Q: Does a large amount of rain dilute the soil barrier created by a weed preventer on my lawn? 

A: If you spread a weed preventing pre-emergent product before a rain, the active chemical is dissolved by rainwater and then adheres to underlying soil particles. Clay and silt hold the chemical tightly in place, ready to thwart seed germination. So unless there is substantial soil erosion from your lawn, the pre-emergent will stay where you put it.