  Given a smooth projective variety $X$ with a smooth nef divisor $D$ and a
positive integer $r$, we construct an $I$-function, an explicit slice of
Givental's Lagrangian cone, for Gromov--Witten theory of the root stack
$X_{D,r}$. As an application, we also obtain an $I$-function for relative
Gromov--Witten theory following the relation between relative and orbifold
Gromov--Witten invariants.
