I have a large (approx. 5' x 7') mirror over my bathroom double vanity with a lower corner that has come loose from the wall. I guess after 25 years the mastic let go of either the mirror, such that now there is 1/8" or so of play when I push on the corner. My question is how to I secure the mirror in the simplest manner?

A couple of notes in case it might be important:

• A few years ago we changed the countertop and backsplash, and there is a 1/8" gap between the bottom of the mirror and the quartz backsplash.
• There are no visible clips on the mirror, however there is a light fixture in the middle that is obviously tied into the wall.

I was thinking of using a tube of mirror mastic and fashioning an extension of thin-walled plastic tubing to get behind the mirror and squirting a glob, but before proceeding I thought it wise to consult some experts!

Thanks in advance for any advice,
WallyH

I would suggest just using a putty knife to spread adhesive behind the mirror and fashion a temporary 2 x 4 horizontal brace from the opposite wall to keep moderate pressure on the mirror until the adhesive sets. A few shims will help to adjust the amount of pressure applied, but be careful not to over do it or you might be buying a new mirror, and one that big is not cheap. Alternatively screw a 2 x 4 into wall studs on either side of the mirror to clamp the bottom of the mirror to the wall until it cures, but this method means patching holes and painting afterwards. If you are going to use a spray adhesive I suggest 3M High Strength 90 Contact Adhesive, which is supposed to be the strongest bonding contact cement. It does not have a spray tube but you might be able to swap spray heads with an aerosol sprayer that does have a tube like Great Stuff Insulation or WD-40.

ADD: Just an after-thought, if the gap is only 1/8" wide, why not fill it with clear silicone caulk?

https://www.homedepot.com/p/3M-17-6-oz-Hi-Strength-90-Spray-Adhesive-90-24/100151277