A home's roof doesn't need to actually sag or collapse, or be missing sections of shingles and tiles, to be in need of major repair work. In some cases, the shingles could be in place and the roof may look somewhat fine, but it may still need major repair work or even outright replacement. Note when your home's roof needs repair so you know not to put off this work and are sure to get it done quickly before your home suffers serious water damage due to leaks or defective shingles.
A discoloured roof can mean one of two things: either algae is growing on the roof or the shingles have become thin and worn. If algae is growing on the roof, you can clean this with a pressure washer and a bleach solution, but the algae will probably just grow back. You need to have damaged shingles replaced, so they don't allow standard water and resultant algae to form.
The same is true for shingles with missing granules; they are not insulating the home against water damage and can also be letting out your heating and air conditioning. Don't assume that dark spots or discoloration on the roof are just stains, but have these areas inspected and then repaired as needed.
Water spots and streaks
You should never have water stains on your home's exterior walls; the gutters should be catching rainwater and directing it to the downspouts, which then direct the water away from the home. However, if your home's roofing shingles have missing granules, the roof may be allowing that water to run down to the gutters too quickly. In turn, those gutters overflow and your home's exterior walls suffer water stains. If the gutters are in good repair, check the roof for needed updates.
Moisture and humidity
If your home's attic or crawlspace seems very damp, humid, and hot and stuffy, this often means that your home's roof is not venting heat and humidity as it should. Since heat rises and carries humidity with it, a home's roof will typically have some type of vent to allow that heat to escape. If the vents are blocked, insufficient for your home or otherwise damaged, they need immediate repair; this humidity in the attic can allow mould to grow and spread through the entire home. This could also mean that the roof itself is getting older; if the vents are old enough to need repair, the roofing shingles may also be ready for an inspection and replacement.
Contact a roofing contractor for more information and assistance.