Q: We are considering installing 26 gauge standing seam sealed concealed fastener roofing panels over our existing shingles. We had planned to lay a 30 lb. felt down first but the question I have is that some of the shingles are curling on the tab ends, will this show through the panels? I would appreciate any help you can provide.
A: Oil canning, or ripples, in standing seam more commonly occur because either the roof deck itself is uneven (dips or swales), the panels are produced poorly, the metal has inherent unavoidable stresses in it, or the panels are not fastened properly and evenly.
Unless the curling is real bad, it should not cause oilcanning. However, I caution you, with the other things above in mind, that does not mean you will not have oilcanning.
30-pound felt as underlayment should be good. You may want to specify a slip sheet on top of that since you’re in a warm climate I believe.
Also, you may as an alternative wish to consider metal shingles which will get you the same durability as standing seam but will not have issues with oilcanning and are very forgiving going over uneven surfaces.