I beg to differ, but I’ve seen too many metal roofs that do leak. I love how everyone here says “if the system is properly installed it won’t leak,” which can be said about any roof for the most part. Problem is, there is the human factor when dealing with roof installations, and sometimes there are other elements that can not be foreseen. For example, if you have a building that has negative pressure, water can actually be sucked into the building around vents, wall joints, etc. You also have to deal with wind-driven rain, which I’ve seen cause roof penetrations, standing seams, and ridges leak in metal roofs. There are also some poorly designed systems. I recently dealt with a school system near Houston that was suing a metal manufacturer because several of the manufacturer’s metal roof systems leaked. The case was settled out of court, and the manufacturer agreed to remove a large portion of the metal panels, install additional layers of 30# felt, and then replace the metal valleys that were rusting from the underside, before re-installing the metal panels.
I have another client that has several buildings with the BulterRib roof on them, and he is constantly having people go out to repair the roofs. So, don’t believe anyone that trys to tell you that metal roofs don’t leak, because it is just not true. That would be like me telling you that EPDM roofs don’t lead “if installed properly.” That would be a true statement, but in all likelihood someone will poke a hole in the membrane or something else will cause it to leak in the next 15 years. Same with metal, there are many things that can cause a metal roof to leak, and your best protection is either a layer of ice & water shield or two layers of 30# felt. The reason most designers prefer ice & water shield to felt is because it will generally seal around any penetrations. Remember, when you install screws or nails to fasten the panels in place the underlayment is being punctured.
The only other advice I will leave you with is this: If you decide to have RoofingProFrank or someone similar install your metal roof without the proper underlayment, demand not only a 20-year leak-free warranty, but make sure it is an NDL (no dollar limit) warranty. Also, make sure the warranty covers damage to the building contents and losses, or make sure you are well insured if you have valuables in the building like computers, etc. Like I said before, you also want to make sure the roof has an FM rating of at least I-90, and depending on your location you may need an FM I-120 rated system. Also, you want the contractor to not only comply with the roofing panel manufacturer’s recommendations and requirements, you will also want them to comply with SMACNA.
Now, I will agree with RoofingProFrank that ice & water shield will increase the price of the job. I highly recommend the ice & water shield as a consultant, but if you can’t afford it over the entire roof then I would suggest the following. Install ice & water shield centered over any valleys in the roof, and target patches around roof penetrations such as soil stacks. Install 2, yes two, layers of 30# felt underlayment with side laps that are staggered. Then install the metal roof system in accordance with the manufacturer’s specifications. I would recommend that the contractor install a cleat along the eave edge of roof, and fasten the panels along the ridge. Roofing contractors will know what you mean if you tell them to float the eave and fix the ridge. Assuming you are using a variation of a standing-seam system, you will then want the contractor to install Z-flashings along both sides of ridge, and seal the flashings with a high-quality sealant. Install a metal ridge cap along the ridge, with the cap attached to the Z-flashings on both sides of ridgeline.
With regard to type of system, I would recommend that you use a system that utilizes a mechanically hemmed standing seam, especially a double-locked seam. There are several other options, but when choosing it would be prudent to select a high standing seam to help prevent moisture intrusion. Once again, a mechanically hemmed seam or soldered seam is best, but the attached sites gives you some other options:
mbci.com/mbci/docs/catalog/2 … Panels.pdf
Here is the SMACNA link also: