I've been trying to research various methods on the joints between the winglet and the wingtip out of interest. I'm aware that the airfoil is attached to the underbody of the fuselage by means of a butt joint, but with companies like Aviation Partner who offer options such as BBJ's, how are these winglets attached?
There are a number of winglets available in market for retrofitting and their method of attachment will also vary. Boeing details the procedure for attaching a winglet to their BBJ and it is quite instructive. According to the article the main issue is modifying the wing to accommodate the winglet, rather than the attachment itself.
The more time-consuming part of the retrofit is installation of the wing modification to accommodate the winglet.
For this particular aircraft, the following steps are given:
Removal and replacement of the outboard upper and lower skin panels
Removal and replacement of rib 25, which is third from the outermost rib
Installation of stiffeners across rib 25.
Cutting of the closure rib (rib 27) and trimming of the two spars
Installation of the new center section of rib 27 and the new winglet attach fitting
Installation of the spar attach fittings
Installation of the aft-position light.
which is followed by installation of the winglet.
Installation of winglet, from Boeing Aero.