Hello, I really need some advice about the installation of a range hood soffit. The way our kitchen is laid out, the only way that we can have exterior ventilation for our range hood is to run a soffit in an L shape along ceiling out to an exterior wall. We have a 600 CFM hood with an 8 inch duct. The contractor built the part of the soffit that is closest to the actual range 16 inches off the wall (that is, it measures 16 inches deep by 12 inches tall) which will overhang our top kitchen cabs by 4 inches since they are only 12 inches deep. Once the soffit reaches the longer part of the L, it's only 12 x 12 which is much more aesthetically pleasing. The contractor doesn't want to change the 16 inch deep part to 12 inches deep because he says that the soffit nearest the range has to be considerably wider than the duct in order to avoid a fire hazard because that's where the ducting is hottest.

I don't know enough about construction to know whether this is a reasonable statement, and we need to live with the soffit overhanging our cabinets, or whether he's just being lazy and / or dishonest. I think it's BS but I'd really appreciate input from someone who knows better. Help!