20 years? In that case I'd be real tempted to just do whatever was done
last time. Your climate etc. must be right for rolled roofing. Besides,
it's over a garage, so an occational leak (during a bad storm or
something) wouldn't really be a big issue anyway.

Here on a 2 over 12 pitch on our dining room we get constant leaks,
with all sorts of terrible degredation on the roof. And its only 3
years old! (from the previous owner -- we are putting in membrane this
summer).