My bathroom above my tub is getting some black spots on it that seems to be growing. I assume it's mold. I am wondering how should I deal with this? What should I expect it to cost?
First find out whether it is mold or not. What you do next depends on this.
Assuming it is mold you need to do two things.
Clean it off - disinfectant/bleach solution should do the job.
Find out why it's forming. This is the important bit as if you just do the first step it will come back.
The usual cause is condensation which in a bathroom means you have insufficient ventilation.
So the ultimate cure is to fit an extractor fan above the bath to remove the steam. You can get ones that wire up to the light circuit and then "run on" after the light is switched off to provide additional ventilation.
Once you've done this redecorate.
Every answer here suggests using a water-bleach solution, though I am suggesting cleaning the mould (if that is what it turns out to be) with just soap and water.
And I have my friends Health Canada and the Canadian Mortgage and Housing Council to back me up. CMHC is not a health authority, but they do have an interest in damage to mortgaged-backed homes and their typical clients are first-time home buyers who have little to no experience with home maintenance.
Both advise against using bleach to clean mould. Soap and water works fine and is safer (for you) to use, especially in poorly-ventilated spaces (you have a mouldy bathroom after all).
The CDC does not outright discourage the use of bleach for cleaning small areas of mould (it discommends it for for areas larger than 10 sqft) and does provide advice and instructions should you decide to use bleach.
The assumption we make here is that this is "typical" house hold mould and not something you'd need professionals to clean. Please don't attempt to use soap and water for black mould or mould that covers an area of more than 10 sqft.
You can start off with taking a 50/50 solution of bleach and water, then spraying the mold with it. Wait a few days.
To prevent it in the future, when showering you should be using the bathroom fan to allow the hot air to be exhausted outside rather than condense on the walls and ceiling (which are made of drywall of course).
They also make mold-inhibiting paint and mold-inhibiting drywall. Depending on your bathroom and funds, this may be a long term option. But bleach/water mixture will be your first step.
Yup, probably mold. First you'll want to clean it up. A weak solution of water and bleach should do the trick.
To make sure it stays gone, you'll want to make sure your bathroom is properly ventilated. If you don't have a vent fan, you'll want to install one. If you already have one, you'll want to make sure you use it. Hooking it up so it automatically turns on with the light is a good way of doing that.
They also have a mold-preventative that you can add to paint (pretty sure home depot carries it). I would strongly recommend using that and repainting the ceiling. I've used it myself several times and it works very well.
Although cleaning the surface like others have suggested would be good, if this was my house I would want to make sure the source mold is not coming from the attic. It could be your attic is not properly vented and moisture is forming of the other side of the ceiling and is so bad that it is coming though to the finished side. This would indicate a major issue. I would take a look up there above the bathroom and see if you see any mold. If so, you will want to bring in the pros.