Mold in vaulted ceiling
I previously posted about venting a vaulted ceiling and got one answer. An add on to that question is whether there could be a mold issue in the area between roof rafters if the area is not correctly vented.
Re: Mold in vaulted ceiling
With a properly vented vaulted ceiling, mold should not be a problem but I guess anything is possible. I presume you are speaking of ceilings that are applied directly to the rafters; the usual method of creating these. The following will not apply if there is attic space above the ceiling and below the roof itself!
Some contractors will 'overstuff' insulation on vaulted and cathedral ceilings so they can use a cheaper fiberglass insulation to meet code compliance instead of using the thinner 'high-R' insulation made specifically for this application. This can crush the cardboard or styrofoam 'ventilation channels' that should have been installed above the insulation, so that when the sheetrock is installed over it these 'channels' collapse rendering them ineffective. If you have vinyl soffits you're in luck- they can usually be removed easily enough to get your head in there to look up the rafter bays so that you can see what is going on, then replaced without damage to anything. Other forms of soffit material will make this inspection harder to do, so for those I usually pull the ridge vent and look inside from the top down. With these kinds of ceilings you must use a ridge vent since there is no other method that can ventilate the top of each bay as they are otherwise enclosed and thus unventilated.
Any mold you find up there could also be from a roof leak admitting moisture so that deserves a good look as well. I'd more suspect that if the visible mold is localized at one spot. I'd suspect a ventilation problem if it involves the larger part of the ceiling width parallel to the ridge. A physical inspection is the only way to know for sure.