Parashath Noah, Laws of Ziziyoth, Page 15-16:
8 (continued).['Odh Yosef Hai (18) If the corner of the tallith (with the zizith attached) gets torn off at a point further than three finger-breadths from the edge, one may not simply sew it back on as it is, as that would constitute preconstruction. One must first remove the zizith from the corner and affix it only after sewing the corner back on.
If less than three finger-breadths is torn from the corner of the tallith, the piece which has been torn off may not be replaced, because it is too small to be significant. A piece measuring larger than three by three finger-breadths must be sewn back on to the corner, and the zizith affixed to that.
If, however, the edge is not torn off completely, and something - even a few threads - remains attached to the tallith, it may be resewn, even if the torn portion measures less than three by three finger-breadths.
When doing any sewing to the corners of the tallith, it is important to remember not to use wool on a woolen tallith (the reason for this will be given later). Only silk thread should be used on a woolen or cotton tallith; and only cotton thread on a silk tallith.]
This learning should be in memory of Maran, HaRav HaGaon Ovadia Yosef, ztz'l.