On asphalt roofs, check the shingles. Fading color, especially with darker shingles, is normal wear. However, the shingles should be lying flat against the roof and should not be cracked or warped. This kind of damage is caused by wind, cold weather, and sometimes animals. Cracked shingles increase the possibility of water leaking and causing more severe water damage. Check the gutters and downspouts occasionally for an excessive amount of shingle granules, as this may be another indicator of your roof needing repairs. These granules are added to shingles for extra protection, and it is normal for them to fall off. Without them, your roof is more prone to sun and water damage and will cause roof leaks and deterioration. 
This is usually a sign of rot because of trapped moisture. This moisture may be the result of several things, including poor water runoff design, poor ventilation in the attic, and not maintaining the roof’s surface.  Look for sagging throughout the entire roof. Note any pools of standing water or snow accumulation that is concentrated in one area. It is worth hiring a professional to first, deal with the water damage, then correct the sagging roof and rafters as soon as possible. Delaying on such repairs can result in further damage such as a collapsed roof.
Moss on Roof
No matter the type of roof, moss growth is not something to ignore. Moss grows quickly in damp, shaded environments through the spread of spores, and needs immediate attention. As it grows, moss can push under the shingles, causing them to lift up. A common method of cleaning both moss and algae is by spraying directly onto it with a bleach and water solution. After allowing the solution to sit for 15 to 20 minutes, scrub from the top down with a soft-bristle brush and rinse off with low water pressure. 
Tadlock Roofing, “Granular Loss and What it Means for Your Roof,” Tadlock Roofing, February 26, 2018, https://tadlockroofing.com/granular-loss-and-what-it-means-for-your-roof/ (assessed February 25, 2021).
Henry Parker, “Causes of a Sagging Roof and Repair Options,” Home Reference, https://homereference.net/sagging-roof/ (assessed February 25, 2021).
“Algae & Moss Prevention and Cleaning for Asphalt Roofing Systems,” Asphalt Roofing Manufacturers Association, February 22, 2012, https://www.asphaltroofing.org/algae-moss-prevention-cleaning-asphalt-roofing-systems/ (assessed February 25, 2021).