What causes a rearview mirror to fall off the windshield in summer heat?

Hey folks! This is Patrick from Chip Happens Auto Glass.

Welcome to another Q&A session where I answer questions from y'all on the internet.

Today's question is:

"What causes a rearview mirror to fall off the windshield in summer heat?"

Summary:

Patrick explains that rearview mirrors tend to fall off windshields in summer heat because the adhesive was originally applied too thickly. As temperatures rise, the thick glue loses its tension and can no longer support the mirror, which weighs a few pounds; once the adhesive softens, gravity pulls the mirror down. Counterintuitively, his solution is to use only a very small amount of adhesive when reattaching it. He reports excellent results with this minimal application, noting it holds far better than the thick, puddle-like glue jobs he often encounters. The takeaway is that a falling mirror usually signals it was over-glued initially, and the proper fix is a clean, thin re-application rather than adding even more adhesive.

Answer:

What causes a rearview mirror to fall off a windshield in the heat of summer is that the glue is sometimes applied a little too thick. As it heats up, that thick glue loses its tension, and that lets the rearview mirror fall -- remember, there are actually a few pounds hanging on that small mount, so once the adhesive softens, gravity does its job and the mirror comes down. When we reapply the adhesive, we actually use just a very small amount, and we've had great luck with that holding on. It seems counterintuitive, but a thin application bonds better and holds longer than some of the glue jobs we pull off that are basically a legit puddle of adhesive. So if your mirror falls in the heat, it's usually a sign it was over-glued in the first place, and the fix is a clean, minimal re-application rather than piling on more glue.