Every spring we get calls from Hampshire homeowners who’ve tried bleaching their roof themselves — usually with a garden sprayer and a few bottles of supermarket bleach. The roof looks instantly cleaner. Three months later, it looks worse than before. Here’s why.
What Bleach Actually Does
Household bleach (sodium hypochlorite at 4–6% concentration) is a strong oxidiser. Sprayed on a mossy roof, it:
- Discolours the moss — the green pigments break down within hours, giving the appearance of a clean roof
- Kills the surface growth — but bleach has poor penetration; it doesn’t reach the rhizoids (root-like structures) underneath
- Strips colour from the tiles themselves — particularly on coloured concrete tiles, leaving them looking patchy
- Corrodes metal flashings — lead, zinc, and copper all degrade under repeated bleach exposure
- Kills surrounding plants — the runoff into garden beds is genuinely toxic to most plants for 6–12 months
What Biocide Does
Professional roof biocide is a different class of chemical entirely — usually a quaternary ammonium compound (e.g. didecyldimethylammonium chloride) at carefully calibrated dilution. Sprayed on a mossy roof, it:
- Penetrates the rhizoid layer — kills moss, algae and lichen at the root, not just the surface
- Stays active for months — continues to kill regrowth as it appears, for 6–12 weeks post-application
- Doesn’t damage tile colour or finish — designed specifically for roof surfaces
- Is metal-safe — no flashing corrosion
- Has manageable runoff — we still rinse plants below the work area, but the chemistry is far less ecologically aggressive
How Long Each Lasts
The real test. Apply both to the same roof, come back in 24 months:
- Bleached roof: visible regrowth within 6–12 months, full regrowth by 18–24 months. Often worse than the original, because surface damage gives new spores easier purchase.
- Biocide-treated roof: minimal regrowth at 24 months, with a 5-year usable window before another clean is needed.
Why People Still Use Bleach
Three reasons:
- It’s cheap — £10 of bleach vs £80 of professional biocide
- It’s available — any supermarket vs a trade supplier
- It looks like it’s working — the colour change is dramatic and immediate
The math only works in the short term. Over a 10-year window, repeated bleach treatments cost more in labour and chemical than two professional biocide cleans.
The Safety Angle
One thing worth flagging: bleach + biocide should never be mixed. If you’ve recently bleached your roof and then hire a pro, tell them. Mixing the two produces chlorine gas in trace amounts — not dangerous outdoors at the dilutions involved, but worth knowing.
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