Signs your roof genuinely needs cleaning
Visible moss tufts on the lower tiles, especially on the north-facing pitch.
Lichen — silvery, scaly patches on individual tiles. These trap water and freeze in winter.
Black streaking on south-facing pitches — usually airborne algae. Cosmetic only at first, but it traps moisture.
Moss debris in the gutters every autumn. The moss falling off the roof is the moss currently doing damage.
Cleaning frequency by situation
Standard concrete tile, exposed roof, biocide treatment included: 5–7 years between cleans.
Same roof without biocide: visible moss back within 18 months, full clean needed in 3–4 years.
Roof under or near trees: 3–5 years even with biocide. Leaf litter holds moisture and feeds growth.
Slate roof: less prone to moss; deep cleans every 7–10 years are usually enough.
Flat felt or rubber roof: visual inspection every 2 years, clean only if growth is visible.
What a biocide top-up does
A biocide-only application costs around 25–35% of a full clean and extends the life of the existing clean by 2–4 years.
Year 0: full clean + biocide. Year 3: biocide-only top-up. Year 6–7: full re-clean. That cycle keeps total spend roughly half of cleaning every 3 years from scratch.
When you do NOT need a clean
A few black streaks alone — cosmetic, no harm to the roof.
A single lichen patch on one tile — fine to leave for now.
Anyone knocking the door telling you the roof needs urgent cleaning is a sales pitch, not a survey. Ask for photos from your own roof before agreeing to anything.