Personally, I wouldn't use a heat lamp. The risk of fire is too high, and the birds really don't need it. If you have a coop where they can stay dry, they can tolerate temps well below freezing just fine. Those feathers are remarkably insulating. I feel using a lamp keeps them acclimated to warmer temps and makes for less hardy birds. Then, if the power goes out, etc, your birds aren't used to the cold and can suffer ill effects. They can truely handle sub-zero temps at night, as long as they're dry and have each other to snuggle with.
If you're determined to use a lamp, I'm really not sure how you could mount it above them in most coops. It would need to be I'd imagine at least three feet above the roost, and you'd need to have enough space so they could move out from under it, just like brooding baby chicks.
