if your roofer won't warranty it- RUN! find someone else who will stand behind their work. a roof HAS to perform WITHOUT fail, every single day, year after year, otherwise you might as well move your lazy boy and flat screen into the yard.
unfortunately there is a lot of fly-by-nighters in the trade...some dude helped his cousin Billy "tar some shingles" and all of a sudden they think there a master roofer..puttin' shingles down on a strait, open go is not hard, it's when they hit the details, stacks, valleys, walls, etc that piss poor workmanship shows up. i would bet a good 80-90% of probs i deal with can be traced back to shoddy work, or they just didn't know how to do it.
i've seen it all, and i mean all-right up to a homeowner who put his shingles on upside down...
guys that won't warranty their work do it for a reason, ie lack of knowledge, a corner cutter or whatever...it gives guys who have dedicated their life to roofing a bad name.
a seasoned roofer can give you a good idea of the shape of your deck just by walking around it and doing a little poking around. if you can't see any rot from inside your attic, 99% of the time there will be nothing to worry about topside of your deck. rot doesn't just stay on the top, it goes right thru your wood...you'd be surprised at some of the materials i've seen nailed over top of rot by "professional" roofers...
a vaulted ceiling is even easier to check before ripping off or going over top.
this is a torch-on roof i did in '81, when torch-on first started hitting the market, 40 yrs and still going. if your roof is done right you will get good performance and life from it
Attachment 2093389