The longest allowable span I could find on line was 18' 5" -18' 11"
using 2 x 12 size of #2 douglas fir 12 nches apart.

Maybe you could inset a foot on each side with posts versus down the
middle and span the 2 remaining feet on each end.

RicodJour wrote:
James wrote:
Thanks for the input Anthony, and your experience is interesting and on
point.

From all the input received here, I now have no doubt that I can span 20
feet easily, using #2 southern pine 2 X 12 beams , on 16 inch
centers.

I'd have doubts. http://www.cwc.ca/design/tools/calcs/SpanCalc_2002/
Choose US species and the loads you indicated - you can't span 20'
easily or even barely. Your house, your call.

R