You should not apply Roundup when its raining or if rain is expected with 30 minutes to 6 hours of application. The rain may wash off the Roundup and reduce the performance, so you would  then need to re-apply. Dependant on the formulation, Roundup needs between 30mins (for Ready-To-Use formulations) and 6 hours (Concentrates) to penetrate the weed leaf. So it's best not to apply if rain is expected in this period.