Underlayment for roofs

15# felt will work if you install the shingles correctly. It wouldn't be a bad idea to throw a row or two of ice & moisture guard along the eave depending on your climate.

I've seen people Ice the whole thing on a 3/12, so if you do that, it won't be the first time. 3/12 is technically a low slope roof.

There are three basic types of underlayment used beneath roofing materials:

asphalt-saturated felt; rubberized asphalt; and non-bitumen synthetic.

One of the most common types of underlayment used in residential, steep-slope applications is black, ashphalt-saturated felt paper. Felt underlayment may be made from either organic or fiberglass substrate, although the organic is much more common. It's called "organic" underlayment because it has a cellulose base.

Felt underlayment is water-resistant, but not waterproof. It’s available in two thicknesses: 15-pound and 30-pound. Fifteen-pound felt has a perm rating of about 5, although this number can rise in high-humidity conditions.

Thirty-pound felt is more resistant to damage during installation of the roof-covering material, and will protect the roof longer if it should somehow become exposed to weather. The difference is obvious, once you see them together. Thirty-pound felt is much thicker and stiffer.

more info here : http://www.buyliquidroof.com/liquid-coatings.html