Rafter Span - Purlins

Need confirmation from some framing specialists. I am getting different numbers from online calculators and tables.

Question: What is the maximum rafter span for a roof built in the 1970’s with the following info:

2x6 Douglas Fir
24" O.C.
Dead Load 15.5 (Concrete tile + Plywood)
Live Load 20 psf
Span 20’
Slope 4:12

No Snow load (southern CA)

Trying to determine if one or two purlins are required.

I think I found my answer which is that rafters are calculated based on the horizontal run, not the actual length of the board. This is the horizontal distance from the inside surface of the supporting wall to the inside surface of the ridge board.


Good link Phil, Thanx for posting !