Is moss on my roof a problem?
A small amount of moss does not automatically mean a roof is damaged, but heavy growth can retain moisture, block drainage and contribute debris to gutters and valleys. Whether removal is worthwhile depends on the covering, amount of growth and whether it is interfering with drainage or hiding defects.
When moss is worth acting on
Moss itself is not a defect. What matters is whether it is holding moisture against the covering, blocking the path water takes off the roof, or breaking away and collecting in gutters and downpipes.
North-facing and shaded elevations tend to hold moss most heavily, and roofs surrounded by trees collect more debris generally. Those are the areas worth keeping an eye on rather than the roof as a whole.
Removing it without causing damage
Aggressive removal can do more harm than the moss. High-pressure washing in particular can drive water under the covering, strip surface material from concrete tiles and disturb bedding and pointing.
Careful manual removal and clearing the gutters afterwards is the safer route, along with dealing with whatever is keeping the roof damp — overhanging branches, blocked outlets or poor drainage.