  We consider flat epimorphisms of commutative rings $R\to U$ such that, for
every ideal $I\subset R$ for which $IU=U$, the quotient ring $R/I$ is semilocal
of Krull dimension zero. Under these assumptions, we show that the projective
dimension of the $R$-module $U$ does not exceed $1$. We also describe the
Geigle-Lenzing perpendicular subcategory $U^{\perp_{0,1}}$ in $R\mathsf{-Mod}$.
Assuming additionally that the ring $U$ and all the rings $R/I$ are perfect, we
show that all flat $R$-modules are $U$-strongly flat. Thus we obtain a
generalization of some results of the paper arXiv:1801.04820, where the case of
the localization $U=S^{-1}R$ of the ring $R$ at a multiplicative subset
$S\subset R$ was considered.
