roof design

I am building a four bent, 16x24' shop with a 6:12 roof pitch. Principal rafters are connected to tie beams and posts with a modified english tying joint. Do I need to use a ridge pole? or will tongue and fork joints at the rafter peak (along with collar ties) be adequate?

There are a number of variables that affect the design of a roof. For example local snow and wind loads, roofing material and dead loads. In addition, details of the tying joint are critical. Outward thrust at the rafter feet is significant, increasing exponentially as the roof pitch is lowered.
In many tying joints the ties extend over the plates and the principal rafters rest on top of these. This helps prevent the rafters from rolling the plates off the posts. In addition the rafter feet need to be housed and tenoned into the tie to prevent them from slipping off the end of the ties.... be sure to leave enough relish on the end of the tie to withstand the strong outward thrust.

You may find a structural ridge a simpler solution. The safest bet is to have an engineer go over your plans. You may also try the Timber Engineering forum on this web site.