Is SPF under the roof a smart move or roof killer?

[RigidChrix]

SPF offers excellent air sealing and can turn the attic into a more controlled environment. The big question is would spraying SPF under the roof actually harm the roof structure over time? or is that mostly an installation/ventilation issue? The alternative I’m considering is skipping the foam entirely and building a proper ceiling, air sealing everything meticulously, and then blowing in a thick layer of fiberglass insulation. It’s the more traditional approach, but it might avoid some of the risks associated with closed attics

 

[Harrity]

The foam applied correctly and moisture is managed, it shouldn’t harm the roof structure. Problems usually happen when it’s sprayed on a roof that already has moisture issues or when ventilation details are ignored

 

[thec00l bill]

@RigidChrix yeah, SPF being a roof killer is real, but it’s mostly about moisture and inspection.
When you spray foam directly onto the underside of the roof deck, you essentially seal off the wood deck from the conditioned air inside the house.
If any water does get past the shingles, it can't dry out. It gets trapped between the foam and the roofing material, leading to rotting and structural deck decay that you can’t see until it’s catastrophic.
It also makes it almost impossible for a roofer to inspect the plywood for damage during shingle replacement

 

[RigidChrix]

@thec00l bill that's what worries me about it. So if I already have SPF up there, should I be doing any kind of regular moisture checks? or is it just a hope for the best situation until the next roof replacement?
Also, what do roofers usually do when they find a foamed deck, just refuse the job or charge way more for the extra work?