A homeowner by the name of Ed wrote the following to me:

Todd,

I have removed all the old shingles from the roof boards (not plywood) the roof is in great shape and have installed an ice shield over the roof. Can I install the metal roof over the Ice shield or do I need the 1 X 4 boards every 2 feet as well, It’s a low slope roof thus the ice shield for extra protection.

Ed

Here’s my response:

Ed, that is a great question. Thanks for contacting me. I have several thoughts:

1. Not all metal roofs can be installed over battens like that. Before even considering it, make sure the product you’re installing is approved by its manufacturer for that type of application.
2. Basically all metal roofs that can be installed on battens can also be installed on solid decking with underlayment. Well, there are some exceptions with some of the batten mounted tile and shake profiles but that is about it.
3. Ice and Watershield products are a vapor barrier & they do not breathe. This makes attic ventilation a special concern when those products are used. They can trap moisture inside an attic and that moisture can then find a place to condense.
4. It sounds like the metal panels you’re installing probably have a great deal of contact with the underlayment, unless they are installed on top of battens. This is the case with most vertical seam products.
5. If you do not have good ventilation in the attic, then I have some concern about installing the metal roof without battens. The low pitch of your roof indicates to me that perhaps you may not have much ventilation. And, keep in mind that good ventilation requires both intake of fresh air (usually at the bottom of the roof) and exhaust (usually at or near the ridge of the roof).
6. Installing metal panels without battens can reduce the temperature of your roof decking during cool evenings. This is because the metal panels touch the underlayment which touches the decking and cold is transmitted by conductance. This could cause the decking to reach dewpoint and especially if you have a lot of moisture in the attic (caused by inadequate ventilation), this will set off condensation in the attic. Having the metal panels up on battens though stops that conduction of cold and can help avoid this situation. Keep in mind also that some metal roofs have airgaps between the metal and the underlayment / roof deck, and those airgaps can serve the same purpose as battens in this scenario.

I hope this helps.

Source (AskToddMiller.com)