The expansion and contraction of the roof will cause the shingle to cut into the coating on the metal roof and it'll rust from the bottom.
The way it should be done if shingles are left on is to overlay with runs of 1x4's to screw the metal down to

Ive seen alot of shingle roofs replaced with metal...never the other way around. I have a brand new metal roof on my house...and bout ever member of my family has went to one..we likem with nary an issue yet.

You've gotten some good advice to consider so far.

Screws can back out and washers break down leading to leaks.
Original installation may vary depending on area of practice and if roofing contractor is current and up to date on manufacturers installation guidelines.

Shingles...well nails can back out, tar strips break down and installation can vary too. Shingles are more prone to be impacted by storms. But wind driven rain can walk up a metal roof much easier than shingles and find an entry point.

Lifespan on metal is longer overall.

I think the aesthetics are the main thing, at least probably for your wife. Does she like the way that it looks, will determine whether you buy the house or not...

^^

I recall one of the Aldeer roof installers stated you need to keep an eye out on bolts that may need to be replaced with larger ones over time. Should be able to get 50 years out of the metal roof unless there is a moisture problem. I would have it checked out by a reputable long time roofing installer just as a precaution now and every 5 years for screw problem. Shouldn't charge you to come look at it.

Two questions Irishman, is the R15 the only insulation you have overhead? Won't the space between the double vapor barrier sweat and have condensation that is trapped ?

I had my roof replaced with a metal roof this year. They removed the shingles. I went with a standing seam. It was more expensive, but since it is installed with clips, the metal can expand and contract without elongating any screw holes. We went with galvalume also. Cell phone reception went down a little. It is really no louder in the rain than when we had asphalt shingles. We love our roof. I also worked a deal where they installed seemless gutters as part of the job. The quotes were all over the place. It was obvious that some knew what they were doing and others didn’t. Some were seemingly just trying to price it as high as they could. My point is just because it is metal doesn’t mean it was installed properly or that it will last longer than an asphalt roof. My 2 cents.

We had one at my last church. Just had to get out the buckets every time it rained because it leaked and nobody could fix it. My uncle just had one installed on his camp house. It's leaked every time it has rained so far, but the installer assured him that it's fixed now. They killed cell reception in both places; have to go outside to make a phone call.

I put shingles on my shooting houses because of my dislike of metal roofing. I've had much better luck with the old corrugated or 5v tin than the metal roofing they use now.

Don't think the metal was the problem in both those cases.

They look horrible after about 5 years and they really don't help the resale value of the house

I don’t have a problem with phone reception and have a metal roof there might be some leaks though 😡

I think they look terrible after a few years of sun damage. I wouldn’t have one on my house but that’s just me.

Great question based on the description. I'm assuming this is an unvented sloped roof on the cabin and the R-15 is an air-permeable batt insulation? I've followed your build, Irishguy, but never looked closely at any roofing details. 2 Dogs has some legit questions. Materials used will help determine if there is a potential issue down the road.

I'm going to list layers in descending order of what you listed:

------- Metal Roofing
~~~~~Self Adhered/Peel & Stck type waterproof barrier, like and Ice & Water Shield applied to 1/2" roof decking (Non-permeable barrier?)
_____ 1/2 Roof decking
XXX R15 insulation inbetween ____ 2x4 blocking (Type? Batt Insualtion?)
~~~~~ Vapor Barrier (air permeable?) such as felt or house wrap, on top of T-111
-------- T-111 used as exposed ceiling
===== Rafters visible.
{----} Conditioned space below

The questions to ask are this:
Where does environmental moisture/humidity from the conditioned space go?
What happens to it once there?

Kitchen use, showers, outside air, people emit water vapor just by breathing and living..are all sources of moisture.
Closed off roof systems can also have environmental moisture due to simple temperature changes.
Moisture will act as a vapor and travel vertically, known as vapor drive. Every house ever built has this. Physical laws of nature.

As normal and typical vapor rises to the T-111 surface, it goes through sheathing at joints or penetrations, Vapor drive then goes through the air-permeable barrier, then through the batt insulation, then finds its way to and through the roof decking joints and then out to the waterproof membrane.---but in this case there is a non-permeable membrane type system I assume, so the vapor will stop there and hold which often results in sheathing rot over time. Any moisture in the roof system from any other source will turn to vapor and do the same thing.

[Edit, added]: Also when vapor drive gets turned around and holds up in the roof system, you can get a build up of microbial growth on the ceilings in spots pretty easily. I've had a number of experiences with this too. However....

IF the roof is vented, then this is a non issue because the vapor drive has a route to escape while the insulated roof system still works for thermal protection. Is the roof vented?
IF the lower barrier (on the backside of the T-111) is a non-air permeable material, then the conditioned space vapor drive never gets to the roof sheathing, stays in the space where the HVAC and natural air movement takes care of it.
If the R-15 insulation is non-air permeable (rigid foam-sealed and taped- or spray foam), then any vapor drive stops there and never makes it to the roof sheathing. Vapor will "turn around" and hold in that space between the decking---not always a good thing---but it can also get back to the conditioned space depending on how tightly sealed the roof system is. Rigid foam board that is layered, taped and sealed or closed cell spray foam will create a non-permeable barrier. Batt insulation does not.

I love your cabin and your work has been fantastic so far. But....may want to double check some things now before any more time goes by--err on the side of caution. Hopefully what you have in place will handle vapor drive perfectly!!!!

I've had about a dozen cases involving Grace Ice & Water Shield (or similar) waterproof self adhered barriers where vapor drive cause roof decking rot in roof systems that were either:
A: Non vented sloped or cathedral roofs
B: Over non-conditioned and non-vented attic space

Both situations allow vapor drive to reach membrane and then hold where roof decking deterioration will occur.

Ok, so based on all the posts in this thread..........how do we make sure an existing metal roof was properly installed? This one is 6-7 yrs old, for what it's worth. Would a "typical" home inspector know what to look for, or would we need to have the roof inspected separately (and if so, by whom?)

I have a question for your question...

Does a cooler or an insulated cup sweat between the inner and outer layers?

I think I might read straycat's post. They're trying to help you out, not be asses about it....just saying. If you're all good, then you're all good can't hurt to have someone who deals with it daily give it a little sanity check. Sorry - my 2 cents that ain't worth a penny.

I was asking a legitimate question. That's all.

If It's a multi ply cooler top with open joints and air permeable topped in metal...then maybe so.
Solid where cooler moisture could not penetrate the top...then no.