Moss and algae on a roof usually show up where conditions stay damp and shady. A little bit in a very wet climate isn’t unusual, but heavy growth in specific spots suggests that those areas don’t dry out properly.
This can mean that water is lingering due to poor drainage, sagging sections or debris. Constant moisture, over time, can degrade roofing materials faster, weaken protective layers and eventually lead to leaks.
It also points to shade patterns – maybe an overhanging tree keeps that part of the roof damp, or nearby structures block sun. Trimming branches, cleaning gutters and improving drainage can help reduce growth.
Noticing where moss or algae appears gives you clues about how water and light behave on your roof, so you can act before it turns into a bigger problem.