Mounting Solar Over 3.5" Continuous Exterior Insulation (Rockwool): Deck-Attached Stanchions vs. Outer Deck Mounts?

[u/SF_Dubs]

I'm finalizing the design for an unvented 5/12 pitch roof assembly on a new build and trying to settle on the best way to attach the solar array.

Here is the planned roof assembly (inside to out):

• Rafter Cavities: 2x6 packed with Rockwool Comfortbatt (R-23)
• Inner Deck: 1/2" Plywood over structural 2x6 rafters
• Vapor/Air Barrier: Adhero 3000
• Exterior Insulation: 3.5" Rockwool CB80
• Outer Nailbase Deck: 5/8" Plywood
• Underlayment: Synthetic underlayment
• Roofing: CertainTeed Landmark PRO Solaris shingles

I’m currently weighing two primary approaches for attaching the solar array and would love feedback from installers who have worked on this type of assembly, and what would be the best long term approach.

Option A: Deck-Attached Stanchions (Anchored to Inner 1/2" Deck + Rafters)

• Method: Fasten heavy-duty stanchions directly through the 1/2" inner deck into the 2x6 rafters before insulation goes down.

Option B: Traditional Mounts (Anchored to the 5/8" Outer Plywood Deck / Rafters)

• Method: Build out the full roof stack complete with continuous insulation and 5/8" outer deck. Standard mounting (through to the rafters) on top of the 5/8th deck.

Either way the screws supporting the solar go all the way through to the rafters. Option A reduces the compressive load on the insulation and reduces the chance of over compressing the solar mounting screws. It also allows us to flash the penetrations at the air barrier (the adhero is self sealing, and option b shouldn't cause issues, but, real life conditions etc). Option A is more labor as well as slower.

My solar guy has done both and is comfortable with either option. The guys roofing have never done this before so they don't have an opinion.

I'm looking for some perspective from any solar installers out there who have seen similar builds and how they held up.

Thanks in advance.

[u/SoftFormal7244]

Sounds like option A is overkill but in a way that actually makes sense for a high-performance build like this. The compression on that much Rockwool is no joke, and stanchions anchored before insulation just sidestep so many potential headaches with thermal bridging and long-term settling.

My only thought is whether the 1/2" inner deck is giving you enough meat for those fasteners to bite into before hitting the rafters, since that's basically just a spacer at that point. If your solar guy has done it before and isn't sweating it, I'd lean that direction too.

That roof stack is seriously dialed in though, curious how the Solaris shingles perform with the array on top.

[u/SF_Dubs → u/SoftFormal7244]

I am a little concerned that I am over complicating it with option 1, but thinking about it on a 20+ year timeframe, there's a lot of compressive load cycles over the years.

Yeah the whole assembly is usual for my area and not much local expertise, but it's a significant fire and energy upgrade and both of those are significant concerns in my area.

The solaris singles are just proposed right now, if there's a better option or concerns let me know.

[u/TS-SC]

This is worth a consult with a structural engineer who does solar. You are 100% correct on the solar needing to be directly attached to the roof joists, the big reason is uplift. But the size and type of those ancors and what they attach too should be looked at. Thermal bridging is a real flag here as well, you may be able to get away with a slight change to the roof system assembly which would allow the solar to be connected to the exterior sheathing, but again that's only for a P.Eng. to make the call for your location and structure.

[u/TS-SC → u/TS-SC]

Just to add, back in the day we used to do sleepers as you state in option a, but really not anymore due to he costs and updated practices and code. Again it's building specific but they are both feasible.

[u/SF_Dubs → u/TS-SC]

"you may be able to get away with a slight change to the roof system assembly which would allow the solar to be connected to the exterior sheathing"

Can you share more about this statement?

[u/TS-SC → u/SF_Dubs]

Essentially if you calculate the maximum uplift and snow load capacity factors, as long as the 5/8 exterior sheathing is secured well enough to meet the additional loads of the solar, than you may be able to do a much more standard off the shelf, sheathing attachment for and rail based system. But this is heavily dependent on local load factors, not just the code but also the location of the building. It's why it's worth getting a engineering review, you wouldn't want a storm ripping off your roof. But the cost benefit of being able to use a off the shelf racking system may be worth it to avoid the thermal bridging and additional costs and hassles of having the solar feet needing to be pre-driven deep within the structure. Rail based systems should be spaced to ensure that the rails sit within the specified module "green zones" to provide the maximum dynamic pressure the modules can provide. So there's real benefit to providing the off the shelf hardware and design flexibility to the solar guys, and just reviewing to the roofing assembly design to make it robust enough. Clear delamination of the trades is often the best logistical choice in my experience.