Cement board for above a furnace?

[Raspy]

If you use the right metal, like corrugated galvanized roofing, or polished aluminum, you don't need an air gap because the metal stays cool. Mine is screwed directly to the backing and stays at room temp. The idea is to reflect the radiant energy. Painted sheetrock can get quite hot even 24" away from the stove, but with a radiant reflector the gap can be way less and be fine. Mine is about 8" away and could be closer.

 

[s219]

In new home construction, they often use foil-faced paper (stapled over insulation/etc) to buffer combustible surfaces from fireplace boxes, furnaces, hot chimney/exhaust pipes, etc. You really just need a shiny reflective surface that is properly installed.

For the generator shed I recently finished, I used a sheet of galvanized steel to act as the reflector above the muffler to shield the roof framing from any direct radiation. Picked up a 2'x3' piece for about $8 at Home Depot or Lowes (can't remember which, but the both should sell it). That would probably work fine for the OP's needs.

 

[chopped]

Just to update and to maybe clarify the reason i was looking to add this is. I already had 5/8 drywall over the wood furnace. and a small section of it extended over the oil furnace. in the back section there was nothing at all just open floor joists. My in intention was to add some sort of fire shield more than a concern for the heat.I used 1/2 in rockboard and the special screws. Went up easy and used plenty of screws so I doubt it will sag at all.
Both of these don't get as hot as they would if it were a straight wood stove with the heat. The higher heat is blown through the ducks so the ceilng temp is lower