Is it acceptable to install underlayment over skip sheathing alone, or should solid decking have been installed?
Appreciate any feedback.
How the heck are the nailing down the shingles? Perfect alignment?
Generally, shingle manufacturers will only accept a 1/4" gap. You should look up a few installation guidelines like GAF or others.
And staples to boot.
A lot of cities around me have banned the use of staples to install shingles.
Looks as if they are just using staples to the skip sheathing?
I’ve come across similar setups before—though rarely—and in most cases, the underlayment was sagging or deteriorating between the gaps
I brought up the irregularity to the clients and tried to explain why a missing solid decking would be an issue.
No doubt for good reason.
My thoughts as well! I wonder if he walked the roof and what that felt like? Hopefully he checked the attic before climbing the roof.
Something I never do. Perhaps it is time for me to reconsider my plan of action 
The staples you see are from the old shakes. Those are narrow crown staples, shingles required wide crown staples.
But it’s easier to bang them down after tearing them off then to pull them out.
Most likely that’s the old 30lb felt. I’m sure the OSB was nailed over the top of it before Installing the shingles.
It’s easy to check by pushing on the underside, or by lifting a shingle at the gable or peeking under the drip edge.
Never assume anything. As if you are wrong and report it, you might just be paying the contractor for his waisted time.
Wouldn’t installing OSB over felt still be considered improper? If any moisture gets through the shingles, the OSB would absorb it before the felt can serve its protective function, defeating the purpose of the underlayment
Of course it’s improper, but it’s better then no OSB. Roofers these days are getting pretty lazy, Lots won’t even remove the old underlayment or dripedge.
Man, this type of install is crazy, and a laziness which can lead to a lawsuit. If someone installed my roof that way i’d be giving them a call.
What I don’t get is If someone is gonna take the time to install new OSB and underlayment, why not just do it in the right order? Might as well of not even installed the underlayment.
You have any images of the roof deck surface? Type of shingles?
Typically/Normally/Usually, it is generally not recommended by roofing manufacturers, nor building codes, for asphalt composition shingles to be laid atop skip sheathing with underlayment on top.
Typically/normally/Usually, asphalt single manufacturers recommend solid rigid panel sheathing be it plywood or OSB is preferred. It provides a more consistent suitable stable surface for fasteners.
My 2 cents.
If William is correct, and they simply didn’t remove the old felt prior to installing OSB, that doesn’t mean that they didn’t apply new felt over the new OSB. However, in my opinion, I would be surprised if they installed the OSB over the old felt. It would be pretty hard to remove all the old shingles without also removing at least some of the felt underneath.
I agree with Ryan, it would take a special kind of stupid to install the OSB over the underlayment. But then again it takes a special kind of stupid for the OPs install.
I think it safe to say that if the roof were to get blown off in a storm that insurance could deny coverage for improper installation.
Write hard. 
So… worth less than one cent USD!!
Not sure how OSB being installed is determined from that photo. Looks like a great candidate for the WTF thread.
The house was really old, I think 1960s-ish, and this paper looked to be nearly new.
I said something along these lines to clients in summary. Narrative I used was “Roofing underlayment observed installed above skip sheathing – configuration is uncommon and
may not contribute meaningfully to moisture protection – recommend evaluation by a licensed roofer to review roof system installation and material arrangement.”
I agree. That does not appear to be old felt.
To answer your question.
Yes, it is generally acceptable to install underlayment over skip sheathing for a tin roof. Seen it several times many years ago. It provides good ventilation and allows for the expansion and contraction of metal panels.
Not for composition or three tab shingles.
