Roof Underlayment Over Zip – Belt and Suspenders or Unnecessary Redundancy?
Is it a good idea to install underlayment over Zip roof sheathing or am I wasting my time?
The roof is sheathed with 5/8 in. T&G Huber Zip. When the panel joints are sealed with Zip tape the roof is essentially dried in except for the ridge where a 3-in. space is left for attic ventilation.
Taped Zip panels qualify as roof underlayment in most cases. 
This means that you don’t need to install tar paper or synthetic underlayment over the roof sheathing before installing the roofing. The exceptions are where the local building code or roof covering manufacturers’ require a membrane type underlayment.
In our case the IRC calls for ice dam protection in regions prone to ice dams — and homes in our region get ice dams. So we installed a self-adhering waterproof membrane along the eaves. And the shingle manufacturer calls for a valley lining of either a self-adhering membrane or roll roofing. I think of this as part of the underlayment process and it would be in addition to the taped Zip sheathing.
So with self-adhering membrane applied along the eave for protection from ice dam leaks and om in the valleys there’s technically no need for me to install underlayment over the rest of the Zip panels on the roof. 
But I did. I could rationalize that I installed the underlayment as a belt and suspenders approach to water management or because I had a couple of half-rolls of underlayment laying around or so the students helping on the job get experience properly installing underlayment. It’s probably out of habit — a 35-plus year habit of installing rolls of underlayment over roof sheathing before installing roof shingles. Am I wasting my time or is there an argument to be made for installing underlayment over Zip System roof sheathing?
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I noticed a new home being constructed in my neighborhood recently. It's walls were sheathed with Zip. I thought to myself, nice to see new product being installed instead of just typical OSB. A couple day's later I noticed they installed house wrap over the taped Zip System walls. I thought is this a "belt and suspenders" approach or would the addition of a typical house wrap over the zip be a place to trap moisture?
Absurd-re wall -either you install zip and tApe correctly or go with cdx and a SA WRB , Henry blueskin being one of the least expensive. Re roof, don’t see the point of using zip go with adavantech sheathing -will hold nails better and you won’t run into reverse shingling of tape horizontally and cover with a synthetic. Of course if you are putting ace ap builders grade shingle on, why even bother? Best to use tar pa[eer- it will disintegrate and come off easier when u replace that roof in 15 years or less !