Replies

1. 
   user-2310254 | Jul 17, 2020 11:16am | #1

   Tyler,

   You should check with the manufacturer of your metal roofing. They have different requirements and will tell you if you are going to void your warranty or create performance issues.

2. 
   creativedestruction | Jul 17, 2020 11:30am | #2

   Tyler,
   Make sure to choose an appropriate location for your primary air barrier. I would recommend at the structural sheathing underneath the polyiso. Taped plywood works and should connect with your wall air barrier. 2x4 flat furring strips above the polyiso or even nailbase with integrated sheathing/polyiso may work for metal roof attachment depending on the product, as Steve noted.

   I wouldn't recommend smushing polyiso against rafters. This would compromise the structure, air and weather tightness of the assembly.

   Best,

3. Expert Member
   Dana Dorsett | Jul 17, 2020 02:44pm | #3

   >"...can i even flip it and lay the poyiso directly onto the rafters, then 1/2″ plywood, then ice & water, then metal roof?"

   Nope. The polyiso will compress against the much narrower rafter edge area over time, and with 1.75" of polyiso between the roof deck and rafters the fasteners would have to be timber-screws on some pre-engineered spacing, not nails.

   Also, half inch plywood doesn't quite meet the structural requirements for roofs in many areas (but 5/8" usually does.)

   Preferred stackup, from the bottom up:

   -Structural roof deck nailed directly to the rafters per code prescriptive fastener schedule.

   -Ice & Water Shield goes directly on the structural roof deck

   -1.75" polyiso, cap-nailed in place, seams taped

   - nailer deck or 2x4 purlins, timber-screwed to the structural roof deck (not to the rafters) with pancake head timber screws.

   - if nailer deck (not purlins), weather resistant semi-permeable underlayments prescribed by the steel roofing vendor. If purlins (not nailer deck) use the roofing vendors preferred fastening method.

1. 
   dustin_7022224 | Jul 31, 2020 01:40pm | #9

   Dana,
   I too am curious about the comment on attaching 2x4 purlins to the roof deck (not to the rafters). We are planning to run 2x4s diagonally to create a 'rain screen' vent space just below the standing seam metal roof.

2. 
   merlinm | Jul 31, 2020 10:04pm | #11

   Dana, thanks for this as we are planning a similar roof except with 2" of foil-faced polyiso over structural sheathing. There will be 2 layers of polyiso foam with staggered seams. On the underside of the roof deck, we plan 2" of closed cell spray foam and then 6 or 8" of open cell spray foam. Are the 2x4 purlins on top of the polyiso adequate without a nailer deck on top of the polyiso? Should the purlins be oriented at 45° or vertically?

1. 
   pakrat1 | Jul 31, 2020 10:33pm | #12

   FYI foil faced poly is vapor closed and 2" of ccspf has very low permiability IE vapor barrier on both sides of deck "risky"

   Dana needs to chime in on this

1. 
   merlinm | Jul 31, 2020 10:55pm | #13

   Good point, Michael. Thanks.

   Are ice & water shields low permiability, which Dana recommends on the structural deck, as well?

1. 
   maine_tyler | Aug 01, 2020 10:42am | #14

   John, Ice and Water is low permeability. But the typical stack-up would not include a 'sandwiching' of sheathing between two low permeability layers.*

   This is accomplished by forgoing ccspf under the rafters and using vapor (and usually air) permeable insulation instead. (Why use spray foam of any kind in the rafter/truss bays if using exterior foam anyways?)

   But do make sure to hit minimum R-value ratios for the above-deck rigid foam, based on climate.

   *That said, the 'sandwich' assembly is occasionally used and considered not a concern by some despite others' nervousness about it. I personally don't see why one would design that from the start, when other options exist.

   https://www.buildingscience.com/documents/insights/bsi083-mea-culpa-roofs

   https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers

3. 
   dustin_7022224 | Sep 07, 2020 09:32pm | #15

   Anyone seen Dana? I'm still hoping he can answer my question (post #9) and a few others have asked for his advice/clarification as well.

1. 
   b_coplin | Sep 08, 2020 01:44am | #16

   This question was answered in another thread: "Instructions from some of the nailbase panel manufacturers explicitly state it should not be screwed into the rafters."

   There is a lengthier hypothesis on the rationale, if you're curious:
   https://www.greenbuildingadvisor.com/question/question-for-dana

   Asking a new question will often yield more answers than reviving an old thread, especially if commenters have not uniformly signed up for email notifications.

1. 
   dustin_7022224 | Sep 21, 2020 11:24am | #17

   Thanks! I'll look there.

4. 
   pakrat1 | Jul 17, 2020 04:09pm | #4

   Q for Dana

   why screw into the roof deck, not to the rafters

   Mike Lind

5. 
   tyleralex1 | Jul 18, 2020 09:46am | #5

   dana, yes its 5/8" plywood.

   you have some what answered this, but it seems to me its best to tape the seams of the polyiso, not the plywood seams underneath as I will be coming from the inside and spray foaming open cell foam in all the corners of the underside of the plywood where they meet the rafters.

   MBCI, the stand seam roof manufacturer, recommended using a plate (top hat as he called it) to nail thru the polyiso and into the plywood sheathing as this would eliminate overdriving nails into the polyiso. we also would apply a synthetic underlayment directly over the polyiso.

   so outside to inside:
   standing seam roof
   plates nailed through synthetic underlayment, polyiso and into plywood.
   open cell spray foam from underside of plywood in corners of rafters and plywood
   batt insulation
   sheetrock

   any and all advise is greatly appreciated...thx so much!

1. 
   Jon_R | Jul 18, 2020 12:30pm | #6

   So no wood above the foam. Do you have a link to such a plate?

2. 
   maine_tyler | Jul 31, 2020 09:37pm | #10

   Any particular reason for the open cell spray foam, rather than just taping (or ice and water) the sheathing for air barrier?

6. 
   tyleralex1 | Jul 20, 2020 09:31am | #7

   This is from MBCI:

   Tyler,
   Please see pic in the middle marked "bearing plate". Please let me know if you have any questions.
   Thanks

   Respectively,

   Quote. Order. Build. | Order Metal Roof and Wall Products Online at Shop.MBCI.com
   P: 770-948-7568 |
   Direct/Mobile: 812-322-4283| F: 770-739-6228 | http://www.mbci.com

   
7. 
   pakrat1 | Jul 20, 2020 01:46pm | #8

   Dana

   Not trying to butt in to Tyler's post, Just wondering why you say not to screw nailboard into rafters, only primary deck. comment # 3

   Sorry for intruding, but this applies to my project also

   Thanks Mike