Wood – Can 1/4″ bead board be used horizontally without sagging

I agree with answer #1 with the following exception: Do not use longer screws to attach the fixture. Find a suitable box with extensions as needed to make the box opening flush with the ceiling surface. You do not want a gap between the box and fixture the could allow heat and spark from a fault in the box to escape. Here are products that I would use in your situation. These may not be available at home improvement store but are available at electrical suppliers who service contractors.

• For support between studs -- Erico/Caddy TSGB24 or simply use a 2x4

• Box attached to support --4" octogon box

• Two extension boxes as need to bring opening flush -- 4" octogon extension box 2 1/8" deep

This combination gives you a box 5 3/4" deep. Adjust the support on the attic studs to make this flush with ceiling.

Remember that it is important to not leave a gap that could cause a smoldering fire in the voids of the ceiling structure. This is the reason that most electrical codes require that box faces be flush with the finished surface.

Go ahead and place the drywall over the top edge of the crown moulding. Sounds like the wall was already re-plastered with it in place. If you tried to remove it, you may end up re-doing the walls as well. Even forgetting that, it's almost impossible to remove moulding without sustaining some loss. Without replacement material available, removal with intents of preservation is extremely risky, especially with stain grade material. You don't want to go there.

If your joists were flat, you'd still only need to shim down the first 4 feet or so along the walls, no one will see the resulting slight slope. Keep this in mind when planning your shims. It doesn't have to be perfectly flat, unless that gives you pleasure. You also don't need to shim to completely fill the gap. An 1/8" gap makes a clean shadow line that is not detrimental (assuming it is perfectly uniform) at all. It also makes finishing easier. The taped joints of drywall are normally not perfectly flat, despite the tapered edges. There is a slight bulge which is usually invisible. But against straight moulding, depending on the profile, this bulge may be visible. You might consider grinding down the edge tapers at the board corners against the wall so the joint tape can be completely buried and the yet the joint against the moulding can still be perfectly flat.