I have a 26×45 garage with a 1:12 shed roof.  It is currently shingled and not surprisingly, it leaks.  I had considered reframing the roof to match an adjacent structure with a 9:12 pitch, but priorities don’t allow it right now.

In lieu of a fully adhered two layer rolled roof system, I decided to go with exposed fastener metal roofing that is good down to 1:12 pitch (fabral mighti-rib, aka PBR).  The metal is only a little more expensive than the rolled roofing system, will be a lot easier to install, and I don’t have to remove the existing shingles.  Fabral has a tech bulletin that details how to install metal roof over existing shingles using furring strips run horizontally.  This is where I run into a bit of an issue.

The Fabral bulletin says to use an underlayment between the old shingles and the furring strips.  I am fine with this recommendation; the problem is that all of the roofing underlayment I have looked into is only good down to a 2:12 pitch.  Even fully adhered systems like ice and water indicate that they are not generally recommended for anything less than 2:12.  Not to mention ice and water cannot be installed over shingles.

Basically, I just need to decide whether or not to install an underlayment over the shingles.  I would likely use a synthetic felt-replacement product offered by all of the big manufacturers, and Fabral.  At low slopes, the synthetic underlayment must be sidelapped by more than half the 48″ sheet width.  This means I would need twice as much as a normal roof installation, probably 3 1000sqft rolls.  Does the underlayment offer any added value and protection since it is not rated for my roof pitch?  or should I just punt and leave off the underlayment for this application?  Suggestions?

Lastly, I plan to put a call in to Fabral next week to see what their recommendation is as well.

Thanks.