Mold is an extremely dangerous substance. It can form just about anywhere when conditions are right, and it poses a health hazard to humans. For this reason, it's important to eliminate mold and prevent it from forming wherever possible. So if you have mold growing on your ceiling, it's definitely a cause for concern.
If you're not sure how the mold got there, then read this guide to find out the most likely cause.
How It Gets There
You're probably not going to be happy to hear this, but the most likely way that the mold got on your ceiling is from a roof leak. Pipe leaks can sometimes cause mold to develop on the ceiling, but usually, it will become evident in a day or two if that's the case as the leak will become bad enough to start dripping through your ceiling.
With a roof leak, that may not necessarily be the case. It's possible that you have a very minor leak right now in your roof that's causing just a little rainwater or even just plain moisture to come in from outside. If you have an attic or crawlspace above your house, this is even worse news, because if the damage is appearing on the ceiling in the main part of your house, it likely means that water has already been seeping in and then pooling in your attic, where it's been inducing mold.
What to Do About It
The good news is that if you act quickly, you may be able to get away with your roof being repaired, not fully replaced. It will, of course, depend upon a variety of factors, like the age and condition of your roof, but that will be up for the roofer to decide.
For now, contact a roofer and set up an appointment for them to come out as soon as possible. If you're physically able to do so safely, putting a tarp over your roof in the meantime is a good idea. This will help to prevent more water from getting inside if you have a rainy day or just condensation accumulating on your roof in the mornings.
Your roofer will evaluate your roof to determine if it needs a repair or replacement. Make sure to show them all the parts of your house that have mold so that they can make sure to thoroughly check those parts of the roof.
Having a roof break down is a common thing to happen to homeowners a decade or so into owning a home. Don't ignore the problem as that's just likely to make it worse and require a full replacement. Get help now.
To learn more about residential roofing and roof leaks, contact a roofer in your area.