stability, avoiding condensation

I am planning to put insulation and a metal roof over my 2x6 tongue and groove cathedral ceiling. I thought I had a plan, but after talking more am not sure. I plan to strip the roof, put 2x6s on 4' centers, use foam insulation panels (polyiso, 4" deep) and then either 2x4 purlins or plywood before underlayment and metal roof. The main thing to attach the addition to are the purlins that support my roof. they are at 4" spacing (on centers) also. 1 If I get a 26 guage screw through metal roof, will that provide enough stability for the 2x6s with the 2x4 purlins on top or would plywood or OSB be better? 2. Is it a problem to use a membrane or felt over a solid surface like plywood or does it need some ventilation under it to keep the roof cool? (I'm considering getting a bare roof because I have seen and heard about so much failing paint with ceramic, Kynar, etc.) 3. I live in an area that usually goes over 100 in the summer, but can have snow for a week or two and freezing temperatures for longer in the winter. I intend to have space over the insulation and under the plywood or purlins. I understand that together with a ridge vent this arrangement should help prevent moisture problems, by giving the moisture somewhere to go. However, I have read a lot about crawl spaces with insulation under the floor that are not sealed having huge problems with condensation. This vented situation seems like the crawlspace one just flipped over and with less air space. Would the venting bring in moisture that would condense on the upper part of the 2x6s as they match the temperature of the roof over them? Thanks a lot for any help you can offer. There is still a lot for me to understand. Margaret

Make sure that you have a proper air/vapor barrier over the T&G prior to the poly-iso. You need to make sure that you stop all the air loss from between those board and get that area buttoned up properly. Overdeck venting is a great option but may not be necessary if you get the roof sealed up properly and insulated. Todd knows a whole bunch about this stuff and I would be happy to help you craft an approach as well. Painted roof finishes are fine. Do not believe that they will fail prematurely and they will give you the look you want. At the end of the day, guide your decisions by this tennant..."an ounce of prevention is worth a pound of cure". Do the roof properly the first time and know that the roof will last a lifetime.