  We study 5 dimensional grand-unified theories in an orbifold geometry by the
method of effective field theory: we match the low and high energy theories by
integrating out at 1-loop the massive Kaluza-Klein states. In the
supersymmetric case the radius dependence of threshold effects is fixed by the
rescaling anomalies of the low energy theory. We focus on a recently proposed
SU(5) model on $M^4 \times S^1/(Z_2\times Z_2^\prime)$. Even though the
spectrum of the heavy modes is completely known, there still are corrections to
gauge unification originating from boundary couplings. In order to control
these effects one has to rely on extra assumptions. We argue that, as far as
gauge couplings are concerned, the predictive power of these models is similar
to conventional GUTs.
