1. We took a yard stick and marked with a Sharpie the measurements of the new mirror where we would need to make our cut.. 2.

Then we took a board which acts a straight edge for scoring the mirror and … Measure it anyway.
Re: Cut Mirror On The Wall About the only way to do this is to take the mirror down before it gets cut. This is assuming that you cut across the shortest end (42") If you have to cut across the 78" obviously you will need a bigger piece, but it is MUCH easier if you can cut across the shortest side.

You might not be able to do this if the 78" side of the mirror is resting on a vanity top, or splash guard. The mirror is probably 1/4" thick. Go to a tile shop and get some smooth tile a bit thinner than the mirror. If it is glued you can use some piano wire, high pound test fishing line or something similar to slide it back and forth behind the mirror and down the wall to cut it lose.

The reason you want thinner is the mastic on the tile and the mastic on the back splash will add some thickness to the total and you … To begin we set the mirror on a large flat surface. STEPS.