Is your backsplash a marble slab or tile? Marble tile is usually cut with a wet saw to lessen the chance of cracking from the heat caused by the friction of the blade. Cutting stone also produces a lot of dust.

It is a slab.

it is a for the notch for an above range microwave, which is already mounted to the wall, so I can’t use a wet saw or a circular saw.

I don’t have the option of taking the marble off the wall without cracking it, so I need to cut it in place. I could use a squirt bottle or sponge.

I don’t have a choice about dust.

What to use and how to do it?

Shim the wall for the bracket and set the microwave in front of the backsplash please. Yeah, the microwave will protrude a bit. No one will notice.

Thank you very much for your suggestion, but there is a bottom bracket that needs to screw into to the wall studs where the slab is.

Even if I shim, I’d still need to drill holes through the slab for the bracket to anchor to the wall.

Drilling marble is a piece of cake compared to cutting a splash accurately on site.

If suggest looking at doing a vent hood instead of an OTR. It’s shorter, and more effective. It’s more cohesive with the style direction of a marble slab backsplash as well.

This is an apartment in a high rise concrete building with 8 foot ceilings. There are cabinets above the stove and no exhaust available to the outside....and no other place for a microwave.

Joseph Cornett: since the area to drill and hang the mounting plate is all within 1”, and the weight of the microwave is meant to hang in the plate, won’t this put more stress on the marble leading to cracks?

Is getting a shorter microwave an option? I saw them in various heights when I was shopping.

I looked. Most direct vent only microwaves can be smaller, but I don't have a direct vent.

Who does a marble backsplash with an ORT? Skip that ugly thing and just do a hood.