Just be make sure anyone else is reading this for info.... there are some errors in here. first, unless my book is wrong, Page 84 in D1.1:2015 has the Prequalified joint B-L2c-S that you are referencing.
Also, your question asked about the "root face" which is 'f' in the sketch.
it is NOT the +1/16, -0 line (that is for root OPENING ' R' ) rather the +/-1/6 is for 'f'
Also, this question is a little misleading because even though there IS a tolerance stated for f, it also says "f= 1/4 MAX" and that MAX part is not always used in other diagrams, usually its a range, or just a single amount. Because of that little word, I don't believe you can't apply a fit up tolerance that would increase it MORE than 1/4". BUT if they had detailed it at 3/16" (because if you look at as detailed, they can't go more (0+) but can go less (f-) then you CAN apply the +/- 1/16" that is listed in as fit up, and make the face anywhere from 1/8 up to 1/4. Hope that helps anyone who is reading this.