Decal: Can I have effect on Roughness, but on on Metal

Hello, I’m making water puddle decals, and would like them to make the ground all shiny!

Using the normal+roughness mode of the decals allows me to create little waves, as well as have smooth water.

However, I don’t know how I could tell that decal to not also affect the metalness of whathever it is on. Right now, the metalness of what is under is overridden, which is not ideal.

Is there a way to have a decal affect roughness, but NOT affect metalness?

Thank you.