Mirror above fireplace

unlikely to be plywood or your house would have gone up in flames by now. Is the builder still around?

I will check with the builder.

Keep in mind that it's a pre-manufactured fireplace. It doesn't have a brick & mortar chimney. I assume that it's a steel tube like set-up, like ductwork.

of course...I was being facetious. But there should be something the drywall was screwed to

Do you know what types up materials they might use to screw to the drywall?

Whenever I installed a prefab fireplace the wood studs were set at least 3 inches from the netal, by code. Sometimes we turned the studs sideways for more clearance. But we always screwed the drywall to studs. Are you sure there's no studs?

Anyways 20 lbs is light. Use two mollybolts.That should support the mirror.

Thank you for the reply.

I am not certain that there are no studs. I just know that I can't drive a nail past the drywall anywhere near the area I want to hang the mirror. Actually, I'm sure I could drive the nail further if I hit it hard enough, but I don't want to damage whatever is behind the drywall.

Should I drive the mollybolt into the stud, or whatever is behind the drywall?

No, mollybolts are made to pierce the drywall and then expand. They don't go into any wood. Ask your home despot salesman and they'll sell you the right ones.