(GMAW process .045" Metalcore wire)
Welding in a 1G position, in a T configuration (turn the T on its side and that is the joint in question), the Vertical parent metal extends approx. 8" above the joint to be welded and the other parent metal extends horizontaly several feet from the joint.  The vertical member is 4" thick and the horizontal is 1/2" thick.  Because of lack of access to the "other" side, there is a 1/2" backing bar placed first to the vertical member with 1/4" extending from the tip of the 45 degree bevel that is placed on the vertical member for a single bevel full pin joint config.  Now the question is, if we pre-weld a root pass joining the backing bar to the parent metal (at the edge of the bevel), and THEN place the Horizontal member against the backing bar and tack into place, then weld a second root pass joinint the backing bar to the vertical member, and then finish out the bevel with the necessary hot passes, IS the first root pass we place considered part of the horizontal member now and we then need another 3/16" root opening beyond the toe of that, OR is the first root pass just considered pre-welding a portion of the joint before it is assembled? 

The reason for doing the pre-pass is because of the gun angle not being sufficient enough with the restriction from the vertical member extending above the joint.