  Disformal transformation provides a map relating different scalar-tensor and
vector-tensor theories and gives access to a powerful solution-generating
method in modified gravity. In view of the vast family of new solutions one can
achieve, it is crucial to design suitable tools to guide their construction. In
this work, we address this question by revisiting the Petrov classification of
disformally constructed solutions in modified gravity theories. We provide
close formulas which relate the principal nulls directions as well as the Weyl
scalars before and after the disformal transformation. These formulas allow one
to capture if and how the Petrov type of a given seed geometry changes under a
disformal transformation. Finally, we apply our general setup to three relevant
disformally constructed solutions for which the seeds are respectively
homogeneous and isotropic, static spherically symmetric and stationary
axisymmetric. For the first two cases, we show that the Petrov type O and
Petrov type D remain unchanged after a disformal transformation while we show
that disformed Kerr black hole is no longer of type D but of general Petrov
type I. The results presented in this work should serve as a new toolkit when
constructing and comparing new disformal solutions in modified gravity.
