key with SIPs is making sure they are properly sealed so that moisture from the living space can’t migrate into the panels or through the seams. If that is done correctly, I have not seen issues."

http://www.roofaquaguard.com/products/RoofAquaGuard-BREA/documents/RoofaquaguardBREAbrochure-June2012.pdf
and
http://www.roofaquaguard.com/products/RoofAquaGuard-BREA/


The corrugated panels will be fastened thu the high point of the metal, not the valley, I think that will provide a better chance of water/moisture not sitting and getting past the screw gasket.

There are tools to get the screws centered on those ridges by Simpson Tie.
Same tool can be used for siding, floors and drywall

Now as far as holding ability in osb, it is true that years ago it was real problem, but the osb made to day is close in holding ability to plywood, if I recall 350# vs 285#,

gosolar, I'm sure the manufacturer of that corrugated roof panel probably makes a blocking strip in the same profile to close the ends, guarding against birds' nests and squirrels/rats, et cetera.

Posted By Lbear on 06 May 2015 04:47 PM
The roof on a standard truss roof home doesn't need the roof furring because it can dry to the interior via the attic. Unless you spray foam the attic ceiling, then that is another topic altogether.

If you were to spray foam the attic ceiling of a conventional truss roof with an open cell foam, wouldn't you get away from the furring requirement as well? (Update: following the posting of this question, I found the following article - http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing. It's worth a read, but I remain curious as to what the folks on this board think)

http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing is well worth the read.

Posted By Alton on 21 May 2015 07:27 AM
http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing is well worth the read.

Yes, very definitely worth the read. In summary, green building scientist Martin Holladay states: Here are my recommendations for builders who use spray foam to create a sealed, conditioned attic:

•     Recognize that this type of roof assembly carries more risk than a vented, unconditioned attic. Keeping ducts within the conditioned space of a building (not in the attic) is preferable to installing spray foam on the underside of roof sheathing.

•     If you want to lower the risk of damp OSB, choose closed-cell spray foam, not open-cell spray foam, to insulate the underside of the roof sheathing.
• 
  
•     To further limit your risk, consider installing ventilation channels above your roof sheathing.
• 
  
•     If you choose to install open-cell spray foam against the underside of roof sheathing in Climate Zone 5 or colder zones, make sure that you include an interior vapor retarder.
• 
  
•     If you choose to install open-cell spray foam against the underside of roof sheathing in a humid climate, Joe Lstiburek recommends that the HVAC system be designed to condition the attic air and lower humidity levels in the attic.