I have a section of roof on my house that has a slight downward pitch, approximately 10" over 15' which is currently a tar roof, but needs replacing. I would like to have it shingled, as one roofer has suggested, but everyone else I've talked to has recommended a rubber roof. I live in New England, so I am worried that the snow, water & ice will not properly drain off if there's not enough pitch and will back up under the shingles. Is there a standard or rule of thumb for this? Thanks.